Ohm's Law & Power Calculator

The four relationships

Ohm's law states that the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance. Combined with the power law, that gives you twelve equations relating four quantities — but you only ever need the four that use the pair you already know.

If you know voltage and current

R = V / I and P = V × I

If you know voltage and resistance

I = V / R and P = V² / R

If you know current and resistance

V = I × R and P = I² × R

If you know power and one other quantity

V = P / I, I = P / V, R = V² / P, R = P / I²

Worked example: sizing a resistor for heat, not just value

Suppose you are dropping a 12 V rail to feed a 5 V load drawing 40 mA through a series resistor. The resistor drops 7 V at 40 mA, so R = 7 / 0.04 = 175 Ω — call it 180 Ω from the E24 series. Power is P = 7 × 0.04 = 0.28 W.

That last number is the one people skip. A standard 0805 chip resistor is rated at 125 mW, so it would run at more than twice its rating and fail within hours. You need a 1206 at minimum, and in practice a 0.5 W part for margin. As a rule of thumb, pick a resistor rated at twice the calculated dissipation, and more if it sits near other hot components or inside a sealed enclosure.

Where Ohm's law stops working

Ohm's law describes ohmic devices — ones whose resistance is constant regardless of the voltage across them. Plenty of things you will put in a circuit are not ohmic:

• Diodes and LEDs have an exponential V–I curve. You cannot compute an LED's current from a resistance; you subtract its forward voltage and apply Ohm's law to the series resistor instead. That is what the LED resistor calculator does.
• Incandescent lamps have a cold resistance around a tenth of their hot resistance, which is why they draw a large inrush current at switch-on.
• Thermistors and varistors are non-linear by design — that is the entire point of them.
• Semiconductor junctions in transistors and regulators follow their own device equations, not Ohm's law.

Ohm's law also assumes DC or low-frequency AC. Once reactance matters you need impedance rather than resistance, and voltage and current stop being in phase.

Practical notes

• Watch the units. Most errors in this calculation are factor-of-a-thousand errors from mixing milliamps with amps. The unit selectors above exist for exactly that reason.
• Resistance changes with temperature. Copper rises about 0.39% per °C. A trace or winding that is warm carries measurably more resistance than the same part on the bench.
• Measure across the component, not the supply. Voltage drop in wiring and connectors is real, and on high-current low-voltage rails it can dominate.

Frequently asked questions

What is Ohm's law in simple terms?

Voltage equals current times resistance: V = I × R. Push harder (more volts) and more current flows; add resistance and less current flows. Rearranged, I = V / R and R = V / I.

How do I calculate power from voltage and current?

P = V × I. If you know resistance instead, P = V² / R or P = I² × R. All three give the same answer for an ohmic component.

What resistor wattage do I need?

Calculate the dissipation with P = I² × R, then pick a part rated for at least twice that. A 0805 chip resistor handles about 125 mW, 1206 about 250 mW, and 2512 about 1 W — derate all of these above 70 °C.

Why doesn't Ohm's law work for LEDs?

LEDs are not ohmic — their current rises exponentially with forward voltage, so there is no fixed resistance to plug in. Instead you subtract the LED forward voltage from the supply and apply Ohm's law to the series resistor. The LED resistor calculator handles this.

Does resistance change with temperature?

Yes. Copper increases roughly 0.39% per °C, so a winding or PCB trace at 85 °C carries about 25% more resistance than at 20 °C. Precision resistors specify a temperature coefficient in ppm/°C for exactly this reason.