Ohm's Law Calculator
Solve for voltage, current, resistance, or power from any two known values using Ohm's law.
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About the Ohm's Law Calculator
Ohm's law is the backbone of practical electrical work, tying together voltage, current, and resistance in a single relationship that technicians, electronics hobbyists, and engineers use dozens of times a day. This calculator takes the two quantities you can most easily measure or specify, voltage across a component and the current flowing through it, and returns the resistance and the power being dissipated. From sizing a current-limiting resistor to checking why a circuit runs hot, these four numbers cover a huge share of everyday troubleshooting.
The power result is what turns Ohm's law from an academic formula into a design tool. Components fail not because current alone is high but because the power they dissipate exceeds their rating and they overheat. A quarter-watt resistor asked to dissipate half a watt will scorch. By showing power alongside resistance, this tool tells you both what value part you need and how much heat it must survive.
Enter voltage in volts and current in amps. The tool assumes a purely resistive DC circuit, so it does not model inductance, capacitance, or AC phase angle. For DC loads, LED circuits, heating elements, and resistive sensors, that assumption holds well and the numbers you get are directly usable.
How the Ohm's Law Calculator works
Ohm's law states that voltage equals current times resistance, or V = I times R. Rearranged to solve for the unknown here, resistance equals voltage divided by current, R = V / I. That is the primary result: given the voltage measured across a component and the current through it, you get its effective resistance in ohms.
Power is the second output and comes from the power equation P = V times I, the product of voltage and current in watts. This is equivalent to the other forms P = I squared times R and P = V squared divided by R; all three give the same watts, and the tool uses the voltage-times-current form because both values are already entered. Watts represent the rate at which electrical energy converts to heat in the component.
If you enter zero current the tool shows a dash for resistance rather than dividing by zero, since a resistance is undefined when no current flows. Both resistance and power update the instant you change either input, letting you quickly test how a different supply voltage or a changed load current affects the part you need.
Worked example
Imagine you probe a small DC load and read V = 12 volts across it while I = 2 amps flows through it. Resistance is voltage divided by current, 12 / 2 = 6 ohms. So the load behaves like a 6-ohm resistor at this operating point.
Power is voltage times current, 12 x 2 = 24 watts. That is a substantial amount of heat for a small component, so whatever part carries this load, a power resistor or a heating element, must be rated well above 24 watts, commonly a 50-watt device to run cool with margin.
Now suppose you only wanted 1 amp through that same 6-ohm load. Ohm's law says you would need V = I times R = 1 x 6 = 6 volts, and the power would drop to 6 x 1 = 6 watts. Halving the current cut the power to a quarter of its value, a direct consequence of power scaling with the square of current for a fixed resistance.
Tips & common mistakes
- Always size a component's power rating above the calculated watts, typically at least double. A resistor dissipating 24 W should be a 50 W part so it runs at half its rating and stays reliably cool.
- This tool assumes DC or purely resistive loads. For AC circuits with motors, transformers, or capacitors, true power involves the power factor and V times I gives volt-amps, not watts, so the reading will overstate real power.
- If resistance shows a dash, your current entry is zero. A real energized circuit always draws some current, so a zero reading usually means an open circuit or a meter on the wrong range.
- Measure voltage directly across the component whose resistance you want, not across the whole circuit, or you will include wiring and source drops and compute a resistance that is too high.
- For LED current-limiting resistors, subtract the LED's forward voltage from the supply first, then apply Ohm's law to the remaining voltage and your target current to get the resistor value.
Ohm's Law Calculator: frequently asked questions
Can I use this for AC circuits?
Only for purely resistive AC loads like heaters or incandescent lamps. For loads with motors, transformers, or capacitors, current lags or leads voltage and the product V times I gives apparent power in volt-amps, not real power in watts. You would need the power factor to find true watts, which this calculator does not account for, so treat it as a DC and resistive-load tool.
How do I find the resistor I need to limit LED current?
First subtract the LED forward voltage from the supply voltage. With 12 V supply, 2 V LED drop and 0.02 A target, R = 10 / 0.02 = 500 ohms and P = 10 x 0.02 = 0.2 W. A nominal 0.25 W resistor has little margin and is not automatically adequate. For example, 0.5 W gives more nominal headroom, but check the actual datasheet, temperature derating, tolerances and enclosure rather than treating any wattage as a safety approval.
Why does the tool show power in addition to resistance?
Because a component's failure usually comes from heat, not from the resistance value itself. Power in watts tells you how much energy the part must dissipate. Knowing you need a 6-ohm resistor is not enough; you also need to know it must handle 24 watts so you can pick a physically large enough part that will not overheat.
What if I know voltage and resistance instead of current?
This particular tool asks for voltage and current. If you have voltage and resistance, you can find current yourself with I = V / R, then enter both values. For example 12 volts across 6 ohms gives 2 amps, which you would type in to see the same 6-ohm and 24-watt results.
Is the resistance value the same as what an ohmmeter would read?
For an ideal linear resistor, yes. For nonlinear devices like LEDs, diodes, or lamps, the computed value is the effective resistance only at that operating voltage and current. A cold incandescent filament reads far lower on an ohmmeter than the effective resistance it shows when hot and passing current, so the operating-point value from this tool can differ from a bench measurement.