Index / Units
Ohm's Law Calculator
Volts, amps, ohms and watts. Fill in any two and the other two follow. An empty box here means unknown, not zero — it is the thing being solved for.
What you know
Waiting for two values
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Fill in any two boxes.
Voltage
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Current
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Resistance
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Power
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The circuit, worked out
The four equations, and what they assume
V = I × R and P = V × I. Everything else on this
page is those two rearranged: P = I²R, P = V²/R,
R = V²/P, I = √(P/R), V = √(P R).
The answer is checked three ways in front of you. Once both a voltage and a current are known, the power is worked out as V × I, as I²R and as V²/R. All three must agree; the largest disagreement between them is printed below with the tolerance it should sit under, which is 1 part in 109. Rounding in binary floating point is the only thing that should ever put a number in that row.
This is direct current, or the RMS values of an alternating one into a purely resistive load. With any reactance — a motor, a transformer, a capacitor, a long cable at high frequency — the current and the voltage stop being in step, and the real power is V × I × cos φ rather than V × I. This page has no power factor in it and does not pretend to.
Resistance is taken as constant. It is not: a filament lamp's cold resistance is roughly a tenth of its hot resistance, and a copper winding gains about 0.4% per degree. The figure you type is the resistance at whatever temperature it was measured.
Zero is a real answer and infinity is not. Zero current with a voltage across it is an open circuit, where the resistance is unbounded rather than a large number, and zero resistance with a voltage across it is a short circuit, where the current is limited by something this page cannot see. Both are said in words instead of being printed as a figure.
The energy row assumes the load stays exactly as it is for the whole hour. One watt for one hour is 3,600 joules, and a kilowatt-hour is 3.6 megajoules.
The resistor rating line applies the usual 50% derating: it names the smallest part in the standard series — 1/8, 1/4, 1/2, 1, 2 and 5 W — that would be running at half its rating or less. A part run at its full rating gets hot enough to drift and to discolour a board, so almost nobody specifies one that way.
The answer is checked three ways in front of you. Once both a voltage and a current are known, the power is worked out as V × I, as I²R and as V²/R. All three must agree; the largest disagreement between them is printed below with the tolerance it should sit under, which is 1 part in 109. Rounding in binary floating point is the only thing that should ever put a number in that row.
This is direct current, or the RMS values of an alternating one into a purely resistive load. With any reactance — a motor, a transformer, a capacitor, a long cable at high frequency — the current and the voltage stop being in step, and the real power is V × I × cos φ rather than V × I. This page has no power factor in it and does not pretend to.
Resistance is taken as constant. It is not: a filament lamp's cold resistance is roughly a tenth of its hot resistance, and a copper winding gains about 0.4% per degree. The figure you type is the resistance at whatever temperature it was measured.
Zero is a real answer and infinity is not. Zero current with a voltage across it is an open circuit, where the resistance is unbounded rather than a large number, and zero resistance with a voltage across it is a short circuit, where the current is limited by something this page cannot see. Both are said in words instead of being printed as a figure.
The energy row assumes the load stays exactly as it is for the whole hour. One watt for one hour is 3,600 joules, and a kilowatt-hour is 3.6 megajoules.
The resistor rating line applies the usual 50% derating: it names the smallest part in the standard series — 1/8, 1/4, 1/2, 1, 2 and 5 W — that would be running at half its rating or less. A part run at its full rating gets hot enough to drift and to discolour a board, so almost nobody specifies one that way.