What if LED forward voltage exceeds the supply?
There is no positive resistor solution; reduce the series LED count or use an appropriate driver/topology.
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LED Resistor Calculator — Series Resistance & Power
Calculate a current-limiting resistor for one or more series LEDs from supply voltage, forward voltage, target current and whole-number LED count.
Result
300 Ω
0.12 W resistor dissipation
Supply 12 V · 3 LED(s) × 2 V · 20 mA.
Formula and worked example
R = (Vs − n×Vf) ÷ I; resistor power = I²R.
A 12 V source with three 2 V LEDs at 20 mA needs (12−6)/0.02 = 300 Ω, dissipating about 0.12 W.
Assumptions
- LEDs are modeled in series with a stated forward-voltage drop.
- This is a DC steady-state estimate; drivers, PWM and thermal behavior require separate design checks.
Last updated: 2026-09-16
Frequently asked questions
Why calculate resistor power?
The resistor converts the remaining voltage drop into heat, so dissipation matters as well as resistance.