Electronics CalculatorsLED resistor calculator
Enter the supply voltage, the LED voltage and the current you want. You get the exact resistor and the standard values to buy.
The formula
An LED has an almost fixed voltage across it when it is on, its forward voltage (Vf). A resistor in series takes up the rest of the supply voltage, and Ohm's law gives its value: R = (Vs − Vf) / I. With several LEDs in series, subtract all their voltages: R = (Vs − n × Vf) / I.
Standard values
You cannot buy every value. The tool shows the next value above the result in the E12 and E24 series, so that the current is a little below the target. A value below the result would give more current than you asked for. The page on E-series values explains the series.
How it was tested
We checked 400 random cases: the resistor value, that the E12 and E24 values are the smallest standard values at or above the result (found by brute force), that the real current never exceeds the target, and that the power equals I² × R.
Find the forward voltage
The forward voltage depends on the LED, its colour and its current. Read it in the data sheet. Do not guess from colour alone if the LED is expensive or the current is high.
Can I connect LEDs in parallel with one resistor?
It is better to give each LED its own resistor. Small differences between LEDs make one take more current than the others.
Is the resistor power important?
Yes: it must be above I² × R. The result shows it. A 0.25 W resistor is fine for the usual small LED currents.
This is an independent guide. It is not connected with any standards body or company named here. The formulas are the usual textbook ones; the colour code, E-series and AWG values are those printed in common references, because the standards IEC 60062, IEC 60063 and ASTM B258 are paid documents that we did not read. Tables and examples were calculated with Python on 14 November 2026. Check data sheets and local safety rules before you build.