Ohm's law states that the voltage across a resistor equals the current through it times its resistance, written V = I times R. Combined with electrical power P = V times I, any two of the four quantities voltage, current, resistance and power determine the other two.

How does the formula wheel work?

The wheel groups the twelve standard rearrangements of Ohm's law and the power equation. Whatever pair of values you enter, the calculator selects the matching forms — for example V squared over R for power from voltage and resistance — and highlights the active segment so you can see which formula was applied.

How do I enter milliamps, kilohms or milliwatts?

Use the unit dropdown next to each input. Selecting mA, kΩ or mW scales your typed number to base SI units automatically, so 4.7 kΩ and 0.5 mA are handled correctly without you converting them by hand.

How do you find power without knowing the current?

Use the derived forms. If you know voltage and resistance, power is P = V squared over R. If you know current and resistance, power is P = I squared times R. The calculator applies whichever form matches the pair you selected.

How do you find resistance from power and current?

Rearrange to R = P over I squared. For example, 50 W flowing as 2 A needs R = 50 over 4 = 12.5 ohms. Choose the current-and-power pair and the tool computes this, then derives the voltage as V = I times R.

What power rating should a resistor have?

Size it above the dissipated power P = I squared times R, with margin. A 100-ohm resistor carrying 0.1 A dissipates 0.1 squared times 100 = 1 W, so a quarter-watt or half-watt part would overheat — you would pick at least a 2 W resistor for headroom.

Ohm's Law Calculator

Ohm’s law links voltage (V), current (I) and resistance (R) through the single relationship V = I × R, and electrical power (P) follows as P = V × I. Because these four quantities are tied together, knowing any two of them fixes the other two exactly. This calculator lets you enter whichever pair you have to hand — volts and amps, watts and ohms, or any other combination — and instantly returns the remaining values, complete with the worked steps and the classic formula wheel so you can see which equation was used.

It is built for everyday electronics: sizing a current-limiting resistor, checking that a component stays inside its power rating, working out the load a power supply must drive, or sanity-checking a measurement on the bench. Every input has an SI-prefix selector, so milliamps, kilohms and milliwatts are handled without you converting anything by hand, and the results are re-expressed in sensible engineering units automatically.

How it works

You select which two of the four quantities you know, type their values, and the tool solves for the rest. Depending on the pair, it applies the matching derived form of Ohm’s law and the power equation:

You know	Voltage	Current	Resistance	Power
V and I	—	—	V ÷ I	V × I
V and R	—	V ÷ R	—	V² ÷ R
I and R	I × R	—	—	I² × R
V and P	—	P ÷ V	V² ÷ P	—
I and P	P ÷ I	—	P ÷ I²	—
R and P	√(P × R)	√(P ÷ R)	—	—

The full set of relationships is V = I × R, P = V × I, and the two derived power forms P = I² × R and P = V² ÷ R. The calculator keeps everything in base SI units internally (volts, amperes, ohms, watts) and only applies engineering prefixes when displaying the answer.

Worked example

Suppose you are driving an LED-style load from a 12 V rail and you measure 2 A of current (the default pair). Ohm’s law gives the effective resistance, and the power equation gives the heat the load must dissipate:

Resistance: R = V ÷ I = 12 ÷ 2 = 6 Ω
Power: P = V × I = 12 × 2 = 24 W

So a load drawing 2 A at 12 V behaves like a 6-ohm resistor turning 24 W into heat — enough to need real thermal management. Switch the known pair to Resistance and Power with R = 6 Ω and P = 24 W and the tool runs the calculation in reverse: I = √(P ÷ R) = √(24 ÷ 6) = 2 A, then V = I × R = 12 V, recovering the same numbers from the opposite direction.

Formula note

These relationships assume an ohmic, linear resistance held at a steady temperature and a DC (or instantaneous-value) circuit. For AC circuits with reactive components, resistance R is replaced by impedance Z and real power involves the power factor, so treat the result as the resistive-load case. Every figure is computed locally in your browser — nothing is uploaded or stored.