LED Resistor Calculator: Series, Parallel and Power Rating Worked Examples
Quick answer: one formula covers every case — R = (Vs - n×Vf) / If, where n is the number of series LEDs. Round up to the next standard value, then check the resistor's power rating.
Single LED
5 V supply, white LED 3.2 V, 15 mA: R = (5-3.2)/0.015 = 120 Ω exactly. Power = 0.015² × 120 = 27 mW — any 0.125 W part works.
Series String
12 V, three red LEDs (2.0 V each), 20 mA: R = (12 - 3×2.0)/0.02 = 300 Ω. Power = 0.12 W → use 0.25 W. Rule of thumb: keep total Vf below ~80% of the supply so the resistor can absorb variation.
Parallel Strings
Never parallel bare LEDs on one resistor — Vf spread unbalances current. One resistor per series string, then parallel the strings.
Rounding and Tolerance
- Round R up to the next E12/E24 value (slightly dimmer is safe; lower R over-drives).
- Budget worst case: Vs max, Vf min → current must stay below If max.
- For precision brightness, constant-current drive beats resistors.
Power Check
P = I²R. Under 100 mW → 0.25 W; under 250 mW → 0.5 W; above that, use a driver or a higher-wattage part with margin.
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