Ohms Law Calculator
Use this Ohms law calculator, also called an Ohm calculator, to find voltage, current, resistance and power from any two known values. Enter two of V, I, R or P with the unit prefix you need and the other two are worked out with V = I × R and P = V × I. A resistor series and parallel tool is included.
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Enter any two values. The other two are calculated.
Two values are entered, so the other two are calculated. Clear one of the entered values or press Reset to change which values you enter.
Voltage
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Current
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Resistance
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Power
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V = I × R, P = V × I, so P = I² × R = V² ÷ R. Pick a prefix for each unit: µ (micro, 10⁻⁶), m (milli, 10⁻³), k (kilo, 10³), M (mega, 10⁶). Results are shown with an automatic prefix. Valid for ohmic (constant-resistance) parts in DC or for resistive AC loads; not for diodes, LEDs or reactive loads.
Ohm's law formulas (all 12)
| Voltage (V) | V = I × R | V = P ÷ I | V = √(P × R) |
|---|---|---|---|
| Current (A) | I = V ÷ R | I = P ÷ V | I = √(P ÷ R) |
| Resistance (Ω) | R = V ÷ I | R = V² ÷ P | R = P ÷ I² |
| Power (W) | P = V × I | P = I² × R | P = V² ÷ R |
Resistors in series and parallel
Series total
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R = R₁ + R₂ + …
Parallel total
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1/R = 1/R₁ + 1/R₂ + …
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How it works
- What is Ohm's law?
- Ohm's law says the current through a conductor is proportional to the voltage across it: V = I × R, where V is voltage in volts, I is current in amps and R is resistance in ohms. For example, 12 V across a 6 Ω resistor drives 12 ÷ 6 = 2 A.
- What are the formulas for electrical power?
- Power in watts is P = V × I. Combined with Ohm's law this gives P = I² × R and P = V² ÷ R. For example, 2 A through 6 Ω dissipates 2² × 6 = 24 W, the same as 12 V × 2 A.
- How do I find resistance from voltage and current?
- Divide voltage by current: R = V ÷ I. A 120 V supply that drives 2 A has R = 120 ÷ 2 = 60 Ω. If you know power instead, use R = V² ÷ P or R = P ÷ I².
- What units does Ohm's law use?
- Voltage is in volts (V), current in amperes (A), resistance in ohms (Ω) and power in watts (W). Prefixes scale them: 1 kΩ = 1,000 Ω, 1 mA = 0.001 A, 1 MΩ = 1,000,000 Ω. Convert to base units before applying a formula, or let this calculator do it.
- When does Ohm's law not apply?
- It holds for ohmic materials, where resistance stays constant as voltage changes, such as most metal wires and resistors at steady temperature. It does not describe diodes, LEDs, transistors or filament lamps whose resistance changes with voltage or heat, and AC circuits with inductance or capacitance need impedance instead of simple resistance.