Ohm's Law Calculator

Known values

First value

Second value

Result

Voltage, V12
Current, A2
Resistance6 Ом
Power, W24
Resistance, Ω6
Energy per hour, kWh0.024

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. Know any two and the third follows in one step, and power joins as a fourth — the product of voltage and current.

In practice resistance is what people usually want. A 12 volt supply, a load that must draw no more than two amps, so a 6 ohm resistor. Or the other way round: what current will flow through a known load.

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 get power from current and resistance?

P = I²·R. From voltage and resistance it is P = U²/R. All three forms give the same answer; each suits a different set of known values.

Does Ohm's law hold for alternating current?

For a purely resistive load, yes, using RMS values. Once coils or capacitors appear you need impedance rather than resistance, and a phase angle to go with it.

Why does the resistor get hot?

Every watt it drops becomes heat. A 6 ohm resistor at 2 amps dissipates 24 watts — comparable to a small soldering iron, and it needs a heatsink.

What happens as resistance approaches zero?

Current heads for infinity — that is a short circuit. In reality the source's internal resistance limits it, but not before a fuse blows or a conductor melts.

Worked example

A load on a bench supply

Input: 12 V and 2 A

Output: 6 Ω resistance, 24 W of power

Note: The power figure matters more than the resistance: a 6 ohm resistor here has to be rated for at least 25 watts or it simply burns out.

Useful links

All tools in Electrical

Ohm's Law Calculator

Result

Related tools

Watts Amps Volts Calculator

LED Resistor Calculator

Resistor Colour Code

Useful links