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About this tool
Enter any two of voltage, current and resistance to find the third — plus power.
The Ohm's Law Calculator solves V = I × R for whichever of voltage, current or resistance you leave blank, then reports the power dissipated as P = V × I. Fill in any two of the three fields and it returns the missing quantity plus a full breakdown in volts, amps, ohms and watts. It is built for electronics hobbyists, students and anyone sizing a resistor or checking whether a supply can carry a load.
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Enter the known values of voltage (V) and current (A) in the respective fields, leaving the resistance (Ω) field blank to calculate the resistance.
Alternatively, input the known values of voltage (V) and resistance (Ω), and the calculator will determine the current (A).
If you know the current (A) and resistance (Ω), simply fill in these fields and the calculator will compute the voltage (V) and power.
Leave any one of the three fields blank and it rearranges V = I × R for you instead of making you pick a formula.
Every result includes P = V × I in watts, which is the number that decides resistor and heatsink ratings.
Voltage, current, resistance and power are all shown together, so you can sanity-check the whole circuit at a glance.
Ohm's law is V = I × R: voltage in volts equals current in amps multiplied by resistance in ohms. Rearranged, I = V / R and R = V / I, which is exactly what this calculator does depending on which field you leave empty.
Power in watts is P = V × I — multiply the voltage across the component by the current through it. A 12 V supply pushing 2 A delivers 24 W, and substituting Ohm's law also gives the equivalent forms P = I² × R and P = V² / R.
Subtract the LED's forward voltage from the supply voltage, then divide by the target current. For a typical red LED with a 2 V forward drop on a 5 V supply at 20 mA, R = (5 − 2) / 0.02 = 150 Ω, dissipating about 0.06 W, so a standard 1/4 W resistor is ample.
Only in the simple form for purely resistive loads; with capacitors or inductors you replace resistance with impedance (Z) and use V = I × Z. This calculator handles the DC or resistive-AC case, where reactance is zero and the arithmetic stays as V = I × R.
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