Solve Ohm's law and the power formula together — enter any two of voltage, current, resistance and power (watts), and get the other two, with the formula wheel and the exact equation used.
Type any two of the four values — volts, amps, ohms or watts — in any units (mA, kΩ and the rest are handled), and the calculator fills in the remaining two live and shows which formula it used. No Calculate button, no “solve for” menu. Free and entirely in your browser.
Enter any two values to solve the other two.
Ohm's law applies to resistive circuits (resistors and other ohmic elements). For AC with reactance, or non-ohmic parts like diodes and LEDs, use impedance and the device's own characteristics — the simple V = I × R relationship will not hold.
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Worked examples
Open one to load the values, then change anything you like.
Ohm's law (often written without the apostrophe as “ohms law”) states that the current through a resistor is proportional to the voltage across it: V = I × R. Rearranged, that gives the three forms you use most:
V = I × R I = V ÷ R R = V ÷ I
Add the power formula (Watt's law, sometimes written “watts law”), P = V × I, and you can also bring watts into the picture. Together they cover every combination of voltage, current, resistance and power.
The Ohm's law triangle and formula wheel
Many people first learn the three core relationships as the Ohm's law triangle: V sits on top, with I and R beneath it. Cover the quantity you want and the triangle shows the formula — cover V and you get I × R, cover I and you get V ÷ R, cover R and you get V ÷ I.
The full formula wheel extends that idea to include power, collecting all twelve rearrangements in one place. Pick the quantity you want in the centre, and the outer ring shows the three ways to find it from the others.
The twelve Ohm's law and power formulas.
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
Ohm's law examples
A few worked examples — the kind of Ohm's law sample problems and quick mental math that come up on the bench:
Volts from current and resistance: 0.5 A through 24 Ω gives V = 0.5 × 24 = 12 V.
Watts from volts and amps: 12 V at 2 A is P = 12 × 2 = 24 W.
Resistance from power and voltage: a 60 W lamp on 120 V is R = 120² ÷ 60 = 240 Ω.
LED series resistor: a 5 V supply, 2 V LED and 20 mA target gives R = (5 − 2) ÷ 0.02 = 150 Ω.
Watts, volts, amps and ohms — how they relate
Voltage is the push, current is the flow, resistance is what opposes the flow, and power is the rate energy is used. Ohm's law ties the first three together; the power formula brings in watts. Because every pair determines the rest, you only ever need two known values. One thing the wheel cannot do is convert a single quantity into another — you cannot turn volts into ohms or amps into watts on their own, because they measure different things. Supply a second value and the calculator does the rest.
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Frequently asked questions
What is the formula for Ohm's law?
Ohm's law is V = I × R: voltage equals current times resistance. Rearranged, current is I = V ÷ R and resistance is R = V ÷ I. Pair it with the power formula P = V × I and you can move between voltage, current, resistance and power using any two known values.
How do I calculate watts from volts and amps?
Power in watts is voltage times current: P = V × I. So 12 V at 2 A is 24 W. If you have resistance instead of one of those, use P = I² × R or P = V² ÷ R. Enter any two of the four quantities and the watts are filled in automatically with the formula shown.
How do I find resistance from power and voltage?
Use R = V² ÷ P. For example, a 60 W bulb on 120 V has R = 120² ÷ 60 = 240 Ω. If you know power and current instead, use R = P ÷ I². Enter the two values you have and the calculator picks the right rearrangement.
Can I convert volts to ohms?
Not directly — volts and ohms measure different things, so you need a second quantity. With current, resistance is R = V ÷ I; with power, R = V² ÷ P. Enter voltage plus either current or power and the calculator returns the resistance. Voltage alone is not enough.
What's the difference between Ohm's law and Watt's law?
Ohm's law links voltage, current and resistance (V = I × R). Watt's law, sometimes called the power law, links power to them (P = V × I, P = I² × R, P = V² ÷ R). Combined, they make the 12-formula wheel that finds any of the four quantities from any two.