LED Series Resistor Calculator
Resistor value and power needed to run an LED safely from a given supply.
Results
Resistance
145.0
Ω
Power
58.0
mW
Power
0.04200
W
Efficiency
42.00
%
LEDs in series
2
What this tool does
An LED has almost no resistance of its own, so a series resistor is what actually sets the current. Subtract the LED's forward voltage from the supply, divide by the current you want, and round up to the next standard resistor value — a slightly larger resistor is always the safe direction.
Formula
R = (Us − Uf) ÷ I , P = (Us − Uf) × I
Variables
| Symbol | Meaning | Unit |
|---|---|---|
us | Voltage | V |
uf | Voltage | V |
i | Current | mA |
ns | LEDs in series | — |
R | Resistance | Ω |
P | Power | mW |
PL | Power | W |
EF | Efficiency | % |
NM | LEDs in series | — |
Worked example
- Voltage5 V
- Voltage2.1 V
- Current20 mA
- LEDs in series1
- Resistance145.0 Ω
- Power58.0 mW
- Power0.04200 W
- Efficiency42.00 %
- LEDs in series2
Limitations
- The calculation runs at full precision and only the display is rounded. If you copy an intermediate value and retype it, small differences can appear.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.