Ohm's Law Calculator

Calculate voltage, current, resistance, and power using Ohm's Law. Enter any 2 values to solve for the other 2.

Ohm's Law Worksheet

Enter any 2 values to calculate the other 2.

Voltage (V)Volts

Current (I)Amps

Resistance (R)Ohms

Power (P)Watts

Solved for R = V / I, P = V × I

Voltage12 V

Current500.000 mA

Resistance24 Ω

Power6 W

Solve For	Formula	Value
Voltage (V)	V = I × R	12 V
Current (I)	I = V / R	500.000 mA
Resistance (R)	R = V / I	24 Ω
Power (P)	P = V × I	6 W

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Ohm's Law

Ohm's Law is the fundamental relationship in electrical circuits. It states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to the resistance.

Core Formulas

Find	Formula	Variables
Voltage	V = I × R	V (volts), I (amps), R (ohms)
Current	I = V / R
Resistance	R = V / I

Electrical Power

Power is the rate of energy transfer. In DC circuits, power equals voltage times current. Combined with Ohm's Law, power can be expressed three ways:

Formula	Known Values	Use When
P = V × I	Voltage, Current	Both V and I are known
P = I² × R	Current, Resistance	Voltage unknown
P = V² / R	Voltage, Resistance	Current unknown

Common Examples

Scenario	V	I	R	P
LED with resistor	5V	20mA	250Ω	0.1W
Car headlight	12V	4.6A	2.6Ω	55W
USB charging	5V	2.4A	2.08Ω	12W
Space heater	120V	12.5A	9.6Ω	1,500W
EV charger (L2)	240V	30A	8Ω	7,200W

SI Prefixes for Electrical Units

Prefix	Symbol	Factor	Example
Mega	M	10⁶	1 MΩ = 1,000,000 Ω
Kilo	k	10³	4.7 kΩ = 4,700 Ω
Milli	m	10⁻³	20 mA = 0.020 A
Micro	μ	10⁻⁶	100 μA = 0.0001 A

Safety Notes

Household circuits: US standard is 120V/15A (1,800W max) or 120V/20A (2,400W max) per circuit
Wire sizing: Higher current requires thicker wire — use our Voltage Drop Calculator for wire sizing
Circuit breakers: Should be rated for 80% of max continuous load (e.g., 15A breaker → 12A continuous)
Never exceed ratings: Overloading circuits causes overheating, wire damage, and fire risk

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Frequently Asked Questions

What is the Ohms law formula?

Voltage equals current times resistance: V = I × R. Rearranged, current is I = V / R and resistance is R = V / I. Electrical power follows from the same values as P = V × I, which can also be written P = I²R or P = V²/R. The units are volts (V), amperes (A), ohms (Ω) and watts (W).

How many amps does a 1500 watt heater draw on 120 volts?

A 1500 W heater on a 120 V outlet draws 12.5 amps, because current equals power divided by voltage: I = P / V = 1500 / 120. The same rule gives any appliance: a 12 V car battery across a 24 Ω resistor pushes 0.5 A and delivers 6 W. Knowing the current tells you whether a circuit and its breaker can carry the load.

How do you find resistance from watts and volts?

Resistance equals voltage squared divided by power: R = V² / P. A 60 W light bulb on a 120 V circuit has R = 120² / 60 = 240 Ω. The formula comes from combining P = V × I with V = I × R, so any two of voltage, current, resistance and power are enough to find the other two.

Does Ohms law work for AC circuits?

Yes, with impedance (Z) in place of resistance (R). For purely resistive AC loads such as heaters and incandescent bulbs, R and Z are the same, so V = I × R works directly. For capacitive or inductive loads such as motors, impedance also includes reactance, and current and voltage fall out of phase.