Ohm's Law and the Power Triangle

By Anatolie · Updated 2026-09-03

Two relationships cover most everyday electronics maths: Ohm's law, which links voltage, current, and resistance, and the power law, which links voltage, current, and power. Between them they answer "what resistor do I need," "will this wire get hot," and "how much heat does this part dissipate."

Ohm's law

V = I × R

• V — voltage across the component, in volts (V)
• I — current through it, in amperes (A)
• R — resistance, in ohms (Ω)

Rearranged: I = V ÷ R and R = V ÷ I. Keep the units consistent — if current is in milliamps, either convert to amps first or expect the answer in the matching scaled unit. A frequent slip is mixing mA and A in the same calculation.

The power law and the "triangle"

P = V × I

P is power in watts (W). Substituting Ohm's law gives two more forms that are often more convenient:

P = I² × R  (when you know current and resistance)
P = V² ÷ R  (when you know voltage and resistance)

Together the two laws produce six formulas — any one of V, I, R, P from any two of the others. The Ohm's law calculator takes any two values and fills in the rest.

Worked example: an LED series resistor

You want to run a red LED from a 5 V supply. The LED datasheet says it drops about 2.0 V at its target 20 mA. The resistor has to drop the remaining voltage:

V_R = 5 − 2.0 = 3.0 V
R = V_R ÷ I = 3.0 ÷ 0.020 = 150 Ω

Now check the resistor's power so it does not cook:

P = I² × R = 0.020² × 150 = 0.0004 × 150 = 0.06 W

A standard 1/8 W (0.125 W) or 1/4 W resistor is comfortable. The LED resistor calculator does this end to end, and the resistor power calculator checks dissipation on its own.

Worked example: will the wire get hot?

You are pushing 10 A through a length of wire with 0.05 Ω of resistance. Power lost as heat in the wire:

P = I² × R = 100 × 0.05 = 5 W

Five watts in a thin wire is a lot — this is why current rating drops as wire gets longer or thinner, and why the I² term matters: doubling the current quadruples the heating.

Voltage dividers

Two resistors in series split a voltage in proportion to their values:

V_out = V_in × R2 ÷ (R1 + R2)

This is Ohm's law applied twice — the same current flows through both resistors, so the voltage across each is proportional to its resistance. It is exact only when nothing draws current from the midpoint; a real load in parallel with R2 pulls V_out down. The voltage divider calculator handles the loaded case too.

Where these simple laws stop

• Non-ohmic components. Diodes, LEDs, and transistors do not have a constant R — their current-voltage curve is not a straight line, which is exactly why the LED example above used a fixed forward voltage from the datasheet rather than a resistance.
• AC and reactance. With alternating current, capacitors and inductors add impedance that depends on frequency and shifts the phase between voltage and current. Ohm's law generalises to V = I × Z but the arithmetic becomes complex numbers.
• Temperature. A resistor's value drifts with temperature, and a wire's resistance rises as it heats — so the "will it get hot" check can be a moving target for large currents.
• Tolerance. A 150 Ω resistor at 5% tolerance is anywhere from 142.5 to 157.5 Ω. Design with margin rather than assuming the printed value.