Solve for power (W), voltage (V), current (A) or resistance (Ω) — enter any two known values and calculate the rest instantly.
Calculate
Select what you want to solve for, then enter the two known values.
All 12 Power Formulas
Every combination of P, V, I and R — derived from P = V·I and Ohm's Law (V = I·R):
Power Triangle — solve for any variable
Solve P
P = V × I P = I² × R P = V² / R
Solve V
V = P / I V = I × R V = √(P × R)
Solve I
I = P / V I = V / R I = √(P / R)
Solve R
R = V / I R = V² / P R = P / I²
Worked Examples
1 Light bulb power
Voltage V230 V
Current I0.26 A
P = V × I≈ 60 W
2 Resistor dissipation
Current I50 mA
Resistance R100 Ω
P = I² × R0.25 W
3 Find current from power
Power P100 W
Voltage V12 V
I = P / V8.33 A
4 Find resistance from P & V
Power P25 W
Voltage V5 V
R = V² / P1 Ω
Frequently Asked Questions
What is the formula for electrical power?▾
The fundamental formula is P = V × I (watts = volts × amps). Combined with Ohm's Law (V = I × R) you get two more: P = I² × R and P = V² / R. All three give the same result — use whichever matches your known values.
How do I calculate watts from volts and amps?▾
Multiply volts by amps: P = V × I. Example: 12 V × 2 A = 24 W. This works for DC circuits and for AC circuits where you use RMS voltage and current values.
How do I calculate amps from watts and volts?▾
Divide watts by volts: I = P / V. Example: a 60 W bulb on 230 V draws 60 / 230 = 0.26 A. This is useful for sizing fuses and cable ratings.
How do I calculate resistor power rating?▾
Use P = I² × R or P = V² / R. Always choose a resistor rated at least twice the calculated dissipation for a safety margin. Standard ratings: 1/8 W, 1/4 W, 1/2 W, 1 W, 2 W.
What is the difference between watts and volt-amps (VA)?▾
In DC circuits, watts and VA are identical. In AC circuits, watts (W) is real power that does actual work, while volt-amps (VA) is apparent power. The ratio W/VA is the power factor. For resistive loads (heaters, incandescent bulbs) power factor = 1, so W = VA. For motors and capacitors, power factor < 1.