Calculate your Body Mass Index instantly. Supports metric, imperial, and UK (stones/lbs) units. Uses standard WHO cutoffs and shows your healthy weight range.
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Enter your weight and height to calculate BMI
The BMI calculator (Body Mass Index calculator) estimates whether your weight falls within a healthy range for your height, using the same body mass index formula doctors, nurses, and public health agencies have relied on for decades as a first-pass weight screening tool. This free BMI calculator supports metric (kg/cm), imperial (lb/in), and UK (stones/pounds) units, so whether you're looking for a metric BMI calculator, an imperial BMI calculator, or a UK BMI calculator with stones and pounds, one tool covers all three.
Body Mass Index is a simple ratio of weight to height (kg/m²) that correlates, on average, with body fat levels across large populations. It doesn't measure body fat directly — instead it acts as a quick screening indicator that public health bodies like the World Health Organization (WHO) use to flag potential weight-related health risk during routine checkups, without needing specialized equipment like body-fat calipers, bioelectrical impedance scales, or a DEXA scan.
This BMI calculator is designed for adults aged 20 and over who want a quick, evidence-based read on their weight category. It's useful for anyone starting a weight-management journey, tracking progress over time, preparing for a doctor's visit, or simply curious how their weight and height compare to WHO's standard classifications. It is not intended for children, teenagers, or elite athletes, whose body composition needs a different interpretation — see the Limitations section below.
Because BMI is quick to calculate from just two measurements, it remains one of the most widely used weight-screening tools in clinical and public health settings worldwide. Tracking your body mass index over time with this healthy weight calculator can help you notice gradual weight trends before they become significant, set realistic and measurable goals, and have more informed, data-backed conversations with a healthcare provider — though a single BMI reading should never replace a professional health assessment.
How this BMI calculator converts your weight and height into a body mass index score
This calculator converts imperial and UK inputs to metric internally, then applies the metric formula — mathematically identical to using the imperial formula directly, just computed in one consistent unit system behind the scenes.
Underweight: below 18.5 | Normal weight: 18.5–24.9 | Overweight: 25–29.9 | Obese Class I: 30–34.9 | Class II: 35–39.9 | Class III: 40+
BMI Prime = BMI ÷ 25 (1.0 marks the upper limit of normal). Ponderal Index = Weight (kg) ÷ Height (m)³ — less sensitive to height extremes than standard BMI.
For South Asian and East Asian populations, health organizations recommend lower cutoffs: Overweight at BMI 23+, Obese at BMI 27.5+, compared to the standard WHO cutoffs of 25+ and 30+ used above.
From entering your measurements to understanding your BMI category
Select metric (kg/cm), imperial (lb/in), or UK (stones/pounds) depending on which units you know your weight and height in.
Type your current weight and height into the fields for your chosen unit system. Accurate, recent measurements give the most useful result.
Gender fine-tunes the healthy weight range shown in your results, since body composition differs slightly between men and women at the same BMI — it doesn't change the core BMI formula itself.
The calculator converts your inputs to metric internally, then divides your weight in kilograms by your height in meters squared to compute your BMI score.
See your BMI score, WHO weight category, healthy weight range, weight to lose/gain, BMI Prime, Ponderal Index, and two charts visualizing where you land.
Using the calculator's own default scenario — 70 kg at 170 cm
Suppose you weigh 70 kg and are 170 cm (1.70 m) tall.
Explanation: A BMI of 24.2 is just under the 25.0 overweight threshold, placing this example in the WHO "Normal Weight" category with a BMI Prime of 0.97 (just under the 1.0 upper-limit marker). This is a single-point screening result — someone with substantially more muscle mass than average carrying the same weight and height could see the same 24.2 reading despite having a very different body fat percentage, which is why BMI works best alongside other measurements, not in isolation.
What each WHO BMI category generally indicates
| BMI Range | WHO Category | General Health Consideration |
|---|---|---|
| Below 18.5 | Underweight | May be linked to nutritional deficiency or other underlying causes; worth discussing with a doctor |
| 18.5 – 24.9 | Normal weight | Generally associated with the lowest average health risk for most adults |
| 25 – 29.9 | Overweight | Modestly elevated average risk; lifestyle changes are commonly recommended |
| 30 – 34.9 | Obese (Class I) | Increased average risk of weight-related health conditions |
| 35 and above | Obese (Class II/III) | Significantly elevated average risk; professional guidance is strongly recommended |
If your BMI falls in the Normal range: this generally aligns with the lowest average population health risk — maintaining your current activity level and eating pattern is typically a reasonable goal, alongside periodic re-checks.
If your BMI falls below or above the Normal range: the healthy weight range shown in your results gives you a concrete target window. Gradual, sustainable changes to activity and nutrition are generally favored over rapid or extreme approaches.
This result is for informational purposes only and is not a medical diagnosis. BMI is a population-level screening tool, not an individual diagnostic measure — it does not know your muscle mass, bone density, medical history, or fat distribution.
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.
Where a fast, standardized body mass index reading is genuinely useful
Get a quick BMI reading before an annual physical so you can discuss trends with your doctor.
Establish a baseline BMI before beginning a diet or exercise program.
Recalculate periodically to see whether your weight trend is moving toward your healthy range.
Use as a quick, private self-check alongside employer wellness screenings.
Trainers can use BMI as one baseline data point when onboarding new clients.
Check your BMI against both standard WHO and adjusted Asian population cutoffs.
Students and educators can use it to illustrate BMI calculation and classification.
Calculate your BMI in advance to have it ready for a virtual doctor's visit.
Bring an objective starting BMI figure into a conversation with a dietitian.
Use the healthy weight range shown to set an evidence-based target range, not a guess.
Recheck your BMI after a major life change like a new job, move, or lifestyle shift.
Switch between metric, imperial, and UK units — useful for international travelers or expats.
What this BMI calculator does well, and where it can't replace professional advice
Two classification systems this calculator lets you compare
| Category | WHO Standard Cutoff | Asian Population Cutoff |
|---|---|---|
| Underweight | Below 18.5 | Below 18.5 |
| Normal weight | 18.5 – 24.9 | 18.5 – 22.9 |
| Overweight | 25 – 29.9 | 23 – 27.4 |
| Obese | 30 and above | 27.5 and above |
Summary: This BMI calculator gives you an instant, free, WHO-standard body mass index reading in metric, imperial, or UK units, along with your healthy weight range, BMI Prime, and Ponderal Index — a useful screening starting point, not a medical diagnosis. Pair it with related tools like the Body Fat Calculator and BMR Calculator for a fuller picture of your body composition and energy needs.
Common questions about BMI calculations
Official guidance to complement this calculator — not a substitute for licensed medical advice
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