Calculator
LED resistor calculator
Estimate a series resistor for one LED using your supply voltage and LED assumptions.
Size the series resistor
One resistor for one LED on a DC supply. Forward voltage is an assumption at the intended current, not a constant guaranteed by this tool.
Worked example
For 5 V supply, assumed 2 V LED drop and 10 mA target: R = (5 − 2) V / 0.010 A = 300 Ω ideal. The next E12 nominal resistor at or above 300 Ω is 330 Ω. Estimated current = 3 V / 330 Ω ≈ 9.09 mA; resistor power = 3 V × 0.00909 A ≈ 0.02727 W (27.27 mW).
How the estimate works
Basis: Ohm’s law and the nominal E12 preferred-value series (12 values per decade). Supply must exceed the assumed forward voltage. Nominal values, LED forward-voltage variation, supply variation and resistor tolerance can change the actual current.
An LED always needs its current limited. Check the LED’s current rating and the resistor’s rated power with appropriate margin for your operating conditions; this calculation does not determine thermal limits or guarantee safety. These idealized calculations do not check every real-world limit. Work with safe low voltages and verify your design independently.
For the circuit assumptions, common mistakes and practical checks, read the worked guide.