BMR Calculator — Basal Metabolic Rate Calculator (Mifflin-St Jeor)

Guide

About the BMR Calculator

Last updated: July 2026 · Reviewed by the NeftCal editorial team

The BMR calculator (Basal Metabolic Rate calculator) estimates the number of calories your body burns each day at complete rest — just to keep your heart beating, lungs breathing, and cells repairing, before you factor in any movement at all. This free BMR calculator supports three well-established formulas (Mifflin-St Jeor, Revised Harris-Benedict, and Katch-McArdle), plus metric, imperial, and UK (stones/pounds) units, so whether you're comparing a Mifflin-St Jeor calculator result against Harris-Benedict or need a kilojoule figure for a food label read in Australia or Europe, one tool covers it all.

What It Measures

Basal Metabolic Rate is the minimum energy your body needs while completely at rest — no exercise, no recent meal to digest, just baseline survival functions like breathing, blood circulation, brain activity, and cell production. BMR is usually the single largest share of your total daily calorie burn, typically accounting for roughly 60-75% of the calories most sedentary-to-moderately-active people burn in a day, with the remainder coming from physical activity and the digestion of food.

Who Should Use This Calculator

This BMR calculator is designed for adults who want an evidence-based estimate of their resting energy needs — whether you're planning a weight-loss or muscle-gain diet, working with a personal trainer or dietitian, or simply curious how your metabolism compares across formulas. It's especially useful before using a TDEE or calorie calculator, since BMR is the starting number those calculations build on. It is not a substitute for clinical metabolic testing and is not intended for children or people with diagnosed metabolic or endocrine conditions, who should rely on individualized guidance from a healthcare provider.

Why It Matters

BMR is the foundation for calculating your Total Daily Energy Expenditure (TDEE) — the number used to plan calorie targets for weight loss, weight maintenance, or muscle gain. Without knowing your basal metabolic rate, calorie targets are just guesses; with it, you can build a more evidence-based nutrition plan around your actual resting energy needs, whether you track in calories or kilojoules, and revisit the number as your weight, age, or body composition changes.

Real-World Applications

Setting a starting-point calorie floor before building a weight-loss or muscle-gain plan
Feeding your BMR into NeftCal's TDEE Calculator or Calorie Calculator to add an activity multiplier
Comparing Mifflin-St Jeor, Revised Harris-Benedict, and Katch-McArdle side by side for the same body stats
Tracking how BMR is projected to shift over the next 10, 20, or 30 years with the BMR-by-age table
Preparing objective numbers ahead of a dietitian, personal trainer, or telehealth consultation

Tips for Accurate Results

Use Katch-McArdle only if you have a reasonably accurate body fat percentage from a DEXA scan or skinfold calipers — a rough guess skews this formula more than choosing Mifflin-St Jeor instead
Re-calculate periodically as your weight or age changes, since BMR shifts gradually over time even without conscious effort
Remember BMR is not your total calorie needs — combine it with an activity multiplier to estimate full daily energy expenditure
Double-check unit toggles when switching between metric, imperial, and UK stones/pounds to avoid mixed-unit entry errors
Choose kilojoules instead of calories if your food labels or region reports energy in kJ, for consistency with what you read on packaging

Formula

BMR Formulas, Explained

The exact Mifflin-St Jeor, Revised Harris-Benedict, and Katch-McArdle equations this calculator uses

Mifflin-St Jeor (Recommended / Default Formula)

Men: BMR = 10×W + 6.25×H − 5×A + 5

Women: BMR = 10×W + 6.25×H − 5×A − 161

Where:
W = weight (kg)
H = height (cm)
A = age (years)

Revised Harris-Benedict

Men: BMR = 13.397×W + 4.799×H − 5.677×A + 88.362

Women: BMR = 9.247×W + 3.098×H − 4.330×A + 447.593

Where:
W = weight (kg)
H = height (cm)
A = age (years)

Katch-McArdle

BMR = 370 + 21.6×LBM

Where:
LBM = lean body mass (kg) = W × (1 − BF% ÷ 100)
W = weight (kg)
BF% = body fat percentage

This calculator converts imperial and UK inputs to kilograms and centimeters internally before applying whichever formula you selected — mathematically identical to using formula variants built directly for pounds and inches, just computed in one consistent unit system behind the scenes. Results can be displayed in calories (kcal) or kilojoules (kJ) using the conversion 1 kcal = 4.184 kJ.

