Calculate voltage, current, resistance, or power using Ohm's law. Free ohms law calculator with the formula shown - runs entirely in your browser, no signup.
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Calculate voltage, current, resistance, or power using Ohm's law. Free ohms law calculator with the formula shown - runs entirely in your browser, no signup.
2 A through 6 ohms gives 12 V and 24 W.
Transparent methodology
How this calculator works
Reviewed August 2026 · BoringToolsKit Editorial Team
Formula
V = I x R (voltage = current x resistance); rearranged to I = V / R or R = V / I for whichever pair you enter.
BoringToolsKit. “Ohm's Law Calculator.” boringtoolskit.com/ohms-law-calculator/ (reviewed August 2026). Free to reference in articles, syllabi, and answer posts with a link.
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Worked example: 2 amps through 6 ohms
Take a heating element drawing I = 2 A with resistance R = 6 ohms. To solve for voltage, multiply the two inputs: V = I x R = 2 x 6 = 12 V. If instead you knew the supply was 12 V and measured 6 ohms, the same relationship runs in reverse: I = V / R = 12 / 6 = 2 A, so any two of the three values fix the third.
Power follows directly. P = V x I = 12 x 2 = 24 W, which you can cross-check two other ways: P = I^2 x R = 4 x 6 = 24 W, and P = V^2 / R = 144 / 6 = 24 W. All three routes agree because they are algebraic rearrangements of the same pair of laws. The 24 W figure is also a practical sanity check: a resistor rated for 1/4 W would burn out instantly in this circuit, so you would need a component rated well above 24 W, ideally 50 W or more for margin.
The inputs in this tool are treated as exact, so a reading of 2.1 A and 5.8 ohms returns 12.18 V with no rounding of the inputs themselves. Only the displayed output is trimmed to two decimal places.
Where Ohm's law stops matching the meter
The formula assumes constant resistance, and real parts push back. A 60 W incandescent bulb rated for 120 V draws 0.5 A when hot, implying 240 ohms, but its cold filament measures roughly one tenth of that, around 20 to 30 ohms. Inrush current at switch-on is therefore several amps, not 0.5 A. The tool's answer is only valid for the conditions at which the resistance was measured.
Component tolerance matters at small values. A resistor marked 100 ohms at 5 percent can legitimately be anywhere from 95 to 105 ohms, so a current prediction from a 12 V supply ranges from 114.29 mA to 126.32 mA, a spread of about 12 mA. For precision work, measure the actual part rather than trusting the band.
The tool also applies only to DC or purely resistive AC loads. A motor winding or a capacitor input power supply has reactance, so dividing measured AC voltage by measured AC current gives impedance, not resistance, and the watts figure overstates real power. Finally, the two positive inputs rule is deliberate: a zero or negative resistance value makes the division for current and resistance undefined or meaningless, and the tool refuses the calculation rather than returning a misleading number.
