Ohm’s law and power calculator
Type any two of the four — volts, amps, ohms, watts — and the other two appear, with the formula that found them. Plain units: convert kW, mA or kΩ first (1.5 kW = 1,500 W).
For resistive loads. On motors and other AC loads V × I is volt-amps; the watts also need the power factor.
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How it works
Two laws do all of it. Ohm’s law: V = I × R. The power law: P = V × I. Put one inside the other and every pair of known values has a formula for the other two. The calculator picks the pair you gave it and shows which formulas it used, so you can do the same on paper.
Worked example
A 1,500 W heater on 120 V
- Known: P = 1,500 W, V = 120 V. Want: I and R.
- I = P ÷ V = 1,500 ÷ 120 = 12.5 A.
- R = V² ÷ P = 14,400 ÷ 1,500 = 9.6 Ω.
- Check: I × R = 12.5 × 9.6 = 120 V. ✓
Only ohms and watts known
- A 10 Ω resistor must not dissipate more than 5 W. What is the most current and voltage it can take?
- I = √(P ÷ R) = √(5 ÷ 10) = √0.5 = 0.707 A.
- V = √(P × R) = √(5 × 10) = √50 = 7.07 V.
Questions people ask
Why can I only enter two values?
Two is all it takes: volts, amps, ohms and watts are tied together by V = I × R and P = V × I, so any two fix the other two. Enter a third and it might disagree with the first two — the calculator would have to pick which one to believe, so it asks you to clear one instead.
Does this work for motors and AC loads?
For resistive loads — heaters, elements, incandescent lamps, resistors — yes, exactly. For a motor or anything with coils or capacitors, V × I gives volt-amps, not watts; the real power also depends on the power factor. The power triangle calculator handles that.
Can I enter kilowatts or milliamps?
Convert to plain units first: 2.5 kW is 2,500 W, 350 mA is 0.35 A, 4.7 kΩ is 4,700 Ω. Keeping everything in base units is the habit that stops the thousand-times errors.
Related
- Ohm’s law and power — the lesson behind this tool.
- What electricity is — the lesson behind this tool.
- Ohm’s law practice drill — do the arithmetic yourself, marked as you go.
- Ohm’s law & power wheel — printable.
- Series & parallel resistance calculator
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