This free Ohm's Law calculator solves for voltage (V), current (I), resistance (R), and power (P) from any two known values. It applies Ohm's Law — V = I × R — together with the three power formulas P = V × I, P = I² × R, and P = V² / R. Enter any two quantities and the calculator instantly returns the other two plus power. Everything runs in your browser; no signup or installation required.
Ohm’s Law relates the three fundamental electrical quantities: V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. Rearranged, I = V / R and R = V / I.
Electrical power adds a fourth quantity, P (watts), through three equivalent formulas: P = V × I, P = I² × R, and P = V² / R. Together these are often drawn as the "Ohm’s Law wheel" — given any two of V, I, R, and P, you can derive the other two. This calculator does that derivation for you.
Enter any two of the four values (for example voltage and resistance, or current and power). The calculator selects the correct formulas, solves for the remaining two quantities, and displays all four. Use consistent units — volts, amperes, ohms, and watts. For milli- or kilo- prefixes, convert first (1 kΩ = 1000 Ω, 1 mA = 0.001 A).
Suppose a 12 V supply is connected across a 100 Ω resistor. Current I = V / R = 12 / 100 = 0.12 A (120 mA). Power dissipated P = V² / R = 144 / 100 = 1.44 W — so a ¼-watt resistor would overheat and you would size at least a ½-watt (or 1 W for margin). This is exactly the kind of check the calculator automates.
Ohm’s Law states that the current through a conductor between two points is directly proportional to the voltage across the two points, with resistance as the constant of proportionality: V = I × R. It is the foundational relationship in circuit analysis and lets you find any one of voltage, current, or resistance given the other two.
Power in watts can be found three ways depending on which values you know: P = V × I (voltage times current), P = I² × R (current squared times resistance), or P = V² / R (voltage squared divided by resistance). All three give the same result for a resistive load. This calculator picks the right formula automatically from your inputs.
Voltage in volts (V), current in amperes (A), resistance in ohms (Ω), and power in watts (W). Convert prefixed units before entering them: 1 kΩ = 1000 Ω, 1 MΩ = 1,000,000 Ω, 1 mA = 0.001 A, 1 mW = 0.001 W.
For DC and purely resistive AC loads, Ohm’s Law and these power formulas apply directly. For AC circuits with inductance or capacitance, resistance is replaced by impedance (Z), and real power must account for the power factor (P = V × I × cosφ). This calculator covers DC and resistive cases; reactive circuits need an impedance/power-factor analysis.