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.
Worked example
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.
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 voltage times 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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