Ohm's Law Calculator

Solve for voltage, current, or resistance from any two values. Also shows power (watts).

By Konstantin Iakovlev · Updated September 2026 · Source: Khan Academy

A
Ω

Voltage

12.000 V

Current

2.000 A

Resistance

6.000 Ω

All Values

Voltage (V)12.000 V
Current (I)2.000 A
Resistance (R)6.000 Ω
Power (P)24.000 W
FormulaV = I × R

Use the Ohm's Law Calculator above to calculate your results. Enter your values and see instant results — all calculations run in your browser.

Disclaimer: This calculator is for informational purposes only. Results are estimates based on the information you provide and the assumptions described on this page.

How It Works

Engineers, technicians, and hobbyists reach for Ohm's Law whenever a circuit needs to be solved or debugged. Give the tool any two of voltage, current, or resistance, and it returns the third while also reporting the power in watts dissipated by the circuit.

The math rests on Ohm's Law, V = I * R, which ties voltage (V), current (I), and resistance (R) together, paired with the power formula P = V * I. Supply any two of V, I, or R, and the tool rearranges these equations to find the missing value before going on to compute the power.

Consistent units are non-negotiable here: volts, amperes, and ohms. Two traps catch people regularly. One is mixing up resistance in ohms with conductance in siemens; the other is blurring DC and AC behavior, given that Ohm's Law applies cleanly to DC or to instantaneous AC values. Verifying your inputs before you read the result spares you most calculation errors.

Example: Current and Power Through a 24-Ohm Resistor

  1. 1 A 24-ohm resistor is connected across a 12 V DC supply. You want to know the current it draws and the power it dissipates.
  2. 2 Choose Solve For: Current (I), then enter Voltage = 12 V and Resistance = 24 Ω. The calculator uses I = V / R and then P = V × I.
  3. 3 Current I = 12 / 24 = 0.500 A; power P = 12 × 0.5 = 6.000 W.
  4. 4 The supply has to deliver at least 0.5 A, and the resistor turns 6 W into heat, so it needs a power rating above 6 W. Ohm's law gives a fixed answer like this only for parts whose resistance stays constant; an LED does not behave that way, so its current comes from its datasheet and its current-limiting resistor.

Source: Khan Academy · Last updated: September 2026

Frequently Asked Questions

What is Ohm's Law?
Ohm's Law states that voltage (V) equals current (I) times resistance (R): V = IR. You can rearrange to find any value: I = V/R or R = V/I. It is the foundational equation for electrical circuits.
How do you calculate watts from volts and amps?
Power (watts) = voltage x current. For a 120V outlet drawing 10 amps: 120 x 10 = 1,200 watts. You can also calculate: P = I²R or P = V²/R.
What is the resistance of a 60-watt light bulb?
At 120V: R = V²/P = 120²/60 = 240 ohms. The current draw is I = P/V = 60/120 = 0.5 amps. Note that actual resistance varies with filament temperature.

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