Ohm's Law Calculator

How to use this calculator

Fill in any two fields — for example voltage and power — and the remaining two are solved automatically. Change units with the dropdowns (mA, kΩ, kW…) and the values convert in place. Use Show steps to see the formula with your numbers substituted.

The formulas

V = I × R P = V × I

Combining these gives every other form you may need:

Find	From V and I	From I and R	From V and R	From P and …
Voltage	—	I × R	—	√(P × R)
Current	—	—	V ÷ R	P ÷ V
Resistance	V ÷ I	—	—	V² ÷ P
Power	V × I	I² × R	V² ÷ R	—

Worked example: 4–20 mA loop

A 4–20 mA transmitter drives a 250 Ω sense resistor. At full scale (20 mA):

• V = 0.020 A × 250 Ω = 5 V
• P = 5 V × 0.020 A = 0.1 W

That is why 250 Ω resistors convert a 4–20 mA signal into a 1–5 V input. The default values above show the 24 V supply case; set Current to 20 mA to try it.

Reading the graph

The chart plots one quantity against another while holding your other inputs fixed. With voltage fixed, current falls as resistance rises — the inverse curve I = V ÷ R. Switch the axes to plot power, or change the range.

Practical tips

• Resistor wattage: choose a resistor rated at least 2× the calculated power.
• Wire and fuse sizing depend on current, so calculate I first.
• Remember to convert mA to A and kΩ to Ω when working by hand — the calculator handles this for you.

Frequently asked questions

What is Ohm's law?

Ohm's law states that the current through a conductor is proportional to the voltage across it. V = I × R, where V is volts, I is amperes and R is ohms.

How do I find resistance from power and voltage?

Use R = V² ÷ P. A 230 V, 60 W lamp has a hot resistance of 230² ÷ 60 ≈ 882 Ω. Enter V and P above and the calculator does this for you.

Does Ohm's law apply to every component?

No. It applies to ohmic (linear) components like resistors. Diodes, transistors and lamp filaments change resistance with current or temperature.

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