Ohm's Law states that voltage equals current multiplied by resistance: V = I x R. It describes the relationship between these three fundamental electrical quantities in a linear (ohmic) conductor. If you know any two of voltage, current and resistance you can calculate the third.

What is the formula for electrical power?

Electrical power in watts equals voltage in volts multiplied by current in amperes: P = V x I. Combined with Ohm's Law this gives three equivalent power formulas: P = V x I, P = I squared x R, and P = V squared divided by R. Knowing any two of V, I, R or P lets you find all four quantities.

What is the difference between watts and volt-amps?

Watts measure real (active) power, which is the rate at which energy is actually consumed. Volt-amps (VA) measure apparent power in AC circuits and equals voltage multiplied by current without accounting for the phase angle between them. In DC circuits watts and volt-amps are equal. In AC circuits watts equal volt-amps multiplied by the power factor.

How many watts does 12 volts at 2 amps produce?

12 volts at 2 amperes produces 24 watts (P = V x I = 12 x 2 = 24 W). The resistance of the load is 6 ohms (R = V / I = 12 / 2 = 6 ohms).

Volts Amps Watts Ohms Calculator

This calculator solves all four fundamental electrical quantities, voltage (V), current (I), resistance (R) and power (P), from any two values you already know. You do not need to remember which formula to use: enter any two fields, leave the others blank, and the calculator applies Ohm's Law (V = I x R) and Joule's Law (P = V x I) to find the remaining two. All 12 possible two-input combinations are handled automatically. The four quantities are linked by a set of simple but powerful relationships. Voltage is the electrical pressure pushing current through a circuit, measured in volts. Current is the rate of charge flow, measured in amperes. Resistance is the opposition to that flow, measured in ohms. Power is the rate at which energy is transferred or consumed, measured in watts. These relationships underpin almost every calculation in electronics and electrical engineering. Whether you are sizing a resistor for an LED circuit, working out the current draw of a motor, checking the load on a power supply, or estimating heat dissipation in a component, you will use some form of these formulas. In a DC circuit the relationships are exact. In an AC circuit they apply to resistive loads; for reactive loads (capacitors and inductors) you also need to account for the power factor. Enter two known values, choosing from voltage in volts, current in amps, resistance in ohms or power in watts, and the calculator returns all four quantities instantly. Leave exactly two fields filled and the other two blank for best results.

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Voltage (V) in volts

Current (I) in amps

Resistance (R) in ohms

Power (P) in watts

24 W

power

Voltage12 V

Current2 A

Resistance6 Ω

Enter exactly two values and leave the other two blank. All four quantities will be calculated from those two inputs.

How it works

The calculator identifies which two fields you have filled and applies the appropriate formula. The 12 combinations reduce to four formula groups. From V and I: R = V / I and P = V x I. From V and R: I = V / R and P = V squared / R. From V and P: I = P / V and R = V squared / P. From I and R: V = I x R and P = I squared x R. From I and P: V = P / I and R = P / I squared. From R and P: V = square root of (P x R) and I = square root of (P / R). If three or four fields are filled the calculator uses only the first valid pair it finds. If fewer than two valid numbers are entered the results show a dash.

Worked example

A 12 V car battery powers an accessory drawing 2 A. From V = 12 V and I = 2 A, resistance is R = 12 / 2 = 6 ohms and power is P = 12 x 2 = 24 W. The accessory dissipates 24 watts and presents a 6-ohm load to the battery. These figures match the default values pre-filled above.

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