How to use the Ohm’s law calculator
- Enter any two values — voltage, current, resistance or power — and leave the other two fields empty.
- Pick a unit prefix next to each value if you need one: millivolts, milliamps, kilohms, megohms, milliwatts or kilowatts. A 20 mA LED current is easier to type as “20 mA” than as “0.02 A”.
- Read the two missing values. They appear first, followed by the two you entered. The line under the results shows the formula and the numbers that produced them, and the table lists all twelve formulas with your values.
Results are rounded to four significant figures and shown with a suitable prefix (µ, m, k, M). The exact values in base units (volts, amperes, ohms, watts) are listed under the cards, together with the conductance, which is simply 1 ÷ R.
Ohm’s law and the power formula
Georg Simon Ohm published the relationship in 1827: the current through a conductor is proportional to the voltage across it. With the resistance R as the constant of proportionality:
- V = I × R — voltage (volts) equals current (amperes) times resistance (ohms)
- I = V ÷ R and R = V ÷ I — the same law rearranged
- P = V × I — electric power (watts) equals voltage times current
Substituting one equation into the other gives the rest: P = I² × R and P = V² ÷ R, V = √(P × R), I = √(P ÷ R), R = V² ÷ P and R = P ÷ I². That is why any two known quantities are enough to find the other two. Many textbooks draw these twelve formulas as a circle split into four quarters, the “Ohm’s law wheel”, or as a V-I-R triangle: cover the quantity you want and the other two show how to combine them.
Worked examples
A 12 V lamp drawing 2 A. R = 12 V ÷ 2 A = 6 Ω, and P = 12 V × 2 A = 24 W. Enter 12 and 2, leave resistance and power empty, and the calculator shows 6 Ω and 24 W.
A 1,500 W heater on a 120 V outlet. I = P ÷ V = 1,500 ÷ 120 = 12.5 A, and its hot resistance is R = V² ÷ P = 120² ÷ 1,500 = 9.6 Ω. A 15 A circuit carries it, but not together with much else (see the watts to amps calculator for AC loads with a power factor).
An LED resistor. A 5 V supply, a red LED that drops about 2 V and a target current of 20 mA leave 3 V across the resistor: R = 3 V ÷ 20 mA = 150 Ω, and the resistor dissipates P = 3 V × 20 mA = 60 mW, so a common ¼ W part is plenty. The resistor calculator turns 150 Ω into its color bands.
Common values at a glance
| Circuit | Voltage | Current | Resistance | Power |
|---|---|---|---|---|
| USB phone charger | 5 V | 2 A | 2.5 Ω | 10 W |
| Car headlight bulb (H4 low beam) | 12 V | 4.6 A | 2.6 Ω | 55 W |
| Space heater, U.S. | 120 V | 12.5 A | 9.6 Ω | 1,500 W |
| Electric kettle, Europe | 230 V | 10 A | 23 Ω | 2,300 W |
The resistance column is the resistance while the device is running. The cold resistance of a filament lamp or heating element is lower, which is why bulbs draw a short inrush current when they are switched on.
Where Ohm’s law applies — and where it doesn’t
Ohm’s law describes linear (ohmic) conductors: resistors, wires and heating elements at a steady temperature. Diodes, LEDs, transistors and gas discharge lamps are not ohmic — their current is not proportional to their voltage — so you apply the law to the resistor in series with them, as in the LED example. For alternating current, V = I × R holds for resistive loads; for motors, transformers and electronic power supplies, the resistance is replaced by the impedance Z and power needs the power factor (P = V × I × cos φ).
The calculator is a tool for learning, checking and quick design work. Work on mains wiring needs a qualified electrician and your local electrical code.
Frequently asked questions
What is Ohm’s law in simple terms?
The current through a resistor equals the voltage across it divided by its resistance: I = V ÷ R. Double the voltage and the current doubles; double the resistance and the current halves.
How do I calculate watts from volts and amps?
Multiply them: P = V × I. A device drawing 2 A from 12 V uses 24 W. For AC motors and other inductive loads, multiply by the power factor as well.
How do I find resistance from voltage and current?
Divide the voltage by the current: R = V ÷ I. 9 V across a resistor with 3 mA flowing gives 9 ÷ 0.003 = 3,000 Ω, or 3 kΩ.
Why does the calculator need exactly two values?
Voltage, current, resistance and power are linked by two equations (V = I × R and P = V × I), so two known values fix the other two. With three values the inputs could contradict each other, so leave two fields empty.
What is the Ohm’s law wheel?
A circle divided into four quarters for V, I, R and P, each holding the three formulas that give that quantity — twelve formulas in total. The table under the result is the same wheel with your circuit’s numbers.
Last updated: October 19, 2026