Volts, amps, ohms and watts from any two, by ohm's law OHM
Any two of voltage, current, resistance and power fix the other two, and the formula used for each is shown. Resistors in series and parallel, and the resistor an LED needs.
Any two of the four
Current in amps: 20 mA is 0.02. DC, or RMS values into a load that is only resistance, such as a heater element or a lamp filament. A motor or anything with a power factor is outside it.
Resistors in series and in parallel
The resistor for an LED
The forward voltage is on the LED’s data sheet: around 2 V for red, 3 to 3.4 V for white and blue. The page rounds up to the next preferred value (IEC 60063), which lets slightly less current through.
The working
How it works
OHM adds resistors in series and in parallel, and works out the series resistor for an LED, rounded up to the next preferred value with the current and power it will actually give. Everything stays in this tab.
Doing this often? OBSCURA Pro saves the result from here, along with the result of every other tool outside the Workspace, and includes the paid applications. Everything is US$5 a month or US$48 a year, 7 days free.