Ohm's Law Calculator
Enter any two values to solve for the other two. Covers voltage, current, resistance, and power.
V = I × R | P = V × I
Voltage (V) · Current (I) · Resistance (R) · Power (P)
V
A
Ω
W
Formula reference
V = I × R
Voltage = Current × Resistance
Voltage = Current × Resistance
I = V / R
Current = Voltage / Resistance
Current = Voltage / Resistance
R = V / I
Resistance = Voltage / Current
Resistance = Voltage / Current
P = V × I
Power = Voltage × Current
Power = Voltage × Current
P = I² × R
Power = Current² × Resistance
Power = Current² × Resistance
P = V² / R
Power = Voltage² / Resistance
Power = Voltage² / Resistance
About this tool
Ohm's Law Calculator
Ohm's Law Calculator solves for voltage, current, resistance, and power. Enter any two of the four and it works out the other two, so you do not have to remember which rearrangement of the formula you need.
How to use it
- Type any two known values into the voltage, current, resistance, or power fields.
- The remaining two are calculated as soon as there is enough information.
- Press Clear All to start a new calculation.
The four relationships
Ohm's Law itself is V = I x R: voltage equals current times resistance. Power adds a second relationship, P = V x I.
Combining them gives the two forms people forget: P = I squared x R, and P = V squared / R. Those are what let the calculator finish from any pair of inputs rather than only from voltage and current.
Worked example
A 12 volt supply feeding a 4 ohm load:
- Voltage (V)12 V
- Resistance (R)4 ohm
Result: Current is 12 / 4 = 3 A, and power is 12 x 3 = 36 W.
When it helps
- Sizing a series resistor for an LED so it gets the current it is rated for.
- Checking whether a resistor's wattage rating is high enough for the power it will actually dissipate.
- Working out how much current a load will pull before you choose a power supply or a fuse.
- Checking a measured reading against what the circuit should theoretically be doing.
Common mistakes
- Mixing milliamps and amps. A 20 mA LED current is 0.02 A, and entering 20 gives an answer that is off by a thousand.
- Confusing a resistor's power rating with the power it dissipates. A quarter-watt resistor asked to dissipate half a watt will run hot and eventually fail.
- Applying Ohm's Law directly to non-ohmic parts. Diodes, LEDs, and transistors do not have a fixed resistance, so the law applies to the resistor in the circuit, not to the semiconductor.
