Power converter

Watts to Ohms Calculator

Calculate resistance in ohms from power in watts and the applied voltage.

How Watts to Ohms works

Ohm's law and the power equation combine to relate power, voltage, and resistance. Since P = V² ÷ R, rearranging gives resistance R = V² ÷ P — the square of the voltage divided by the power.

This is useful for sizing resistive loads such as heating elements or dummy loads: if you know the operating voltage and the target power dissipation, you can calculate the resistance you need in ohms.

Formula

1R(Ω) = V(V)² ÷ P(W)

Derived from P = V² ÷ R (Ohm's law with the power equation).

Watts to Ohms conversion table

Reference Table6 rows

Resistance in ohms for common power ratings at 12 V and 120 V.
Power↕	At 12 V↕	At 120 V↕
10 W	14.4 Ω	1,440 Ω
25 W	5.76 Ω	576 Ω
60 W	2.4 Ω	240 Ω
100 W	1.44 Ω	144 Ω
500 W	0.288 Ω	28.8 Ω
1,000 W	0.144 Ω	14.4 Ω

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

FAQ: Watts to Ohms

How do you convert watts to ohms?

Square the voltage and divide by the power. Ohms = Volts² ÷ Watts. For example, at 120 V a 60 W load has a resistance of 120² ÷ 60 = 240 Ω.

Why do you need voltage to find ohms?

Power alone does not define resistance — the same power can be produced by many voltage and resistance combinations. Voltage fixes the relationship so resistance can be calculated.

Can I use current instead of voltage?

Yes. If you know the current instead, use R = P ÷ I² (power divided by the square of the current) to find resistance in ohms.

Tools related to Watts to Ohms

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.Ohm's LawSolve Voltage, Current, Resistance, and Power from any two inputs using Ohm's Law.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.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.