Ohm’s Law Calculator
Pick the two values you know — voltage, current or resistance — and the calculator fills in the rest, showing the formula with your numbers and the results in the units an electrician would expect.
Resistance and power are calculated from the voltage and current you entered.
Formula and calculation
Worked example
With 12 V across a circuit carrying 0.5 A, the resistance is 24 and the power is 6. Written out: R = V ÷ I = 12 ÷ 0.5 = 24 Ω; P = V × I = 12 × 0.5 = 6 W
Reading the units
Values in electronics cover a huge range, so engineers express them with prefixes: 0.5 A is written 500 mA, 4700 Ω is 4.7 kΩ, and 0.25 W is 250 mW. The results table converts each quantity into a readable form for you. Two checks are worth making before you trust a number. First, power ratings: a 0.25 W resistor running at 0.4 W will overheat, no matter how correct the arithmetic is. Second, temperature: Ohm's law silently assumes the resistance stays where you entered it, and real resistors drift as they heat up — an incandescent bulb's resistance at room temperature is roughly a tenth of its resistance when lit, which is why a bulb draws a large inrush current when switched on.
What the results mean
- Voltage
- The potential difference across the circuit, in volts. It is either one of your inputs or V = I × R when you entered current and resistance.
- Current
- The current flowing through the circuit, in amperes. It is either one of your inputs or I = V ÷ R when you entered voltage and resistance.
- Resistance
- The resistance in ohms — the ratio of voltage to current. It is either one of your inputs or R = V ÷ I when you entered voltage and current.
- Power
- The power dissipated in watts, from P = V × I, V² ÷ R or I² × R depending on what you entered. This is the number that tells you how much heat the component has to get rid of.
- Substituting your numbers
- The calculation written out with your own numbers, so you can check the arithmetic by hand.
Frequently asked questions
How do I use Ohm's law to find resistance?
Divide voltage by current: R = V ÷ I. With 12 V driving 0.5 A, the resistance is 12 ÷ 0.5 = 24 Ω. In the calculator, pick “Voltage + current”, enter both values and read the resistance and power back.
What is the difference between volts, amps, ohms and watts?
Volts measure the electrical push, amps measure the flow, ohms measure the opposition, and watts measure the energy converted per second. Ohm's law links the first three (V = I × R), and power is the product of voltage and current — so any two of the four numbers determine the other two.
Does Ohm's law work for AC circuits?
Only for purely resistive loads. With AC motors, transformers and anything containing coils or capacitors, the opposition to current is impedance, not plain resistance, and voltage and current drift out of step — the power factor. Multiplying volts by amps then overstates the real power, so treat this calculator's output as a DC or resistive-load figure.
Why does my resistor get hot?
Every component dissipates P = I² × R watts as heat, so a resistor carrying a large current can exceed its rated power even when the voltage and resistance look sensible. Compare the calculated power with the resistor's watt rating and leave headroom — replacing a hot 0.25 W resistor with a 1 W part of the same resistance is usually the cheapest fix.