First year Basic circuits Free · works offline once loaded

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).

Remplissez deux cases. Laissez les deux autres vides.

Tension (V) Courant (A) Résistance (Ω) Puissance (W)

Tension—

Courant—

Résistance—

Puissance—

Pour les charges résistives. Sur les moteurs et les autres charges en c.a., V × I donne des voltampères; pour avoir les watts, il faut aussi le facteur de puissance.

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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 fréquentes

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.

Voir aussi

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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