Ohm's law calculator

How much current flows, and how much power is used?

Give any two of voltage, current, resistance and power to get the other two.

You know

Voltage

Resistance

Current

25.53 mA

12 V and 470 Ω give 25.53 mA and 306.4 mW

The whole circuit

Voltage: 12 V
Current: 25.53 mA
Resistance: 470 Ω
Power: 306.4 mW

Formulas used

Current: I = V / R
Power: P = V² / R
For direct current and resistive loads. Motors, LEDs and AC circuits with coils or capacitors do not follow V = I × R alone.

How to use

Choose which two quantities you know, for example voltage and resistance.
Type the two values and pick their units (mV or V, mA or A, mΩ to MΩ, mW to kW).
Read the other two quantities, with the formulas used. Copy a value or share the link.

How it works

Voltage pushes current through a resistance. Ohm's law says how they are tied together: double the voltage across the same resistor and twice the current flows. Power is how much energy the resistor turns into heat each second, and it grows with both voltage and current.

The formulas are V = I × R and P = V × I, where V is the voltage in volts, I the current in amperes, R the resistance in ohms and P the power in watts. Together they give P = I² × R and P = V² ÷ R, so any two of the four fix the other two. For example, 12 V across a 470 Ω resistor drives I = 12 ÷ 470 = 25.5 mA and uses P = 12² ÷ 470 = 0.306 W, so a ¼ W resistor would run too hot. The units are defined in the SI Brochure of the BIPM.

This holds for direct current through resistive parts. LEDs, diodes and motors do not have a fixed resistance, and alternating current through coils or capacitors needs impedance, not resistance.

Worked example

What happens when 12 V is applied across 100 Ω?

Current: I = 12 ÷ 100 = 0.12 A.
Power: P = 12 × 0.12 = 1.44 W.

The current is 0.12 A and the resistor dissipates 1.44 W.

Frequently asked questions

What resistor wattage do I need?

Calculate the power and pick a resistor rated for at least twice that. A resistor using 0.31 W needs a ½ W part, not a ¼ W one.

Why does a short circuit draw so much current?

I = V ÷ R: as the resistance falls towards zero, the current rises until a fuse, the wire or the source gives up.

Can I use this for household mains?

For heaters, kettles and other resistive loads, yes: a 2 kW kettle at 230 V draws about 8.7 A. Motors and electronics draw a different current because of their power factor.

Does doubling the voltage double the power?

At a fixed resistance, power rises with the square of voltage. Doubling the voltage makes the power four times larger.

Can I enter zero resistance?

No. The calculation requires positive values. An ideal short circuit has zero resistance and no finite current at a nonzero voltage.