How do you use Ohm's law?

Cover the quantity you want in the V = IR triangle: voltage is I × R, current is V ÷ R, resistance is V ÷ I. 9 V across 450 Ω pushes 0.02 A (20 mA) — the current through an LED's series resistor. Choose what to solve for above.

How do I calculate power from voltage and current?

Multiply them: P = V × I. 230 V × 8.7 A is 2,000 W — a 2 kW heater. With resistance instead, P = I²R or V² ÷ R gives the same number. The page shows the power for every result.

What resistor do I need for an LED?

R = (supply voltage − LED forward voltage) ÷ desired current. A 2 V red LED at 20 mA from 5 V: (5 − 2) ÷ 0.02 = 150 Ω. Solve for resistance with 3 V and 0.02 A.

What is the difference between voltage, current and resistance?

Voltage is the electrical push (volts), current is the flow of charge it produces (amperes) and resistance is how much the material opposes that flow (ohms). Double the voltage across a fixed resistor and the current doubles; double the resistance and it halves.

How do resistors in series and parallel combine?

In series they add: 100 Ω + 220 Ω = 320 Ω. In parallel their reciprocals add: 1 ÷ (1/100 + 1/220) = 68.75 Ω, less than either one. Work out the combined value, then enter it here as R.

What is a milliamp, and how do I enter it?

A thousandth of an amp: 20 mA is 0.02 A. Enter currents in amps here, so divide milliamps by 1,000.

Ohm's Law Calculator (V = IR, with Power)

Solve V = IR for voltage, current or resistance, with the power in watts and the energy per hour.

Inputs

Solve for

Voltage(V)

0 V1,000,000,000 V

Current(A)

0 A1,000,000,000 A

Resistance(Ω)

0 Ω1,000,000,000 Ω

Include CalculateEveryday mark

Current

2 A

Working: I = V ÷ R = 12 V ÷ 6 Ω = 2 A
Power P = V × I: 24 W
Energy in one hour: 0.024 kWh

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What each Ohm's Law input means

Solve for: The unknown: voltage, current or resistance. The other two boxes are the ones you know; the third is ignored.
Voltage: The potential difference across the resistor, in volts: 1.5 for an AA cell, 5 for USB, 12 for a car, 120 or 230 for mains.
Current: The current through it, in amperes. Milliamps ÷ 1,000 are amps, so 20 mA is 0.02 A.
Resistance: The resistance in ohms. Kilohms × 1,000 are ohms, so 4.7 kΩ is 4,700 Ω.

Ohm's Law formula

V = I × R · I = V ÷ R · R = V ÷ I · P = V × I = I²R = V² ÷ R · energy (kWh) = P (kW) × hours

V = IR, \qquad P = IV = I^{2}R = \frac{V^{2}}{R}

Ohm's law says the current through a resistor is the voltage across it divided by its resistance: I = V ÷ R, or V = IR. It holds for ohmic materials — metals and ordinary resistors at steady temperature — and is the first equation of every circuit. Given any two of the three the page finds the third, then the power the resistor dissipates, P = V × I (equivalently I²R or V² ÷ R), and the energy that becomes in an hour, since electricity is billed in kilowatt-hours. A 12 V headlight drawing 2.5 A has 4.8 Ω of resistance and turns 30 W into light and heat.

Ohm's Law worked example

12 V across 6 Ω: I = 12 ÷ 6 = 2 A, and the power is 12 × 2 = 24 W — 0.024 kWh every hour. OpenStax's headlight runs the other way: 12.0 V and 2.50 A give R = 12.0 ÷ 2.50 = 4.80 Ω and 30 W.

More Ohm's Law examples

The resistor for a red LED (about 2 V) on a 5 V supply at 20 mA: it has to drop 5 − 2 = 3 V, so R = 3 ÷ 0.02 = 150 Ω, and it dissipates 0.06 W — a standard ¼ W resistor is plenty.

0.5 A through 220 Ω: V = 0.5 × 220 = 110 V, and the resistor turns 55 W into heat — far beyond a small resistor's rating, which is why the power line matters.

Where the Ohm's Law calculator can give the wrong answer

Resistance changes with temperature. A lamp filament is a few ohms cold and ten times that when lit, so the cold reading on a meter understates the running resistance.
Non-ohmic devices — diodes, LEDs, transistors — do not follow V = IR; their current–voltage curve is not a straight line and a single resistance figure means little.
The formula is for a single resistor or a whole circuit's equivalent resistance. Resistors in series add; in parallel their reciprocals add.
For AC circuits with inductors or capacitors, impedance replaces resistance and the same form holds only with phase taken into account.
Zero resistance or zero current makes the corresponding division undefined; the page says so rather than reporting infinity.

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Ohm's Law reference table

Current an appliance draws at US and European mains voltage
Power	Current at 120 V	Current at 230 V
100 W	0.83 A	0.43 A
500 W	4.17 A	2.17 A
1,000 W	8.33 A	4.35 A
1,500 W	12.5 A	6.52 A
2,000 W	16.67 A	8.7 A
3,000 W	25 A	13.04 A

Source: Arithmetic: I = P ÷ V, from P = IV (OpenStax College Physics 2e, §20.4), for resistive loads

Ohm's Law FAQ

: Cover the quantity you want in the V = IR triangle: voltage is I × R, current is V ÷ R, resistance is V ÷ I. 9 V across 450 Ω pushes 0.02 A (20 mA) — the current through an LED's series resistor. Choose what to solve for above.
: Multiply them: P = V × I. 230 V × 8.7 A is 2,000 W — a 2 kW heater. With resistance instead, P = I²R or V² ÷ R gives the same number. The page shows the power for every result.
: R = (supply voltage − LED forward voltage) ÷ desired current. A 2 V red LED at 20 mA from 5 V: (5 − 2) ÷ 0.02 = 150 Ω. Solve for resistance with 3 V and 0.02 A.
: Voltage is the electrical push (volts), current is the flow of charge it produces (amperes) and resistance is how much the material opposes that flow (ohms). Double the voltage across a fixed resistor and the current doubles; double the resistance and it halves.
: In series they add: 100 Ω + 220 Ω = 320 Ω. In parallel their reciprocals add: 1 ÷ (1/100 + 1/220) = 68.75 Ω, less than either one. Work out the combined value, then enter it here as R.
: A thousandth of an amp: 20 mA is 0.02 A. Enter currents in amps here, so divide milliamps by 1,000.