🛌

What Is BMR?

Basal Metabolic Rate is the number of calories your body burns at complete rest to maintain basic life functions — breathing, circulation, cell production, and temperature regulation — typically 60-75% of most people's total daily calorie burn.

🧭

Choosing a Formula

Mifflin-St Jeor is the default and most broadly accurate for the general population. Revised Harris-Benedict is an older equation that tends to estimate slightly higher. Katch-McArdle is most accurate if you know your body fat percentage.

🔥

Calories vs Kilojoules

Calories (kcal) and kilojoules (kJ) both measure energy — 1 kcal = 4.184 kJ. Kilojoules are the standard unit on nutrition labels in Australia, New Zealand, and much of Europe. Switch units anytime in Settings.

🧬

Why Katch-McArdle Differs

Unlike Mifflin-St Jeor and Harris-Benedict, Katch-McArdle uses lean body mass rather than total weight, making it more accurate for very lean or very muscular people whose body composition differs from population averages.

⚙️ Why These Formulas Work

All three formulas are regression equations derived from studies measuring actual resting energy expenditure (via indirect calorimetry) in real study populations, then fitting a linear formula to weight, height, and age, or to lean body mass for Katch-McArdle. Because fat tissue and muscle tissue burn calories at very different resting rates, formulas that separate the two — like Katch-McArdle — can outperform weight-based formulas for people whose body composition differs from the study averages.

🎯 When to Use Each Formula

Mifflin-St Jeor: general-purpose, most people, most situations
Revised Harris-Benedict: cross-checking against an older, still-common clinical reference equation
Katch-McArdle: athletes, bodybuilders, or anyone with a known, accurate body fat percentage

📋 Assumptions

Assumes you're an adult with a roughly typical metabolic profile, not a diagnosed metabolic or endocrine condition
Mifflin-St Jeor and Harris-Benedict assume average body composition for your weight and height
Katch-McArdle assumes the body fat percentage you enter is reasonably accurate

⚠️ Limitations of These Formulas

All three are population-average regression equations — an individual's true BMR can differ from the estimate by roughly 10%
None of them measure your metabolism directly; only clinical indirect calorimetry does that
Katch-McArdle accuracy depends heavily on how accurate your entered body fat percentage is
None account for thyroid conditions, certain medications, or other factors that can shift metabolic rate

Walkthrough

Step-by-Step: How to Use the BMR Calculator

From entering your details to understanding your basal metabolic rate

Choose your unit system

Select metric (kg/cm), imperial (lb/in), or UK (stones/pounds) depending on which units you know your weight and height in.

Enter your gender and age

Select your gender and enter your current age in years — both feed directly into the Mifflin-St Jeor and Revised Harris-Benedict formulas.

Enter your weight and height

Type your current weight and height into the fields for your chosen unit system.

Choose a BMR formula in Settings

Open the Settings section and pick Mifflin-St Jeor (recommended default), Revised Harris-Benedict, or Katch-McArdle. If you choose Katch-McArdle, also enter your body fat percentage.

Click "Calculate BMR" and review your results

See your BMR in calories or kilojoules, plus a BMR-by-age projection table showing how your basal metabolic rate is estimated to change at future ages.

Example

Worked Example

Using the calculator's own default scenario — a 30-year-old male, 70 kg, 170 cm, Mifflin-St Jeor

Scenario

A 30-year-old male weighs 70 kg and is 170 cm tall, using the default Mifflin-St Jeor formula.

GenderMale

Age30 years

Weight70 kg

Height170 cm

Step 1 — Weight term: 10 × 70 = 700.

Step 2 — Height term: 6.25 × 170 = 1,062.5.

Step 3 — Age term: 5 × 30 = 150.

Step 4 — Combine and apply the male constant: 700 + 1,062.5 − 150 + 5 = 1,617.5, which rounds to 1,618 kcal/day.

Step 5 — Convert to kilojoules if needed: 1,618 × 4.184 ≈ 6,770 kJ/day.

