Ohms Law Calculator
Calculate voltage, current, resistance or power from any two values.
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Fill in any two of voltage (V), current (A) and resistance (Ω), and this calculator works out the third using Ohm's law, plus the power in watts. It is designed for quick circuit checks: choosing a resistor for an LED, estimating the current a heater draws, or confirming a resistor will not overheat.
The formulas used
| You enter | It calculates | Power |
|---|---|---|
| V and I | R = V ÷ I | P = V × I |
| V and R | I = V ÷ R | P = V² ÷ R |
| I and R | V = I × R | P = I² × R |
Results are rounded to three decimal places. If you fill all three boxes, the calculator uses voltage and current and ignores the resistance you typed, so leave exactly one field empty. A value of 0 is treated as empty.
Worked example: resistor for an LED
A red LED on a 5 V supply drops about 2 V and should run at 20 mA. The resistor has to take the remaining 3 V at that current. Enter 3 in Voltage and 0.02 in Current:
- Resistance = 3 ÷ 0.02 = 150 Ω
- Power = 3 × 0.02 = 0.06 W, comfortably inside a standard ¼ W resistor's rating.
Enter the voltage across the resistor, not the whole supply voltage. An LED is not a simple resistor, so Ohm's law is applied to the part of the circuit that is.
Worked example: checking power rating
Now put 12 V straight across a 220 Ω resistor. With Voltage 12 and Resistance 220, the current is 0.055 A (about 55 mA) and the power is 0.655 W. That is more than twice what a ¼ W resistor can handle, so it would run hot and eventually fail. A 1 W part is the sensible choice. As a rule of thumb, pick a resistor rated for at least double the calculated power.
Mains appliances at 230 V
Household supply in India is 230 V. A 1000 W kettle draws about 1000 ÷ 230 = 4.35 A. Entering 230 V and 4.35 A gives an element resistance of about 52.9 Ω, and the power line reads roughly 1000 W. The calculator does not take power as an input, so when you only know the wattage, divide it by the voltage first to get the current.
For motors, fans and anything with a transformer, V × I is only the apparent power; the real power also depends on the power factor. To turn an appliance's wattage into a monthly cost, the electricity bill calculator is the next step.
Unit tips
- Convert milliamps to amps: 20 mA = 0.02 A.
- Convert kilo-ohms to ohms: 4.7 kΩ = 4700 Ω.
- Because answers are rounded to three decimals, very small currents lose detail. 9 V across 4.7 kΩ shows 0.002 A, while the exact value is about 1.9 mA; work in the milliamp range with care.