This electric power calculator solves the whole Ohm’s-law family at once: enter any two of voltage, current, resistance and power, and it returns the other two — every formula on the famous wheel, in one place.
Fill in any two fields and leave the other two blank:
Sanity checks worth knowing: current above ~15–20 A on a household branch means breaker territory; resistance of a heating element is usually stamped by its rating (a “1,500 W at 120 V” heater is a 9.6 Ω device drawing 12.5 A).
The Wheel of Twelve Formulas
Two seed laws — Ohm’s law (V = I·R) and power (P = V·I) — generate everything: substitute one into the other and P = I²R, P = V²/R, and all the rearrangements fall out. P = I²R is why transmission lines run at extreme voltage (halve the current, quarter the resistive loss) and why a short circuit (R→0 at fixed V) is a current explosion. The components themselves get decoded on our Resistor Color Code Calculator; once power becomes energy over time, bills happen on the Electricity Cost Calculator. Part of our Physics Calculators collection.
Worked Example (Verified)
A 1,000 W appliance on 120 V mains (the widget’s defaults):
- Current: I = P ÷ V = 1,000 ÷ 120 = 8.33 A — comfortable on a 15 A circuit; two of them are not
- Resistance: R = V² ÷ P = 14,400 ÷ 1,000 = 14.4 Ω
- Reverse check: a 10 Ω resistor on 120 V draws 12 A and dissipates V²/R = 1,440 W — small resistances at mains voltage are serious heaters, intended or not
AC Honesty
- These are the DC / purely resistive formulas. For AC motors and anything with inductance or capacitance, real power is V×I×(power factor) — nameplate amps on motors overstate watts drawn, sometimes by 30%+.
- RMS values are already what household “120 V” means — no extra factor needed for resistive loads.
- Three-phase industrial power uses P = √3 × V_line × I × pf — a different wheel; sizing there belongs to an electrician with the actual nameplates.
Related Tools
- Power Calculator — the mechanical forms (W/t, F·v, τ·ω)
- Electricity Cost Calculator — watts into dollars
- Physics Calculators — the full collection
Frequently Asked Questions
How do you calculate electric power?
P = V × I (volts × amps = watts). With Ohm’s law (V = IR) it also becomes P = I²R or P = V²/R — enter any two of V, I, R, P here and the other two follow, e.g. 120 V and 1,000 W give 8.33 A and 14.4 Ω.
How many amps does a 1,500 W heater draw?
At 120 V: 1,500 ÷ 120 = 12.5 A — most of a 15 A circuit by itself, and the reason space heaters trip breakers shared with anything else. At 240 V it would draw half: 6.25 A.
Why do power lines use such high voltage?
Losses in the wire are I²R: delivering a given power at higher voltage needs proportionally less current, and losses fall with the square of that saving. Stepping 100× up in voltage cuts line heating 10,000× — the entire economics of the grid follows from P = I²R.
What is the resistance of a 60 W light bulb?
At its 120 V rating: R = V²/P = 240 Ω (hot). Cold, the filament measures a fraction of that — resistance rises with temperature, which is why bulbs draw an inrush and usually die at switch-on.
Is a watt the same everywhere?
Yes — a watt is a watt (1 J/s) whether electrical, mechanical or thermal. This calculator’s electrical formulas and the mechanical power formulas on the sibling page describe one quantity through different doors.
Is this electric power calculator free?
Yes — free, instant, no sign-up.