It is the relationship between voltage, current and resistance: voltage equals current times resistance, or V = I × R. Rearranged, current is voltage over resistance and resistance is voltage over current. The power law rides alongside it — power is voltage times current — so from any two of the four quantities the other two follow by arithmetic.

How do I find current from voltage and resistance?

Divide the voltage by the resistance: I = V ÷ R. A 12-volt supply across a 4-ohm load draws 12 ÷ 4 = 3 amps. Enter the two you know here and the tool fills in the current, along with the power that circuit dissipates.

How do I work out watts?

Power is the product of voltage and current, P = V × I. If you only have current and resistance, P = I² × R; with voltage and resistance, P = V² ÷ R. This tool gives you the power however you enter the circuit, because it solves the whole set from any two values.

Why does the tool ask for exactly two values?

Two independent quantities pin down a simple DC or resistive circuit completely; give one and it is undefined, give three and they may not agree. So enter any two of volts, amps, ohms and watts, and it computes the remaining pair from first principles.

Ohm's law calculator

This is straight arithmetic on the two figures you type, assuming a steady DC or purely resistive circuit. Reactance, temperature and real-world power factor are not in it, and nothing here replaces a meter reading or the judgement of a qualified electrician on a live installation.

Type any two of voltage, current, resistance and power, and the tool solves the other two. It is the single most-used relationship in electrical work, and having all four laid out at once saves the mental juggling of which form to rearrange.

Exemple détaillé

A 120-volt heater with a 12-ohm element draws 120 ÷ 12 = 10 amps. Its power is 120 × 10 = 1,200 watts. Enter 120 V and 12 Ω and you get exactly that — the current and the wattage filled in without touching a second formula.

Sur le chantier

Amps off a wattage label. A 1,500 W space heater on 120 V draws 1,500 ÷ 120 = 12.5 A. The same 1,500 W as a 240 V baseboard draws 6.25 A — same heat, half the current, which is the reason big heating loads go on 240. Type the watts and the volts; the tool fills the amps.

A resistance you cannot get a meter on. A 4,800 W element on 240 V: current 20 A, and R = 240 ÷ 20 = 12 Ω. Meter it cold and it should read near that; an open element reads infinite, a shorted one near zero. The nameplate watts and volts are all you need.

Why a loose lug gets hot. Half an ohm of bad connection carrying 20 A drops 20 × 0.5 = 10 V and burns 20² × 0.5 = 200 W at the terminal — a soldering iron's worth, inside a panel. Enter 20 A and 0.5 Ω and read the watts; that is the arithmetic behind a thermal scan.

Exercez-vous

Quick referencefor Ohm's law

The four: V = I × R · P = V × I · P = I² × R · P = V² ÷ R
Amps from watts: I = P ÷ V · 1,500 W: 12.5 A at 120 V, 6.25 A at 240 V
Watts from amps: at 120 V each amp is 120 W · at 240 V, 240 W
Unités: 1 kW = 1 000 W · 1 hp = 746 W · 1 mA = 0,001 A
What this page does not do: size a conductor or a breaker — that is the code book's

Mistakes to catchbefore you trust the number

Entering three values and expecting the tool to check them: it solves from two, so the third is ignored.
Using a nominal voltage (120) with a measured current, then wondering why the resistance does not match the meter — measure both at the same time.
Confusing watts with volt-amps on a motor or a ballast; Ohm's law here is for resistive loads and DC.

The formulas

V = I × R · P = V × I · P = I²R = V² ÷ R

Ohm's law ties voltage, current and resistance; the power law ties power to voltage and current. Together they form a closed set — fix any two quantities and the algebra gives the rest. The tool picks whichever pair you supplied and works forward from it.

Questions

What is Ohm’s law?: It is the relationship between voltage, current and resistance: voltage equals current times resistance, or V = I × R. Rearranged, current is voltage over resistance and resistance is voltage over current. The power law rides alongside it — power is voltage times current — so from any two of the four quantities the other two follow by arithmetic.
How do I find current from voltage and resistance?: Divide the voltage by the resistance: I = V ÷ R. A 12-volt supply across a 4-ohm load draws 12 ÷ 4 = 3 amps. Enter the two you know here and the tool fills in the current, along with the power that circuit dissipates.
How do I work out watts?: Power is the product of voltage and current, P = V × I. If you only have current and resistance, P = I² × R; with voltage and resistance, P = V² ÷ R. This tool gives you the power however you enter the circuit, because it solves the whole set from any two values.
Why does the tool ask for exactly two values?: Two independent quantities pin down a simple DC or resistive circuit completely; give one and it is undefined, give three and they may not agree. So enter any two of volts, amps, ohms and watts, and it computes the remaining pair from first principles.

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