Ohm's law says voltage equals current times resistance (V = I × R), and power is voltage times current (P = V × I). Knowing any two of the four values fixes the other two. For example, a 12 V supply across a 6 Ω resistor drives 2 A and dissipates 24 W as heat.
Hobbyists sizing a current-limiting resistor, students checking homework, and technicians confirming a heater element all reach for the same four relationships. Pick which pair of values you already know — voltage and resistance, power and current, and so on — and the calculator returns the remaining two along with current in milliamps. The working is printed line by line so you can see which form of the law was applied, such as P = V² ÷ R or I = √(P ÷ R). It treats the load as an ideal resistor carrying steady DC or RMS AC; it does not model reactance, so motors, transformers and capacitors on AC need an impedance calculation instead.
Use the drop-down to say which two quantities you have measured or read off a datasheet.
Type them in volts, amps, ohms or watts. Only the fields for your chosen pair are shown.
Voltage, current, resistance and power appear together, plus current in mA, with the formulas used below.
Result Circuit values: V 12 V · I 2 A · R 6 Ω · P 24 W
Voltage (V): 12 V
Current (I): 2 A
Current in milliamps: 2000 mA
Resistance (R): 6 Ω
Power (P): 24 W
Knowing the supply voltage and resistance gives the current drawn and the heat the resistor must shed.
Result Circuit values: V 110 V · I 0.5 A · R 220 Ω · P 55 W
Voltage (V): 110 V
Current (I): 0.5 A
Current in milliamps: 500 mA
Resistance (R): 220 Ω
Power (P): 55 W
The voltage drop and dissipation show why a quarter-watt part would burn out here.
V = I × R; P = V × I = I² × R = V² ÷ R
VIRPOhm's law links voltage, current and resistance for a linear resistive load. Combining it with the power relation P = V × I gives twelve rearrangements; the calculator picks the two that turn your known pair into the missing pair. When power and resistance are known, current and voltage come from square roots, which is why those results can carry more decimals.
Voltage, current, resistance and power are tied by two independent equations, V = I × R and P = V × I. With two unknowns left and two equations, any pair you enter fixes the rest. One value alone leaves infinitely many solutions.
Take the power shown and pick a part rated comfortably above it — a common rule of thumb is about double. A resistor dissipating 0.2 W is usually fitted as a 0.5 W part so it runs cooler and lasts longer.
Not directly. An LED has a roughly fixed forward voltage rather than a fixed resistance. For a series resistor, subtract the LED forward voltage from the supply first, then use that leftover voltage and the desired current here to find the resistance.
Yes for purely resistive loads such as kettles, heaters and incandescent bulbs, using RMS values like 230 V or 120 V. For motors and power supplies the current and voltage are out of phase, so real power differs from V × I.
Multiply amps by 1000. The calculator already lists the current in mA, which is the unit most small-signal and LED datasheets quote, so 0.02 A appears as 20 mA.
Power in a resistance is I² × R. Doubling the current through the same wire quadruples the heat, which is why undersized cable on a high-current load warms up and why fuses are rated by current.
Using this tool in an assignment, article or report? Copy a citation, or a link for your website.
Tools Directory Editorial Team. (2026, October 27). Ohm’s Law Calculator. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/electronics-physics/resistors-and-capacitors/ohms-law-calculator/
Tools Directory Editorial Team. “Ohm’s Law Calculator.” Tools Directory, 27 Oct. 2026, https://tools.milunir.com/electronics-physics/resistors-and-capacitors/ohms-law-calculator/. Accessed 27 Oct. 2026.
Tools Directory Editorial Team. “Ohm’s Law Calculator.” Tools Directory. Last modified October 27, 2026. Accessed October 27, 2026. https://tools.milunir.com/electronics-physics/resistors-and-capacitors/ohms-law-calculator/.
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