Ohm's law calculator.
Fill in exactly two fields. Clear the two you want to calculate.
Ohm's law: V = I × R. Power: P = V × I. Enter any two values to get the other two. Example: 12 V across 100 Ω gives 0.12 A and 1.44 W. Use a resistor rated for at least twice the calculated power.
Voltage, current, resistance and power — enter any two, get the other two. Plus the next standard E24 resistor value and a safe wattage rating.
How the math works
V = I × R (Ohm's law) P = V × I (power) P = I² × R = V² ÷ R
These two equations connect the four quantities. If you know any two, the other two follow. The table below gives all twelve forms.
Ohm's law formula table
| To find | Formula | Formula | Formula |
|---|---|---|---|
| V (volts) | I × R | P ÷ I | √(P × R) |
| I (amps) | V ÷ R | P ÷ V | √(P ÷ R) |
| R (ohms) | V ÷ I | V² ÷ P | P ÷ I² |
| P (watts) | V × I | I² × R | V² ÷ R |
Worked examples
| Situation | Known | Result |
|---|---|---|
| LED resistor: 5 V supply, red LED 2.0 V, 20 mA | V = 3.0 V, I = 0.020 A | R = 150 Ω, P = 0.06 W |
| Space heater: 1500 W on 120 V | P = 1500 W, V = 120 V | I = 12.5 A, R = 9.6 Ω |
| Car accessory: 12 V, 5 A | V = 12 V, I = 5 A | R = 2.4 Ω, P = 60 W |
| Pull-up resistor: 3.3 V across 10 kΩ | V = 3.3 V, R = 10 kΩ | I = 0.33 mA, P = 1.09 mW |
Common pitfalls
- Use base units. Enter 20 mA as 0.02 A and 4.7 kΩ as 4700 Ω. A unit mix-up gives an answer that is wrong by a factor of 1,000.
- Check the power rating. A 1/4 W resistor that dissipates 0.3 W gets too hot and fails. Use at least twice the calculated power.
- LEDs and diodes are not resistors. Subtract the forward voltage first, then apply Ohm's law to the resistor only. The LED resistor calculator does this for you.
- Resistance changes with temperature. A lamp filament or heater has a much lower resistance when cold than when hot.
- AC motors have a power factor. For motors, P = V × I × power factor. This calculator assumes a purely resistive load.
Common questions
What is Ohm's law?
Ohm's law says that the voltage across a resistor equals the current through it times its resistance: V = I × R. If you know two of the three values, you can calculate the third.
How do I calculate current from voltage and resistance?
Divide the voltage by the resistance: I = V ÷ R. Example: 12 V across 100 Ω gives 0.12 A (120 mA).
How do I calculate power?
Multiply voltage by current: P = V × I. You can also use P = I² × R or P = V² ÷ R. Example: 12 V at 0.12 A is 1.44 W.
How many amps does a 1500 W heater draw on 120 V?
I = P ÷ V = 1500 ÷ 120 = 12.5 A. Its resistance when hot is R = V² ÷ P = 120² ÷ 1500 = 9.6 Ω.
Does Ohm's law work for every device?
No. Ohm's law describes resistors and other linear (ohmic) parts. LEDs, diodes, motors and lamps do not have a fixed resistance, so V = I × R gives only an operating-point value for them.
Can I use this for AC circuits?
Yes for resistive loads such as heaters, with RMS voltage and current. For motors and other loads with a power factor, P = V × I × power factor, so this calculator overstates their real power.
Sources
- Units: BIPM SI Brochure — definitions of the volt, ampere, ohm and watt.
- Standard resistor values: IEC 60063 — E-series preferred numbers (E24 used here).
- Circuit analysis: Horowitz & Hill, The Art of Electronics, Chapter 1.
Disclaimer. This calculator is for reference and education. For mains wiring, follow your local electrical code and use a qualified electrician.
See also
LED resistor calculator
Current-limiting resistor and wattage for any LED.
Voltage divider calculator
Find Vout from Vin, R1 and R2.
Wire size calculator
AWG for amps, distance and voltage drop.
Resistor color code chart
Decode the bands on a resistor.
SMD resistor code chart
3-digit, 4-digit and EIA-96 codes.
Watts to horsepower
Convert electrical power to hp.