Ohm's Law Calculator

Enter any two of voltage, current, resistance, or power and instantly get the other two — with the formula shown.

Electronics Calculators Free

All 12 Ohm's Law & Power Formulas

V = I × R
V = P / I
V = √(P × R)
I = V / R
I = P / V
I = √(P / R)
R = V / I
R = V² / P
R = P / I²
P = V × I
P = I² × R
P = V² / R

About this tool

What this Ohm's law calculator does

Give it any two of the four core electrical quantities — voltage (V), current (I), resistance (R), or power (P) — and it solves for the other two instantly, using the right combination of Ohm's law and the power formulas. No need to remember which of the twelve formula variations applies to your situation; pick what you know and the tool figures out the rest.

Who it's for

Electronics hobbyists sizing a resistor for an LED, students checking circuit homework, technicians troubleshooting a power supply, and anyone who just needs a quick voltage, current, resistance, or power calculation without digging out a formula sheet.

How to use it

  • Pick the two values you already know from the dropdown — for example "Voltage & Resistance."
  • Enter those two numbers, choosing whatever unit is convenient (millivolts to kilovolts, microamps to amps, ohms to megaohms, milliwatts to kilowatts).
  • The other two values update live, along with the exact formula used to get there.
  • Scroll down for the full reference table of all twelve Ohm's law and power formula variations if you want to work it out by hand too.

Real-world use cases

  • Sizing a current-limiting resistor for an LED: you know the supply voltage and the LED's forward voltage/current, so pick Voltage & Current to find the resistance you need.
  • Checking a power supply's output: know the voltage and the load resistance, get current and power without a multimeter.
  • Estimating power draw for a component when you only know its resistance rating and the current it'll carry.
  • Verifying homework or a datasheet spec by cross-checking a calculated value against what a formula sheet gives you.

Frequently asked questions

What is Ohm's law?

Ohm's law states that voltage equals current multiplied by resistance (V = I × R). Combined with the power law (P = V × I), you can derive all twelve formula variations used to solve for any one of the four quantities given any two others.

Can I calculate power from just resistance and current?

Yes — power equals current squared times resistance (P = I² × R). Pick "Current & Resistance" from the dropdown and the calculator handles it, along with working out voltage too.

What units does this support?

Voltage in millivolts, volts, or kilovolts; current in microamps, milliamps, or amps; resistance in ohms, kilohms, or megaohms; power in milliwatts, watts, or kilowatts — mix and match per field.

Why can't I enter all four values at once?

Ohm's law only needs two known values to solve for the other two — entering all four risks contradicting the math (e.g. values that don't actually satisfy V = I × R). Picking exactly two keeps the result mathematically consistent.

Is this the same as a voltage divider calculator?

No — this solves the basic V/I/R/P relationship for a single component or load. A voltage divider calculator is for finding the output voltage across two resistors in series, a different (though related) circuit calculation.