Ohm's Law Power Calculator
Ohm's Law (V = I * R) and the power formula (P = V * I) describe the fundamental relationships in DC resistive circuits. Together they generate four equivalent equations: V = I * R, I = V / R, R = V / I, P = V * I, P = I^2 * R, and P = V^2 / R. Enter any two known quantities (voltage V in volts, current I in amps, resistance R in ohms, or power P in watts) and the calculator solves for the other two. This is the most-used calculation in electronics, covering LED sizing, heater wiring, motor selection, and battery capacity planning.
Ohm's Law and power formulas
V = I * R
P = V * I = I^2 * R = V^2 / R
V is voltage (volts), I is current (amperes), R is resistance (ohms), P is power (watts). Example: V = 12 V, I = 2 A gives R = 6 ohms and P = 24 watts.
Ohm's Law applications
- Resistor power rating: calculate P = I^2 * R or P = V^2 / R to select a resistor that can handle the dissipated heat safely.
- Wire sizing: I = P / V finds the current a wire must carry for a given power load and voltage, then select wire gauge from NEC tables.
- Voltage dividers: V_out = V_in * R2 / (R1 + R2) derives from Ohm's Law applied to two series resistors.
- Battery capacity: P = V * I can be rearranged to I = P / V to find the load current for a given power draw and battery voltage.
Frequently asked questions
What is Ohm's Law?
Ohm's Law states that voltage V across a resistor equals current I times resistance R: V = I * R. It applies to ohmic (resistive) components at constant temperature. Rearranging: I = V/R and R = V/I.
What are the power formulas?
Power P = V * I (voltage times current). Combining with Ohm's Law gives two additional forms: P = I^2 * R (useful when R and I are known) and P = V^2 / R (useful when V and R are known).
Which two values should I enter?
Enter any two of the four quantities (V, I, R, P) and the calculator computes the other two. Common combinations: V and I (to find R and P), R and P (to find V and I), or V and R (to find I and P).
Does Ohm's Law apply to all components?
Ohm's Law applies to ohmic resistors where V/I is constant. It does not apply to non-linear components like diodes, transistors, or batteries, whose V-I relationship is non-linear. For AC circuits, impedance Z replaces R: V = I * Z.
What are typical resistor power ratings?
Standard resistors are rated 1/8 W, 1/4 W, 1/2 W, 1 W, 2 W, 5 W, etc. Always choose a resistor with a power rating at least double the calculated dissipation for reliable long-term operation.
Official sources
- NIST: NIST SP 811, Guide for SI Units.
- OpenStax University Physics: Resistivity and Resistance, Vol. 2 Ch. 9.
- NFPA 70 (NEC): NFPA 70, National Electrical Code.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.