Ohm's Law Calculator

Enter any two of the four values (V, I, R, P) — the other two are solved automatically.

Quick Presets

Voltage (V)

Current (I)

Resistance (R)

Power (P)

Fill any two fields — the other two solve automatically.

What Is Ohm's Law?

Ohm's Law states that the voltage (V) across a conductor equals the current (I) flowing through it multiplied by its resistance (R): V = I × R. Published by Georg Simon Ohm in 1827, it is the foundational equation of electrical circuits.

The law is empirical — it describes the behavior of resistive (ohmic) materials where resistance remains constant over a wide range of voltages and currents, such as metal wires, carbon resistors, and wirewound resistors at constant temperature.

V = I·R

Voltage in volts

I = V/R

Current in amperes

R = V/I

Resistance in ohms

The Power Triangle — V, I, R & P Relationships

Combining Ohm's Law with the power formula P = V × I gives 12 equivalent equations linking all four quantities. This calculator uses all six two-variable combinations to solve for the remaining pair.

Known	Find V	Find I	Find R	Find P
V & I	—	—	V/I	V×I
V & R	—	V/R	—	V²/R
V & P	—	P/V	V²/P	—
I & R	I×R	—	—	I²×R
I & P	P/I	—	P/I²	—
R & P	√(P×R)	√(P/R)	—	—

How to Use This Calculator

1
Enter any two values
Type a number into any two of the four fields (Voltage, Current, Resistance, or Power). Select the appropriate unit from the dropdown beside each field.
2
Results appear automatically
The calculator detects which two fields are filled and immediately solves for the remaining two, showing all four values plus step-by-step working.
3
Use presets for common scenarios
Click any preset chip (USB Charger, LED, etc.) to populate real-world values instantly.
4
Reset to start over
Click ↺ Reset to clear all four fields and begin a new calculation.

Worked Examples

Example 1 — Find R & P from V & I

Given: V = 12 V, I = 2 A

R = V/I = 12/2 = 6 Ω

P = V×I = 12×2 = 24 W

Example 2 — LED resistor from V & I

Given: V = 2 V, I = 20 mA = 0.02 A

R = V/I = 2/0.02 = 100 Ω

P = V×I = 2×0.02 = 0.04 W

Example 3 — Find V & I from R & P

Given: R = 8 Ω, P = 50 W

V = √(P×R) = √400 = 20 V

I = √(P/R) = √6.25 = 2.5 A

Example 4 — Household appliance

Given: V = 230 V, P = 1500 W

I = P/V = 1500/230 = 6.52 A

R = V²/P = 52900/1500 = 35.3 Ω

Common Component Reference Values Table

Component	Voltage	Current	Resistance	Power
Red LED (typical)	2.0 V	20 mA	100 Ω	40 mW
USB charger (5V 2A)	5 V	2 A	2.5 Ω	10 W
9V PP3 battery load	9 V	19.1 mA	470 Ω	172 mW
Car headlight (12V 55W)	12 V	4.58 A	2.62 Ω	55 W
Kettle 230V 2000W	230 V	8.70 A	26.5 Ω	2000 W

Frequently Asked Questions

Ohm's Law states that the voltage across a conductor is directly proportional to the current through it: V = I × R. It was formulated by Georg Simon Ohm in 1827. It applies to resistive (ohmic) components where resistance stays constant regardless of voltage or current.

Exceeding a resistor's power rating causes it to overheat. The power dissipated is P = I²R. If P exceeds the rated wattage (commonly 0.25W or 0.5W for through-hole resistors), the resistor can burn, change value, or fail open-circuit, potentially damaging other components.

Ohmic conductors have constant resistance regardless of applied voltage. Non-ohmic devices — like diodes, LEDs, transistors, and tungsten filaments — have resistance that changes with temperature, voltage, or current direction. For example, a tungsten filament has ~10× higher resistance when hot than when cold.

Divide milliamps by 1000: 1 mA = 0.001 A. For example, 20 mA = 0.020 A. This calculator handles the conversion automatically when you select the mA unit — just enter the number in mA and the calculator converts it internally.

Power (watts) = Voltage (volts) × Current (amps): P = V × I. You can also express it as P = V²/R or P = I²R. Watts measure the rate of energy transfer — one watt equals one joule per second.

Yes, but with modifications. In AC circuits, resistance becomes impedance (Z), which includes both resistive and reactive (capacitive/inductive) components: V = I × Z. The formula is identical in form, but Z is a complex number combining resistance R and reactance X.

Use R = (Vs − Vf) / If, where Vs is supply voltage, Vf is the LED forward voltage (~2V for red, ~3.2V for blue/white), and If is desired current (~20mA). Example: 5V supply, red LED (2V, 20mA): R = (5 − 2) / 0.020 = 150Ω. Round up to the nearest standard value.

Ω (ohm) is the base unit of resistance. 1 kΩ (kilohm) = 1,000 Ω. 1 MΩ (megohm) = 1,000,000 Ω. Typical resistors range from 1Ω to 10MΩ. Wiring has milliohm resistance; insulators have gigaohm resistance.

Related Calculators

Volts to Watts Watts to Amps Kinetic Energy Calculator Newton's Second Law Calculator Wave Speed Calculator