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🔌 Ohm's Law Calculator

Solve for voltage, current, resistance, or power using V=IR and the power formulas — enter any two known values.

🔌 Ohm's Law & Power
Solve Voltage
Solve Current
Solve Resistance
Solve Power

Fill in any two of the fields below — leave the rest blank.

📊 Result
Voltage
—
Voltage (V)
—
Current (A)
—
Resistance (Ω)
—
Power (W)
—
🔌

Choose what to solve for, enter two known values, and click Calculate

Guide

About the Ohm's Law Calculator

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

This free Ohm's Law calculator solves for voltage, current, resistance, or power in a DC circuit from any two known values. It combines Ohm's Law (V = IR) with the three power formulas (P = VI, P = I²R, P = V²/R) into a single tool, so you never have to look up which formula variant applies to your specific pair of known values.

How It Works

Choose which value you're missing — Voltage, Current, Resistance, or Power — then enter any two of the other three fields. The calculator automatically detects which two values you've supplied and selects the matching formula: for example, if you're solving for voltage and provide current and resistance, it uses V = IR; if you provide power and current instead, it uses V = P/I. Once the missing value is found, the calculator also fills in every other related value for a complete picture of the circuit.

Who Should Use This Calculator

Electronics students and hobbyists working through Ohm's Law problems, DIYers sizing a resistor or checking a component's power rating, and anyone who needs a quick electrical calculation without memorizing all twelve formula variants. It's a formula tool for basic DC circuit analysis — always follow proper electrical safety practices for real-world wiring work.

Real-World Applications

  • Solving Ohm's Law and power homework problems for electronics or physics class
  • Choosing the right resistor value for an LED or simple circuit
  • Checking whether a resistor's power rating is sufficient for a given current
  • Estimating current draw for a device given its voltage and resistance
  • Pairing with NeftCal's Resistor Calculator to decode a resistor's color bands into its resistance value

Tips for Accurate Results

  • Double-check your units before entering values — volts, amps, ohms, and watts, not millivolts or kilo-ohms.
  • If you enter all three of the non-target fields, the calculator uses the first two by priority — clear the third to be sure which formula is applied.
  • Remember Ohm's Law assumes a simple resistive (ohmic) component — it doesn't model reactive AC circuits with inductance or capacitance.
  • For a resistor's actual power rating, always check the component's datasheet rather than relying solely on a calculated value.
Formula

The Ohm's Law and Power Wheel

All twelve formulas this calculator can apply, grouped by target variable

Voltage (V)
V = I × R  |  V = P ÷ I  |  V = √(P × R)

Current (I)
I = V ÷ R  |  I = P ÷ V  |  I = √(P ÷ R)

Resistance (R)
R = V ÷ I  |  R = V² ÷ P  |  R = P ÷ I²

Power (P)
P = V × I  |  P = I² × R  |  P = V² ÷ R
⚡

Ohm's Law Core

V = IR is the foundational relationship — voltage is what "pushes" current through a resistance, and the three quantities are always proportional.

🔥

Power Formulas

Combining P = VI with Ohm's Law substitutions gives two more power formulas, useful whenever you know power plus just one other quantity.

🎯

Twelve Total Combinations

Each of the four quantities can be found three different ways depending on which two of the other three you already know — this calculator picks the right one automatically.

⚙️ Why This Approach Works

All twelve formulas are algebraic rearrangements of just two base relationships — V = IR and P = VI — substituted into each other. Automatically detecting which two inputs are filled and picking the matching rearrangement means you never need to memorize or manually derive the right formula.

🎯 When to Use It

  • Solving basic DC circuit problems for class or a hobby project
  • Sizing a resistor or checking a component's expected power dissipation
  • Quickly cross-checking a hand calculation

📋 Assumptions

  • A simple, purely resistive DC circuit
  • Values entered in standard units: volts, amps, ohms, watts
  • The component follows Ohm's Law (constant resistance regardless of voltage)

⚠️ Limitations

  • Doesn't model AC circuits with reactance (inductance or capacitance)
  • Doesn't apply to non-ohmic components like diodes or transistors
  • Not a substitute for a component's datasheet or a licensed electrician's work
Walkthrough

Step-by-Step: How to Use the Ohm's Law Calculator

From choosing what to solve for to reading your result

Choose what to solve for

Select Voltage, Current, Resistance, or Power depending on which value you're missing.

Enter any two of the other three values

Fill in any two of the three remaining fields — the calculator picks the correct formula automatically.

Click "Calculate"

See the missing value, plus the full set of related voltage, current, resistance, and power values.

