‹ Tools · LED resistor
LED series resistor calculator
The exact value, the standard value you can actually buy (rounded up, so the current lands under target), the current you will really get and a resistor power rating.
Resistor (rounded up) — Actual current — — —
Brightness — — —
About brightness
- Datasheet mcd is on-axis intensity. A narrow-beam LED (15°) reads thousands of mcd from the same die that gives tens of mcd at 120°: the total light is similar, it is just concentrated.
- Green and yellow look brighter than red or blue for the same optical power: the eye is most sensitive near 555 nm. mcd already includes that weighting, so compare parts by mcd, not by radiant mW or by current.
- The eye is logarithmic-ish: half the current looks about 80 % as bright. Indicators rarely need the full test current; 1–5 mA is normal for a status LED, and it saves power and glare.
- The lumen figure assumes a uniform beam inside the viewing angle. Real beams taper, so treat it as a rough estimate for comparing parts, not a photometric result.
The formula
R = (Vs − n · Vf) / I P = I² · R
When to use it
Indicator LEDs from a logic supply, a string of LEDs from 12 or 24 V, a backlight from a fixed rail. The forward voltage depends on colour and current — about 1.8 to 2.2 V for red, 3.0 to 3.4 V for blue and white — so take it from the datasheet at the current you intend to run.
Pitfalls
- Rounding up keeps the LED under its rating. If the actual current drops well below what you want, take the standard value below and check the LED’s maximum instead.
- When Vs − n·Vf is small (a white LED from 3.3 V), the current swings widely with Vf tolerance and supply changes. Leave at least 1 V across the resistor, or use a current regulator.
- A string of LEDs in series shares one current but needs the supply to exceed the sum of the forward voltages, with headroom.
- Forward voltage falls as the LED warms, so the current creeps up: another reason for headroom across the resistor.
Related note
PCB layout mistakes that only show up at assemblyNeed the whole board designed, not just the LED resistor?