BMR

1,618 kcal/day

Formula

Mifflin-St Jeor

Equivalent

≈6,770 kJ/day

Explanation: A 30-year-old male at 70 kg and 170 cm has an estimated BMR of about 1,618 kcal/day using Mifflin-St Jeor — the calories his body would burn lying in bed all day with zero activity. For comparison, the same inputs produce about 1,672 kcal/day under Revised Harris-Benedict (roughly 3% higher) and about 1,580 kcal/day under Katch-McArdle at an assumed 20% body fat (roughly 2% lower) — illustrating how the choice of formula shifts the estimate even when the underlying body stats are identical. From here, this BMR figure feeds into the BMR Projection by Age table (declining to roughly 1,568 kcal/day at 40, 1,518 at 50, and 1,468 at 60 under the same formula and body stats) and into the TDEE Calculator, which multiplies it by an activity factor to estimate total daily calorie needs.

Interpretation

Understanding Your BMR Result

How your BMR relates to daily calorie intake

Daily Calorie Intake vs Your BMR	What It Generally Means	Consideration
Below your BMR	Consuming fewer calories than your body needs for basic survival functions alone	Not recommended without medical supervision — can trigger excessive muscle loss and metabolic slowdown
At approximately BMR (BMR to BMR+200 kcal)	Very low intake relative to total needs, close to the survival floor	Typically appropriate only as a short-term, medically supervised very-low-calorie plan
BMR × 1.2 to BMR × 1.4	Roughly matches typical maintenance needs for lower-activity lifestyles	A reasonable range for weight maintenance if your activity level is low to moderate
Above BMR × 1.6	Reflects the higher energy needs of more active lifestyles	Appropriate for very active individuals — check your exact multiplier with the TDEE Calculator

Your BMR is a floor, not a target: it tells you the minimum energy your body needs at complete rest, not how many calories you should eat in a day. Most people should eat at or above their BMR and use an activity multiplier — see the TDEE Calculator — to find an appropriate total daily calorie target for weight loss, maintenance, or gain.

This result is for informational purposes only and is not a medical diagnosis. BMR estimates are built from population-average formulas, not a personal metabolic measurement — they do not know your individual health history, hormone levels, or medical conditions.

ℹ️

This calculator provides educational estimates only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Results may vary depending on individual health conditions. Always consult a qualified healthcare professional regarding medical concerns.

Use Cases

Practical Use Cases for the BMR Calculator

Where a fast, formula-backed basal metabolic rate estimate is genuinely useful

🩺

Pre-diet baseline

Establish your resting energy floor before starting a structured weight-loss or muscle-gain nutrition plan.

🎯

Setting realistic calorie targets

Feed your BMR into the TDEE or Calorie Calculator to set an activity-adjusted daily target instead of guessing.

🏋️

Personal trainer client intake

Trainers can use BMR alongside body composition data when building a new client's nutrition plan.

📊

Tracking metabolism over time

Recalculate periodically to see how your BMR shifts as your weight, age, or body composition changes.

🧬

Comparing formulas for athletes

Athletes and bodybuilders can use Katch-McArdle for a lean-mass-adjusted estimate more relevant to their physique.

💻

Telehealth consult prep

Calculate your BMR in advance to bring an objective number into a virtual dietitian or doctor visit.

🥗

Weight-loss plan baseline

Use your BMR as the starting point before calculating a sustainable calorie deficit.

💪

Muscle-gain plan baseline

Use your BMR as the starting point before calculating a calorie surplus for lean muscle gain.

🌍

International kilojoule tracking

Switch to kilojoules for consistency with nutrition labels in Australia, New Zealand, or much of Europe.

📚

Nutrition science education

Students and educators can compare how three different BMR formulas estimate the same body stats.

🔁

Life-stage re-checks

Recalculate after a significant weight change, birthday, or body-composition shift to keep your number current.

⏳

Long-range planning

Use the BMR-by-age table to see how your number is projected to shift over the next 10, 20, and 30 years.

