Ohm's Law states that voltage (V) equals current (I) multiplied by resistance (R): V = I × R. It describes the relationship between the three fundamental electrical quantities.

How do I calculate power?

Power (P) in watts equals voltage times current: P = V × I. You can also use P = I² × R or P = V² / R when voltage or current is unknown.

Voltage in volts (V), current in amperes (A), resistance in ohms (Ω), and power in watts (W). Results are automatically scaled to milliamps, kilohms, milliwatts, etc. for readability.

Can I solve for any two unknowns?

Yes. Enter any two known values and this calculator will compute the remaining two using the appropriate form of the equations.

Ohm's Law Calculator - Voltage, Current, Resistance & Power

Ohm's Law explained

Ohm's Law (V = IR) describes the linear relationship between voltage, current, and resistance in a conductor. Discovered by Georg Simon Ohm in 1827, it is the foundation of electrical circuit analysis.

The power equation

Power (P = VI) measures the rate of energy transfer. Combined with Ohm's Law, you get P = I²R and P = V²/R, useful when only resistance and one other quantity is known.

Common calculations

Use this calculator to size resistors, find current draw from a power supply, check LED current-limiting resistors, or estimate heat dissipation in a circuit.

Ohm’s Law triangle

Cover the variable you want to find to reveal the formula. The triangle is organized as Voltage (V) on top, with Current (I) and Resistance (R) on the bottom:

       [ V ]
      ━━━━━━━━━
      [ I ][ R ]

Cover V -> V = I × R
Cover I -> I = V / R
Cover R -> R = V / I

For power: P = V × I = I² × R = V² / R

Common resistor values (E12 and E24 series)

E12 values (×1Ω)	Additional E24 values (×1Ω)
1.0, 1.2, 1.5, 1.8	1.1, 1.3, 1.6
2.2, 2.7, 3.3, 3.9	2.0, 2.4, 3.0, 3.6
4.7, 5.6, 6.8, 8.2	4.3, 5.1, 6.2, 7.5, 9.1
10, 12, 15, 18	11, 13, 16
22, 27, 33, 39	20, 24, 30, 36
47, 56, 68, 82	43, 51, 62, 75, 91

Scale by ×10, ×100, ×1kΩ, ×10kΩ, ×100kΩ, or ×1MΩ to cover the full resistor range. E12 has 12 values per decade (±10% tolerance); E24 has 24 values per decade (±5% tolerance).

Kirchhoff's Laws

Ohm's Law becomes much more powerful when combined with Kirchhoff's circuit laws:

KCL (Kirchhoff's Current Law): the sum of all currents entering a node equals the sum leaving it. Current is conserved at every junction.
KVL (Kirchhoff's Voltage Law): the sum of all voltage drops around any closed loop equals zero. Energy is conserved around any path.

These two laws, combined with Ohm's Law, allow you to solve any linear DC circuit systematically using node analysis or mesh analysis.

AC vs. DC note

Ohm's Law (V = IR) applies directly to DC circuits with resistors. In AC circuits, impedance (Z) replaces resistance: V = IZ. Impedance accounts for capacitive reactance (X_C = 1/(2πfC)) and inductive reactance (X_L = 2πfL) in addition to resistance. The same Ohm's Law structure applies, but all quantities become complex numbers (phasors). This calculator is for purely resistive DC analysis.