Ohm's Law Calculator

Solve a circuit's basic electrical values without doing the algebra by hand. Enter any two of voltage, current, resistance and power, and this calculator derives the other two from Ohm's law (V = I × R) and the power relation (P = V × I). Type values the way they are written on a schematic — 4.7k, 20mA, 4k7 — and the formula used is shown under the answer. It runs entirely in your browser.

Which formula applies

You knowYou get
V and RI = V / R, P = V² / R
V and IR = V / I, P = V × I
I and RV = I × R, P = I² × R
P and VI = P / V, R = V² / P
P and IV = P / I, R = P / I²
P and RV = √(P × R), I = √(P / R)

When to use it

  1. Find the current through a resistor at a given voltage.
  2. Work out how much power a resistor has to dissipate.
  3. Size a resistor for a target current.
  4. Check a measured circuit against the theory.
  5. Learn or teach Ohm's law with the working shown.

How to Use

  1. Enter any two of voltage, current, resistance and power.
  2. Read the other two values in the results.
  3. Check the formula shown beneath the results.

Features

  1. Solves from any two of V, I, R and P
  2. SI unit prefixes and 4k7-style notation
  3. Formula shown for each combination
  4. Refuses over-specified input instead of guessing
  5. Results to four significant figures
  6. 100% client-side

FAQs

How many values do I enter?

Exactly two. Any two of voltage, current, resistance and power fix the other two. If you fill in three, the tool asks you to clear one rather than guessing which to trust.

Can I type values like 4.7k or 20mA?

Yes. The prefixes p, n, µ (or u), m, k, M and G are understood, as is engineering notation such as 4k7. Lower-case m is milli and upper-case M is mega.

Does it calculate power?

Yes. Power follows from P = V × I, or equivalently V² / R and I² × R, depending on which values you supplied.

Why does it say there is no solution?

Some pairs divide by zero — a voltage across zero ohms, for example, implies infinite current. The tool reports that rather than displaying infinity.

Is my data sent anywhere?

No. It is arithmetic in the page and nothing is transmitted or stored.

Technical Details

Voltage, current, resistance and power are tied together by two relations: Ohm's law, V = I × R, and the power law, P = V × I. Any two of the four fix the other two, so the calculator accepts exactly two and solves the remaining pair from whichever of the six combinations was given — for example V and R give I = V / R and P = V² / R, while P and R give V = √(P × R) and I = √(P / R). Inputs are read with SI prefixes, so 4.7k, 4k7 and 4700 are the same resistance and 20m is twenty milliamps; trailing unit letters are ignored. Values are converted to base units before any arithmetic, and the results are shown back with the most readable prefix to four significant figures. Entering three or four values is refused rather than silently ignoring one, because an over-specified circuit is usually inconsistent and picking two of them at random would hide that. A pair that divides by zero — a zero resistance with a voltage across it, for instance — is reported instead of shown as infinity. The formula used for the current pair is displayed beneath the result so the answer can be checked by hand. Everything runs in the page; nothing is transmitted.