Pros & Cons

Benefits and Limitations

What this BMR calculator does well, and where it can't replace professional advice

✅ Benefits

Free, instant, and requires no signup or personal information
Runs entirely in your browser — your health data is never sent to a server
Offers three well-established formulas (Mifflin-St Jeor, Revised Harris-Benedict, Katch-McArdle) in one tool
Supports metric, imperial, and UK (stones/pounds) units
Results shown in calories or kilojoules
Includes a BMR-by-age projection table showing how your number is estimated to change over time
Katch-McArdle option available for lean-mass-adjusted estimates
Mifflin-St Jeor set as the default, since it's the formula most consistently validated as accurate for the general population
Downloadable plain-text summary of your results
Fast-loading and mobile-friendly
Useful as a repeatable tracking tool over time
Based on publicly documented, peer-reviewed regression formulas
Feeds directly into NeftCal's TDEE and Calorie Calculators for a full daily-needs picture

⚠️ Limitations

All three formulas are population averages — individual BMR can differ from the estimate by roughly 10%
Doesn't measure your metabolism directly; only clinical indirect calorimetry does that
Katch-McArdle accuracy depends entirely on how accurate your entered body fat percentage is
Doesn't account for thyroid conditions, certain medications, or diagnosed metabolic disorders
Not intended for children or during pregnancy, whose energy needs require different calculation approaches
A single reading is a snapshot; real BMR shifts gradually with weight, age, and muscle mass
Doesn't calculate total daily calorie needs by itself — requires an activity multiplier
Not a diagnostic or clinical nutrition tool — cannot replace a registered dietitian's individualized assessment

Reference

BMR Formula Comparison

How Mifflin-St Jeor, Revised Harris-Benedict, and Katch-McArdle compare for the same person

Formula	Example BMR (30yo Male, 70kg, 170cm)	Inputs Required	Best For
Mifflin-St Jeor	1,618 kcal/day	Weight, height, age, gender	General population (default, recommended)
Revised Harris-Benedict	1,672 kcal/day	Weight, height, age, gender	Cross-checking against an older clinical reference
Katch-McArdle	1,580 kcal/day (at 20% body fat)	Weight, body fat %, gender	Athletes, bodybuilders, known body fat %

Common Mistakes and Expert Tips

❌ Common Mistakes

Entering an inaccurate body fat percentage when using Katch-McArdle, which skews the entire result
Mixing unit systems — e.g. entering metric weight with imperial height
Treating BMR as your full daily calorie target instead of a resting-only baseline
Recalculating too frequently and reacting to normal day-to-day fluctuation instead of a real trend
Assuming a single formula is "correct" rather than a reasonable estimate with a margin of error
Ignoring the BMR-by-age projection when planning long-term nutrition strategy

💡 Expert Tips & Best Practices

Use Mifflin-St Jeor unless you have an accurate, recently measured body fat percentage for Katch-McArdle
Pair your BMR with the TDEE Calculator to get a full daily calorie estimate, not just the resting floor
Recalculate every few months, or after a meaningful weight or body-composition change, not daily
Cross-check your BMR against the Calorie Calculator when setting a weight-loss or muscle-gain target
If your goals involve weight loss, avoid eating below your BMR without medical supervision

📝

Summary: This BMR calculator gives you an instant, free basal metabolic rate estimate using Mifflin-St Jeor, Revised Harris-Benedict, or Katch-McArdle, in metric, imperial, or UK units, with results in calories or kilojoules and a BMR-by-age projection — a useful starting point for nutrition planning, not a medical diagnosis. Pair it with the TDEE Calculator to add an activity multiplier, or the Calorie Calculator to set a full daily calorie target for your goals.

FAQ

Frequently Asked Questions

Common questions about BMR calculations

Which BMR formula is most accurate?

The Mifflin-St Jeor equation (1990) is the most accurate for most people — validated on a larger, more diverse population than the original Harris-Benedict study. Revised Harris-Benedict (1984) tends to run slightly higher than Mifflin-St Jeor. Katch-McArdle is the most accurate for athletes and people with unusually high or low body fat, because it calculates BMR from lean body mass rather than total weight. Use it if you know your body fat percentage from a DEXA scan or skinfold calipers.

Why does my BMR decrease with age?

BMR decreases by roughly 1-2% per decade after age 20. This happens because of gradual muscle mass loss (sarcopenia), hormonal changes such as lower testosterone and growth hormone, and reduced organ function. Resistance training helps preserve muscle mass and can slow this decline. Use the BMR Projection by Age table in your results to see how your own number is estimated to change over time.

What's the difference between calories and kilojoules?

Both measure energy — 1 kcal (kilocalorie, commonly just called a 'calorie') equals 4.184 kJ (kilojoules). Calories are the standard energy unit on food labels in the US and UK, while kilojoules are standard in Australia, New Zealand, and much of Europe. Change the Results Unit dropdown in the Settings section of this calculator to switch between the two anytime.

