Ohm's Law states that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. The formula is V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms.

Can I calculate power too?

Yes. Power (P) in watts is derived from the same two base equations. The most common form is P = V × I, but power can also be expressed as P = I² × R or P = V² ÷ R. The calculator outputs all four values — V, I, R, and P — whenever you supply any two of them.

What units does the tool use?

The calculator works in SI base units: volts (V) for voltage, amperes (A) for current, ohms (Ω) for resistance, and watts (W) for power. For milliamps or kilohms, convert first — 20 mA becomes 0.02 A, and 4.7 kΩ becomes 4700 Ω.

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How to Calculate Ohm's Law — Voltage, Current, Resistance Free (2026)

By Rui Barreira · Last updated: 18 June 2026

Need to find voltage, current, resistance, or power in a circuit? The free Ohm's Law calculator solves all four quantities instantly — enter any two values and the tool derives the rest.

How to Use the Tool

Choose which two values you already know — voltage (V), current (I), resistance (R), or power (P).
Enter both values in the corresponding input fields.
The calculator immediately displays all four quantities: voltage in volts, current in amperes, resistance in ohms, and power in watts.

How It Works

Ohm's Law states that voltage equals current multiplied by resistance: V = I × R. Power adds a second relationship: P = V × I. Together these two equations cover all six combinations of two knowns and two unknowns:

Know V and I: R = V ÷ I, P = V × I
Know V and R: I = V ÷ R, P = V² ÷ R
Know V and P: I = P ÷ V, R = V² ÷ P
Know I and R: V = I × R, P = I² × R
Know I and P: V = P ÷ I, R = P ÷ I²
Know R and P: V = √(P × R), I = √(P ÷ R)

All calculations happen in the browser — nothing is sent to a server.

Use Cases

Electronics hobbyists sizing resistors for LED circuits or verifying component ratings before soldering.
Electrical engineers doing quick sanity checks on circuit designs without opening a full simulator.
Students working through physics or electronics coursework and checking their manual calculations.
Circuit debugging — measure two values with a multimeter and use the tool to verify what the third should be.
Power budgeting for battery-operated devices, where knowing current draw and supply voltage determines runtime.

Frequently Asked Questions

What is Ohm's Law?: Ohm's Law is a fundamental principle in electronics that describes the relationship between voltage, current, and resistance in a conductor. Stated simply: the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. The formula is V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms.
Can I calculate power too?: Yes. Power (P) in watts is derived from the same two base equations. The most common form is P = V × I, but since V and I are related through resistance, power can also be expressed as P = I² × R or P = V² ÷ R. The calculator outputs all four values — V, I, R, and P — whenever you supply any two of them.
What units does the tool use?: The calculator works in SI base units: volts (V) for voltage, amperes (A) for current, ohms (Ω) for resistance, and watts (W) for power. For milliamps or kilohms, convert first — for example, 20 mA becomes 0.02 A, and 4.7 kΩ becomes 4700 Ω — then enter the converted value.

Frequently Asked Questions

What is Ohm's Law?: Ohm's Law states that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. The formula is V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms.
Can I calculate power too?: Yes. Power (P) in watts is derived from the same two base equations. The most common form is P = V × I, but power can also be expressed as P = I² × R or P = V² ÷ R. The calculator outputs all four values — V, I, R, and P — whenever you supply any two of them.
What units does the tool use?: The calculator works in SI base units: volts (V) for voltage, amperes (A) for current, ohms (Ω) for resistance, and watts (W) for power. For milliamps or kilohms, convert first — 20 mA becomes 0.02 A, and 4.7 kΩ becomes 4700 Ω.