LED Resistor Calculator – Series Resistor Value & Wattage
An LED needs something to limit its current. The simplest choice is a series resistor that drops the difference between the supply voltage and the LED forward voltage at your target current.
V
= 5 VV
= 2 VRed ≈ 2.0 V, white/blue ≈ 3.1 VmA
= 20 mA20, 20mA and 0.02A all mean 20 mAResults
- Exact resistor
- 150 Ω
- Standard E24 (rounded up)
- 150 Ω
- Actual LED current
- 20 mA
- Resistor power
- 60 mW
- Suggested rating (2× margin)
- 0.125 W
- Efficiency (LED / supply)
- 40 %
Pro
PDF export is an Ohmlark Pro feature.
R = 150 Ω (E24) · 60 mW
Formula
- R = (Vs − n·Vf) / I
- P = I² · R
FAQ
- What forward voltage should I use?
- Check the datasheet. Typical values: red ≈ 2.0 V, green ≈ 2.1–3.2 V, blue/white ≈ 3.0–3.3 V.
- Why round up to the next standard value?
- Rounding up keeps the current at or below your target, which is the safe direction for the LED.
- Can I put LEDs in parallel with one resistor?
- It is not recommended: forward voltages vary, so one LED can hog the current. Use one resistor per LED or per series string.