Type any two of voltage, current, resistance and power to get the other two, with unit prefixes and the Ohm's law wheel.
Type any two of voltage, current, resistance and power, and the calculator gives the other two using V = I × R and P = V × I. The page opens with 12 V across 470 Ω: the current is 12 ÷ 470 = 25.53 mA and the power is 12 × 25.53 mA = 306.4 mW. Clear one box and fill another to ask a different question. If you know the power and the resistance, the voltage is √(P × R) and the current is √(P ÷ R). If you know the current and the power, the resistance is P ÷ I². Each answer keeps four significant digits and picks the prefix that puts the digits between 1 and 999, so 0.02553 A is shown as 25.53 mA. The lines under the result repeat the formula with your numbers.
Each box takes a plain number or a number with a prefix and an optional unit. The prefixes are p (pico), n (nano), u or µ (micro), m (milli), k (kilo), M (mega) and G (giga). Capital M is mega and small m is milli, so 5M is 5,000,000 and 5m is 0.005. These all work: 250 mA, 4.7k, 3.3 kΩ, 1.5 MΩ, 20m, 0.5 W. A unit that does not belong to the box, such as volts in the current box, is refused instead of guessed. Each box accepts values from 1 nano to 1 giga of its unit. Results outside 1 pico to 1 peta are not shown, and the page asks you to check the prefixes. Decimal commas such as 4,7k are read as 4.7k.
An LED that should run at 20 mA from a 9 V battery drops about 2 V, so the resistor sees 7 V. Enter 7 V and 20 mA: the resistance is 7 ÷ 0.02 = 350 Ω and the resistor dissipates 140 mW, so a quarter-watt part is enough. A 60 W lamp on 120 V: enter 120 V and 60 W to get a current of 0.5 A and a resistance of 240 Ω. A 5 V supply across 20 mA: 5 ÷ 0.02 gives 250 Ω and 100 mW. Current and resistance work the other way: 2 mA through 4.7k is 9.4 V and 18.8 mW. The wheel in the result lists the three formulas for each quantity, and the letters you were asked for are marked.
Ohm's law says that for a part whose resistance stays constant, the voltage across it is proportional to the current through it. That holds well for resistors at a steady temperature. It does not hold for diodes, LEDs, filament lamps that heat up, or motors, whose resistance changes with current, so for those the calculator gives the value at the one operating point you typed. The page uses DC values or RMS values of a resistive AC load. It ignores power factor, reactance, wire resistance and the tolerance of real parts. For AC loads with a power factor, or for three-phase supplies, use the watts to amps calculator. The result is an estimate for learning and checking, not a basis for choosing fuses, cables or breakers.
Divide voltage by current: R = V ÷ I. Enter 5 V and 20 mA and the calculator returns 250 Ω, plus 100 mW of power. Type the current as 20m, 0.02 or 20 mA.
P = I² × R or P = V² ÷ R. A 12 V supply across 470 Ω gives 12² ÷ 470 = 0.3064 W, shown as 306.4 mW. With current and resistance known, 25.53 mA through 470 Ω gives the same power.
Put the prefix after the number: 250m or 250 mA for milliamps, 4.7k or 4.7 kΩ for kilohms. Capital M is mega and small m is milli, so 2M is 2,000,000 and 2m is 0.002.
Two known quantities fix the other two. With one box filled there is not enough information, and with three or four filled the numbers could disagree, for example 12 V, 470 Ω and 1 A. Clear boxes until two remain.
Compare the calculated power with the part's rating and leave headroom. A resistor that dissipates 140 mW fits a quarter-watt (250 mW) part. The page gives an estimate only; check the datasheet for the real limits.
No. The numbers are processed in your browser and not uploaded, and they are not saved on your device. Reloading puts the 12 V and 470 Ω example back. The page only counts that a result was shown, without your numbers.