An Ohm’s law calculator for any two known values
This Ohm’s law calculator works out voltage, current, resistance and power from any two of them. Type the two values you know — say 12 V and 4 Ω — and it returns the other two (3 A and 36 W), shows the formulas it used on the classic 12-formula Ohm’s law wheel, and writes out each step.
It handles everyday SI prefixes, so you can enter 250 mA, 4.7 kΩ or the resistor code 4k7 directly. A second tab adds up resistors in series and parallel, and, given a supply voltage, shows the current, voltage and power for each resistor. A third tab works backwards: give it a target value and it finds the missing resistor, or the best E12, E24 or E96 pairs, in parallel or in series. It is built for students, hobbyists and technicians checking a circuit on the bench.
How to use the Ohm’s Law Calculator
- Choose Ohm’s law: V, I, R, P.
- Enter any two values and leave the other two empty. Pick a unit (mA, kΩ, kW…) from the menu, or type a prefix such as 4.7k straight into the box.
- Read the two calculated values in the result, all four values in the tiles, and the highlighted formulas on the wheel.
- Use Clear all to start again, or try an example such as an LED resistor or an electric kettle.
- For resistor networks, open Series & parallel resistors, list the values, choose the connection and optionally enter the voltage across them.
Ohm’s Law Calculator features
Any two of four
Six possible pairs — V and I, V and R, V and P, I and R, I and P, R and P — each solved with the right two formulas.
Formula wheel
All 12 Ohm’s law and power formulas in one diagram, with the ones used for your answer highlighted.
SI prefixes and resistor codes
µ, m, k and M in the menus or typed (220m, 4.7k); codes like 4k7 and 2R2 are understood.
Series and parallel
Equivalent resistance for up to 200 resistors, with a per-resistor table of voltage, current and power.
Built-in check
Every answer is re-checked with an independent formula, P = V × I.
Reverse and E-series
Finds the second resistor for a series or parallel target, with the nearest E12, E24 or E96 value and the resulting error.
Ohm’s law and the power formulas
Georg Ohm found that the current through a conductor is proportional to the voltage across it: V = I × R, where V is in volts, I in amperes and R in ohms. Combined with the power law P = V × I (watts), substitution gives every other relationship: P = I²R, P = V²/R, I = √(P/R) and V = √(PR). Those 12 rearrangements form the wheel shown in the calculator.
| Find | From V and I | From V and R | From I and R | From P and one other |
|---|---|---|---|---|
| Voltage V | — | — | I × R | P ÷ I or √(P × R) |
| Current I | — | V ÷ R | — | P ÷ V or √(P ÷ R) |
| Resistance R | V ÷ I | — | — | V² ÷ P or P ÷ I² |
| Power P | V × I | V² ÷ R | I² × R | — |
Worked examples
- 12 V across 4 Ω: I = 12 ÷ 4 = 3 A, and P = 12² ÷ 4 = 36 W.
- A 1,000 W kettle on 230 V: I = 1,000 ÷ 230 = 4.348 A, and its hot element resistance is R = 230² ÷ 1,000 = 52.9 Ω.
- An LED series resistor: a 5 V supply and a red LED that drops about 2 V leave 3 V across the resistor. For 20 mA, R = 3 ÷ 0.02 = 150 Ω, dissipating 3 × 0.02 = 60 mW, so a ¼ W resistor is ample.
- 5 mA through 2.2 kΩ: V = 0.005 × 2,200 = 11 V, and P = 55 mW.
Resistors in series and parallel
In series the resistances simply add: 100 Ω + 220 Ω + 470 Ω = 790 Ω, and the same current flows through each. In parallel the reciprocals add: 1/R = 1/100 + 1/220 + 1/470, so R ≈ 59.98 Ω — always less than the smallest resistor. Two equal resistors in parallel give half the value (two 1 kΩ → 500 Ω); for any two, use product over sum, R₁R₂ ÷ (R₁ + R₂).
For AC power with power factor or three-phase supplies, use the watt calculator; for voltage drop along a cable or a voltage divider, use the voltage calculator. To change units such as kW to horsepower, the power converter does it directly.
Ohm’s Law Calculator FAQ
How do I use an Ohm’s law calculator?
Enter two known values — for example voltage and resistance — and leave the other two boxes empty. The Ohm’s law calculator returns the missing current and power and shows which formulas it used.
What is the formula for Ohm’s law?
V = I × R: voltage equals current times resistance. Rearranged, I = V ÷ R and R = V ÷ I. Add P = V × I to bring power into the picture.
How do I calculate resistance from voltage and current?
Divide the voltage by the current: R = V ÷ I. A device drawing 0.5 A from 12 V has an effective resistance of 24 Ω.
How do I calculate resistors in parallel?
Add the reciprocals of the resistances and take the reciprocal of the sum. For two resistors, multiply them and divide by their sum: 220 Ω and 330 Ω in parallel give 132 Ω.
How do I find the second resistor for a parallel target?
Use R₂ = R₁ × R ÷ (R₁ − R). To get 1 kΩ with a 3 kΩ resistor you need 1.5 kΩ. The reverse tab does this and picks the nearest standard value from E12 (10%), E24 (5%) or E96 (1%), the preferred values of IEC 60063.
Does Ohm’s law work for AC circuits?
For purely resistive loads such as heaters and kettles, yes, using RMS voltage and current. Motors, transformers and electronics have reactance, so you need impedance and power factor; the watt calculator covers power factor.
Why does a light bulb seem to break Ohm’s law?
A filament’s resistance rises sharply as it heats up, so its cold resistance measured with a meter is far lower than its working resistance. Ohm’s law still holds at any instant; R just isn’t constant.