LED Resistor Calculator — Current, Power & Circuit Planner
Choose a standard resistor for small DC LEDs, check a resistor you have and see current, power and wiring for series strings or parallel branches.
SMALL DC LED CIRCUITS
Choose the resistor. See the current.
Pick a real resistor value, check its dissipation and see how to connect your LEDs.
Your circuit
Resistor choice
More LEDs, component tolerances & power margin
Without a voltage range, the model uses the nominal voltage inputs at every corner. Resistor tolerance still applies.
Model upper current 19.74 mA · within your 20 mA ceiling
One branch: one resistor in series with one LED.
- Nominal LED current
- 18.75 mA
- Model current range
- 17.86–19.74 mA
- Maximum resistor dissipation
- 59.21 mW per resistor
- Suggested rating at 2 × margin
- 125 mW per resistor
Constant forward-voltage model for small DC LEDs. These are estimates under your entered assumptions, not a guaranteed real current or temperature. Check the LED limits and resistor derating in the actual datasheets. No mains, high-power LED drivers, PWM or GPIO-drive design.
How the choice and current range are calculated
Each series branch has its own resistor. LEDs in a branch share the same current; parallel branch currents add. Do not share one resistor across bare parallel LEDs.
For n LEDs, supply Vₛ, assumed per-LED voltage Vf and current ceiling I in amperes, nominal ideal resistance is (Vₛ − nVf) / I. With voltage bounds and fractional resistor tolerance t, the design resistance is at least (Vₛ,max − nVf,min) / [I × (1 − t)]. We choose the next E12 or E24 value upward, then calculate current through that real value.
The model current range is max(0, Vₛ,min − nVf,max) / [R × (1 + t)] to (Vₛ,max − nVf,min) / [R × (1 − t)]. Resistor dissipation is voltage drop squared divided by resistance. The power-rating suggestion rounds the chosen margin times the maximum model dissipation up to a common rating from our shortlist. It is a planning suggestion, not a temperature check or stock guarantee.
An LED’s forward voltage changes with current and temperature. Enter measured or relevant datasheet bounds; the example values do not identify a certified LED part. The model does not solve a diode I–V curve, heating, supply impedance, battery discharge or driver regulation.
Scope: 0.1–48 V DC, one LED’s assumed forward voltage 0.1–12 V, current ceiling 0.05–100 mA, 1–50 LEDs per branch and 1–1,000 identical branches. Nominal supply must exceed the series LED voltage. Voltage ranges must contain nominal inputs. A zero lower current means conduction is not predicted at that corner.
Sources: DigiKey’s LED resistor example, DigiKey’s power-rating guidance, Vishay E-series values and Broadcom’s discussion of drive-circuit tolerances. Corner calculations above are derived independently.
