Power converter

Ohm's Law Calculator

Enter any two electrical values to automatically solve for the others using Ohm's Law and the Power Equation.

How Ohm's Law works

Ohm's Law states that electrical potential force (Voltage) equals current (Amps) multiplied by resistance (Ohms). Combined with the Power law, we can describe all four parameters in a circuit.

This calculator solves for any two unknowns. Simply fill in two values you already know, and the system automatically determines the remaining constants using V = I × R and P = V × I equations.

Formula

1V = I × R (Voltage = Current × Resistance)

2P = V × I (Power = Voltage × Current)

3P = I² × R (Power = Current squared × Resistance)

4P = V² ÷ R (Power = Voltage squared ÷ Resistance)

Input at least two fields to solve.

Ohm's Law conversion table

Reference Table4 rows

Examples of Ohm's Law relationships.
Voltage (V)↕	Current (I)↕	Resistance (R)↕	Power (P)↕
12 V	2 A	6 Ω	24 W
24 V	4 A	6 Ω	96 W
120 V	10 A	12 Ω	1,200 W
240 V	15 A	16 Ω	3,600 W

💡 Click any row to auto-fill the calculator aboveClick column headers to sort

FAQ: Ohm's Law

What is Ohm's Law?

Ohm's Law is a fundamental formula in electrical engineering showing the relationship between Voltage (V), Current (I), and Resistance (R). It states that current is directly proportional to voltage and inversely proportional to resistance.

How do you calculate power from voltage and resistance?

Use the formula Power (Watts) = Voltage² ÷ Resistance (P = V² ÷ R). For example, a 12V bulb with 6 Ohms of resistance draws 12² ÷ 6 = 24 Watts of power.

Tools related to Ohm's Law

Watts to Joules ConverterThe full power × time energy calculator with three modes.kWh to JoulesTurn the kWh figure on your electricity bill into joules — 1 kWh = 3,600,000 J.Joules to kWhConvert a joule or megajoule figure into billable kilowatt-hours — divide by 3,600,000.Watts to AmpsCalculate amps from watts and volts for DC and AC circuits — I = P ÷ V.Joules to CaloriesConvert joules into calories and food Calories (kcal) — 1 cal = 4.184 J.Amps to WattsCalculate watts from amps and volts for DC and AC circuits — P = I × V.Volts to WattsConvert potential (volts) to power (watts) given the electric current — P = V × I.Wh to JoulesConvert watt-hours to joules — 1 Wh = 3,600 J.BTU to JoulesConvert heating or cooling thermal units (BTU) to joules — 1 BTU = 1,055.06 J.BTU/hr to WattsConvert heating/cooling power (BTU/hr) to electrical watts — 1 BTU/hr = 0.293071 W.mAh to JoulesConvert battery milliamp-hours (mAh) to joules given nominal voltage — E = mAh × V × 3.6.Battery Run TimeEstimate battery run time under a given load — active hours = capacity × volts × DoD ÷ load.Appliance Energy CostCalculate electricity cost for any appliance — cost = power × hours ÷ 1,000 × rate.Voltage DropCalculate voltage drop over long electrical wire runs — V_drop = 2 × K × I × L ÷ Area.AC BTUCalculate heating/cooling BTUs needed for a room — base BTU = area × 20.Watts to HPTurn watts into horsepower — 1 mechanical hp = 745.7 W.HP to WattsTurn horsepower into watts — 1 mechanical hp = 745.7 W.Joules to WattsTurn joules of energy into watts of power — W = J ÷ seconds.Watts to OhmsFind resistance in ohms from power and voltage — R = V² ÷ P.Joules to eVTurn joules into electronvolts — 1 eV = 1.602176634×10⁻¹⁹ J.Electricity CostFind the running cost of any device — cost = (W ÷ 1000) × hours × rate.Watts to AhSize a battery from a load — Ah = (watts × hours) ÷ volts.Ah to WattsFind the load a battery supports — W = (Ah × volts) ÷ hours.Watts to VoltsFind voltage from power and current — V = P ÷ I.Joules to WhTurn joules into watt-hours — 1 Wh = 3,600 J.Joules to BTUTurn joules into BTU — 1 BTU = 1,055.06 J.Watts to Joules per SecondWatts and joules per second are the same unit — 1 W = 1 J/s.