Ohms Law Calculator

Three arrangements. Current is voltage divided by resistance. Resistance is voltage divided by current. Voltage is current times resistance. 12 and 4 give 3 amps. 2 amps and 4 ohms give 8 volts. Division by zero is refused. A negative resistance is refused. A negative voltage is allowed and the current comes out negative, which is a direction. The mode line names the solved quantity. Do not size a fuse from this page alone. A real circuit has more than two numbers.

Twelve volts across 4 ohms is 3 amps. Two amps through 4 ohms is 8 volts. The unused box can stay empty. Zero resistance is refused when you are dividing by it. A negative resistance is refused. Voltage may be negative, and the current comes out negative, which is a direction, not a broken meter. The mode line names which quantity was solved so a screenshot is not three numbers with no verb. This is ideal Ohm’s law. It is not a model of a battery’s internal resistance or of a wire that heats up. If the numbers disagree with a real circuit, the circuit has something this page does not include. Do not size a fuse from this page alone.

Three amps and eight volts are the two samples to memorise. They use small integers so the arithmetic is obvious. A lab supply that shows 11.8 volts is a different problem. Type 11.8 if that is what you measured. The page will divide it. Do not round to 12 and then complain that the current is not what the meter shows. The ideal law is exact for the numbers you type and silent about contact resistance, meter burden and temperature. Leave the unknown box empty so you do not accidentally solve with a leftover zero. A zero you typed is a real zero. An empty box is an unused input. They are not the same.

Ohm’s law here is the ideal relation voltage equals current times resistance. Choose the quantity you do not know. To find current, type voltage and resistance. Twelve volts and four ohms give three amps, because 12 divided by 4 is 3. To find voltage, type current and resistance. Two amps and four ohms give eight volts, because 2 times 4 is 8. Leave the unknown box empty. An empty box is not a zero. A typed zero is a zero, and dividing by a zero resistance is refused. A negative resistance is refused. A negative voltage is allowed and produces a negative current, which you can read as direction. The mode line says which quantity was solved. This is not a model of a battery, a heated wire, or a fuse. If a meter disagrees, type the meter’s number rather than a rounded one, and still treat the result as the ideal law only. Do not size a fuse from this page. The arithmetic is exact for the numbers you type and silent about everything the circuit adds.

Memorise the two samples as sentences: twelve volts across four ohms is three amps, and two amps through four ohms is eight volts. Use those sentences when you check that you picked the right mode.

Current from 12 volts and 4 ohms

Inputs: Solve for: current | Voltage: 12 | Resistance: 4

Choose current, set voltage to 12 and resistance to 4, then press Calculate. The current is 3 amps.

Result: Mode: current Voltage: 12 V Resistance: 4 Ω Current: 3 A I = V ÷ R.

Voltage from 2 amps and 4 ohms

Inputs: Solve for: voltage | Current: 2 | Resistance: 4

Choose voltage, set current to 2 and resistance to 4, then press Calculate. The voltage is 8 volts.

Result: Mode: voltage Current: 2 A Resistance: 4 Ω Voltage: 8 V V = I × R.

Why is the current 3 amps?

12 volts divided by 4 ohms is 3 amps.

Why is the voltage 8 volts in the second example?

2 amps times 4 ohms is 8 volts.

Can resistance be zero?

Not when the law would divide by it. The tool refuses that.

What does a negative current mean?

A direction opposite the one you called positive. It is not an error.

Does this include internal resistance?

No. Only the ideal product or quotient of the numbers you type.

Are the numbers uploaded?

No. The arithmetic runs in the browser.