How can I increase my BMR?

The most evidence-based ways to raise BMR are building muscle through regular strength training, since muscle tissue burns more calories at rest than fat tissue; eating adequate protein to preserve that muscle; staying well hydrated; avoiding severe or prolonged calorie restriction, which can lower BMR over time; and getting sufficient sleep, since sleep deprivation is linked to a reduced metabolic rate.

What information do I need to enter to calculate BMR?

You need your gender, age, height, and weight, entered in metric (kg/cm), imperial (lb/in), or UK (stones/lbs) units. If you select the Katch-McArdle formula in the Settings section, you'll also need your body fat percentage so the calculator can determine your lean body mass.

How do I switch between metric, imperial, and UK units?

Use the unit toggle buttons above the form — Metric (kg/cm), Imperial (lb/in), or UK (st/lbs) — to switch input fields. The calculator converts your entries to kilograms and centimeters internally before applying the BMR formula, so results stay consistent no matter which unit system you use.

What is the Mifflin-St Jeor formula?

For men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5. For women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161. It's the default formula in this calculator because research has consistently found it the most accurate equation for the general adult population.

How is Revised Harris-Benedict different from the original equation?

The Revised Harris-Benedict equation (1984) recalculated the original 1919 coefficients using updated population data, correcting for people being less physically active on average than the original study group. It remains a widely used option in this calculator but is generally considered slightly less accurate than Mifflin-St Jeor for most people today.

When should I use Katch-McArdle instead of Mifflin-St Jeor?

Use Katch-McArdle if you know your body fat percentage accurately — from a DEXA scan, skinfold calipers, or a reliable bioelectrical impedance scale — and you're notably lean or muscular, such as an athlete or bodybuilder. Because it calculates BMR from lean body mass rather than total weight, it avoids the overestimation that formulas based on average body composition can produce for these body types.

How is BMR different from TDEE?

BMR is the energy your body burns at complete rest with zero movement — just breathing, circulation, and cell repair. TDEE (Total Daily Energy Expenditure) adds your activity level on top of BMR, including exercise, walking, and digestion, to estimate your true total daily calorie burn. Use the TDEE Calculator to turn your BMR into an activity-adjusted daily calorie target.

Why does the BMR calculator ask for gender?

Men and women differ, on average, in body composition and hormone levels, both of which affect resting energy expenditure. Mifflin-St Jeor and Revised Harris-Benedict each use separate constants for men and women to account for this difference. Katch-McArdle sidesteps the question by calculating BMR directly from lean body mass instead of using a gender constant.

Can I export or save my BMR results?

Yes. After you calculate your BMR, click 'Export Result' below your results to download a plain-text file containing your BMR value and, when shown, the full BMR-by-age projection table.

What is a good BMR for my age and gender?

There's no single "good" BMR — it depends heavily on your body size, muscle mass, age, and sex, so comparing your number to a generic target isn't meaningful. A larger or more muscular person will naturally have a higher BMR than a smaller person, even at the same age and activity level. Instead of comparing your BMR to an external benchmark, use it as your personal baseline: track how it shifts over time and pair it with an activity multiplier (see the TDEE Calculator) to set realistic calorie targets for your own goals.

Does exercise change my BMR?

Exercise itself doesn't directly raise BMR during the workout — that calorie burn is counted separately as activity, not basal metabolism. However, consistent strength training can raise your BMR indirectly over time by increasing muscle mass, since muscle tissue burns more calories at rest than fat tissue. Cardio has a smaller long-term effect on BMR but still contributes meaningfully to total daily energy expenditure (TDEE).

Can BMR calculators be wrong?

Yes. BMR formulas like Mifflin-St Jeor are regression equations built from population averages, so any individual's true basal metabolic rate can differ from the formula's estimate by roughly 10% due to genetics, hormone levels, medical conditions, and muscle mass. They're a reliable starting estimate, not a lab-measured value — indirect calorimetry (a clinical breathing test) is the only way to measure your actual BMR precisely.

Is this BMR calculator free and private to use?

Yes. This BMR calculator is completely free, requires no signup or account creation, and runs entirely in your browser — your age, weight, height, and gender inputs are never transmitted to or stored on a server. You can recalculate as many times as you like across metric, imperial, or UK unit systems and any of the three supported formulas.