Ohm's Law Calculator

Written byOleh Soloninupdated 25 September 2026

Known values

First value

The first value of the chosen pair: voltage in volts, current in amps or resistance in ohms.

Second value

The second value of the pair: current in amps, resistance in ohms or power in watts.

Result

Voltage, V12
Current, A2
Resistance6 ohm
Power, W24
Resistance, Ω6
Energy per hour, kWh0.024
WorkingR = V ÷ I = 12 ÷ 2 = 6 Ω; P = V × I = 24 W

Two values fix all four

Finds all four quantities from any two known values, with resistance shown in whichever unit reads most naturally.

Ohm's law ties three quantities together: voltage equals current times resistance, V = I × R. Power adds a fourth, P = V × I. Any two of the four fix the other two, so the calculator takes six pairs, from voltage and current to resistance and power.

For direct current and a purely resistive load such as a heater, the formulas are exact. In an AC circuit real power is V × I × power factor. With a motor the current is shifted against the voltage and the factor is cos φ; a switch-mode power supply also distorts the current, so it needs the full power factor from its data sheet. The watts and amps calculator handles that.

How to use

Choose which two quantities you know: voltage, current, resistance or power.
Enter them as the first and second value, in the order the chosen pair names them.
Read the other two quantities, the energy per hour and the working line with the formula used.

The formula for each pair

V = I × R, I = V ÷ R, R = V ÷ I; power P = V × I = I² × R = V² ÷ R. From voltage and power, the current is P ÷ V and the resistance V² ÷ P; from resistance and power, the voltage is √(P × R) and the current √(P ÷ R).

The row with a 470 Ω, 0.25 W resistor shows its limit: above 10.84 V it would dissipate more than its rating.

Each row is a separate run of the calculator on this page.
Known	Voltage	Current	Resistance	Power
5 V and 0.5 A	5 V	0.5 A	10 Ω	2.5 W
12 V and 1 A	12 V	1 A	12 Ω	12 W
24 V and 2 A	24 V	2 A	12 Ω	48 W
230 V and 10 A	230 V	10 A	23 Ω	2,300 W
230 V and 2,000 W	230 V	8.696 A	26.45 Ω	2,000 W
470 Ω and 0.25 W	10.84 V	0.023 A	470 Ω	0.25 W
0.02 A and 150 Ω	3 V	0.02 A	150 Ω	0.06 W

What this calculator shows

All four quantities at once — voltage, current, resistance and power
Resistance written in ohms, kilohms or megohms as appropriate
The energy consumed over an hour of operation

What to keep in mind

Calculated for direct current and a purely resistive load. Coils and capacitors in an AC circuit add reactance and the formula grows.
Resistance is treated as constant, although in most conductors it rises with temperature.

FAQs

How do I find resistance from voltage and current?

Divide the voltage by the current: R = V ÷ I. 12 volts at 2 amps is 6 ohms.

How do I find current from power and voltage?

I = P ÷ V. A 2,000-watt appliance on 230 volts draws 8.7 amps, and its resistance while running is V² ÷ P = 26.45 ohms.

What voltage can a resistor of a given wattage take?

V = √(P × R). A 470 Ω resistor rated 0.25 W reaches its rating at 10.84 V and 23 mA.

Does Ohm's law hold for alternating current?

For a resistive load, yes, using RMS voltage and current. With a motor the current is shifted against the voltage; a switch-mode power supply also distorts it. Real power is V × I × power factor either way — for a motor that is cos φ, while for a power supply the factor comes from its data sheet, because cos φ does not capture the distortion. The watts and amps calculator does that sum.

How much energy does an appliance use in an hour?

Its power in kilowatts times the hours. 24 W for an hour is 0.024 kWh; the calculator shows that line separately.

How much does power rise if the voltage doubles?

Fourfold at the same resistance: P = V² ÷ R. A 6 Ω heater gives 24 W on 12 V and 96 W on 24 V.

Worked example

A 12-volt heater

Input: 12 V and 2 A

Output: 6 Ω and 24 W

Note: In an hour the heater uses 0.024 kWh.

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