Ohm's Law Calculator
Solve V = IR and P = VI for voltage, current, resistance or power from two known values, with the rearranged formula shown and all 12 Ohm's-law equations.
Ohm's Law Calculator: with the default inputs, solved value is 3.
Potential difference across the component. Not used when solving for V.
Enter milliamps as decimals: 250 mA = 0.25 A. Not used when solving for I.
Enter kilohms as thousands: 4.7 kΩ = 4700 Ω. Not used when solving for R or P.
The quantity you chose to solve for, in V, A, Ω or W.
- Solved for
- I = 3 A
- Voltage (V)
- 12
- Current (A)
- 3
- Resistance (Ω)
- 4
- Power (W)
- 36
Assumptions
- DC or purely resistive circuits (no reactance).
- Resistance is constant — the component is ohmic and at steady temperature.
- Values are RMS if you enter AC quantities.
| To find | Formula 1 | Formula 2 | Formula 3 |
|---|---|---|---|
| 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 |
Every entry comes from combining V = IR with P = VI. Pick the row for the unknown and the column that uses the two values you have.
How this is worked out
The formula
V = I × R (Ohm's law) P = V × I (electrical power) V = voltage in volts (V) I = current in amperes (A) R = resistance in ohms (Ω) P = power in watts (W) Combining them: P = I²R = V²/R
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Solve for
- Choose one of 4 options.Current (I) — from voltage & resistance · Voltage (V) — from current & resistance · Resistance (R) — from voltage & current · Power (P) — from voltage & current
- Voltage (V)
- Potential difference across the component. Not used when solving for V.defaults to 12
- Current (I)
- Enter milliamps as decimals: 250 mA = 0.25 A. Not used when solving for I.defaults to 2
- Resistance (R)
- Enter kilohms as thousands: 4.7 kΩ = 4700 Ω. Not used when solving for R or P.0 or more · defaults to 4
What you get back
- Solved valuemain answer
- The quantity you chose to solve for, in V, A, Ω or W.
- Solved for
- Voltage (V)
- Current (A)
- Resistance (Ω)
- Power (W)
What this assumes
- DC or purely resistive circuits (no reactance).
- Resistance is constant — the component is ohmic and at steady temperature.
- Values are RMS if you enter AC quantities.
About this calculator
Ohm's law says the current through a resistor is proportional to the voltage across it: V = IR. Add the power relation P = VI and you can get any of the four electrical quantities from any two. Pick what you want to solve for, enter the two values you know, and the calculator shows the rearranged formula with your numbers substituted, plus the power the component is dissipating.
How to think about it
Voltage is the push, current is the flow, resistance is how hard the material resists that flow. Double the voltage across a fixed resistor and the current doubles; double the resistance and the current halves. Power is the rate at which electrical energy turns into heat (or light, or motion), and because P = VI = I²R, the heat in a resistor grows with the square of the current — that is why thin wires carrying big currents get hot and why transmission lines run at very high voltage and low current.
Common mistakes
- Unit prefixes. 4.7 kΩ is 4700 Ω, 250 mA is 0.25 A. Mixing kΩ with mA happens to cancel (V = kΩ × mA), but mixing kΩ with A does not.
- Power rating. Solving for current is not the whole job — check that P stays under the component's rating. A ¼ W resistor with 12 V across 100 Ω would dissipate 1.44 W and burn.
- Series vs. parallel. Ohm's law applies to a single resistance. Combine series (R = R₁ + R₂) or parallel (1/R = 1/R₁ + 1/R₂) resistors first, then apply it.
Where the model breaks
Ohm's law describes ohmic materials — metals and carbon-film resistors at roughly constant temperature. Diodes, LEDs, transistors and gas-discharge tubes are non-ohmic: their current is not proportional to voltage, so a single "resistance" does not describe them. Even a plain incandescent filament is about ten times more resistive hot than cold. For AC circuits with capacitors and inductors, replace R with impedance Z, which also depends on frequency.
Frequently asked questions
▸What is Ohm's law?
V = IR: the voltage across a conductor equals the current through it times its resistance. It holds for ohmic materials such as metal wires and ordinary resistors at constant temperature.
▸How do I find current from voltage and resistance?
Divide: I = V ÷ R. A 12 V supply across a 4 Ω load pushes 12 ÷ 4 = 3 A.
▸How do I calculate power from voltage and resistance?
P = V² ÷ R. If you have current instead, use P = I²R; if you have voltage and current, P = VI. All three are the same law.
▸Why does my resistor get hot?
Because it converts electrical power into heat at P = I²R. Compare that number with the resistor's rating — common through-hole resistors are 0.25 W, and running above rating shortens their life or burns them out.
▸Does Ohm's law work for AC?
Yes for pure resistors. Once capacitors or inductors are involved you use impedance Z in place of R (V = IZ), and Z depends on frequency and has a phase angle.
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