Example

Worked Example

Solving for current, then rearranging to solve for power

Scenario 1 — Finding Current From Voltage and Resistance

Voltage12 V
Resistance100 Ω
Step: I = V ÷ R = 12 ÷ 100 = 0.12 A.
Bonus — Power: P = V × I = 12 × 0.12 = 1.44 W.
Current
0.12 A
Power
1.44 W

Scenario 2 — Finding Voltage From Current and Resistance

Current2 A
Resistance50 Ω
Step: V = I × R = 2 × 50 = 100 V.
Bonus — Power: P = I² × R = 4 × 50 = 200 W.
Voltage
100 V
Power
200 W
Use Cases

Practical Use Cases for the Ohm's Law Calculator

Where solving for voltage, current, resistance, or power comes up

🎓

Electronics & physics homework

Solve Ohm's Law and power problems and verify your work instantly.

💡

LED resistor sizing

Find the current-limiting resistor value needed for an LED circuit.

🔋

Battery & power budgeting

Estimate current draw and power consumption for a battery-powered project.

🛠️

DIY electronics projects

Check whether a resistor's power rating is sufficient before wiring a circuit.

🔍

Quick hand-calculation checks

Cross-check a manual Ohm's Law calculation before finalizing a design.

Pros & Cons

Advantages and Limitations

What this Ohm's Law calculator does well, and where it has boundaries

✅ Advantages

  • Free, instant, and requires no signup
  • Runs entirely in your browser — no data is sent to a server
  • Automatically picks the right formula from any two known values
  • Covers all twelve formula variants across V, I, R, and P
  • Shows the complete set of related values, not just the one you asked for

⚠️ Limitations

  • Doesn't model AC circuits with reactance (inductance or capacitance)
  • Doesn't apply to non-ohmic components like diodes, transistors, or filament bulbs
  • Not a substitute for a component's datasheet or professional electrical work
Reference

Common Mistakes and Expert Tips

Get an accurate Ohm's Law calculation

❌ Common Mistakes

  • Mixing up millivolts/milliamps with volts/amps without converting first
  • Applying Ohm's Law to a non-ohmic component like a diode or LED, which don't follow a simple linear relationship
  • Forgetting to square current or voltage in the power formulas (P = I²R, not P = IR)
  • Assuming a resistor's power rating is automatically sufficient without checking the datasheet

💡 Expert Tips & Best Practices

  • When choosing a resistor, always add a safety margin above the calculated power dissipation
  • Use NeftCal's Resistor Calculator to decode a physical resistor's color bands into its actual resistance
  • For LED circuits, subtract the LED's forward voltage from the supply voltage before calculating the resistor value
  • Double-check your answer using a second known-value pair when possible, as a sanity check
📝

Summary: This Ohm's Law calculator solves for voltage, current, resistance, or power from any two known values, using V=IR and the power formulas — free, instant, and covering all twelve formula variants automatically. Pair it with the Resistor Calculator or Voltage Drop Calculator for related electrical work.

FAQ

Frequently Asked Questions

Common questions about Ohm's Law

What is Ohm's Law?
Ohm's Law states that voltage equals current multiplied by resistance: V = I × R. It describes the relationship between voltage (V, in volts), current (I, in amps), and resistance (R, in ohms) in an electrical circuit.
How do I calculate voltage, current, or resistance?
Voltage = Current × Resistance (V = IR). Current = Voltage ÷ Resistance (I = V/R). Resistance = Voltage ÷ Current (R = V/I). This calculator also supports solving each of these from power and one other value, using the extended power formulas.
How is power related to Ohm's Law?
Power (P, in watts) equals voltage times current: P = VI. Combining this with Ohm's Law gives two more useful forms: P = I²R (power from current and resistance) and P = V²/R (power from voltage and resistance).
What if I only know power and one other value?
This calculator handles that case too. For example, if you know power and current, it finds voltage using V = P/I, or resistance using R = P/I². Enter power alongside whichever other value you have, and the calculator selects the matching formula automatically.
What are the standard units in Ohm's Law?
Voltage is measured in volts (V), current in amperes/amps (A), resistance in ohms (Ω), and power in watts (W). This calculator expects and returns values in these standard SI electrical units.
Does Ohm's Law apply to all electrical components?
Ohm's Law applies precisely to "ohmic" components, like standard resistors, where resistance stays constant regardless of voltage or current. Some components, such as diodes, transistors, and light bulb filaments at high temperature, are "non-ohmic" and don't follow a simple linear V=IR relationship, so Ohm's Law only approximates their behavior.
Learn More

Authoritative Resources on Electrical Circuits

Standards references to complement this calculator

🏛️ NIST — SI Units: Electric CurrentU.S. federal reference on electrical units ↗ 🦺 OSHA — Electrical SafetyU.S. federal electrical safety standards ↗

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