Size the perfect current-limiting resistor for any LED. Enter your supply voltage, the LED’s forward voltage and target current, and get the exact resistance, nearest E12 value, color bands, power and wattage with a circuit diagram that redraws as you type.
Circuit values
All fields are required.
V
V
mA
Live circuit diagram
Redraws automatically from your inputs.
Supply
Resistor
LED
Ground
Results
Recommended resistor and full breakdown.
Enter your values to see the recommended resistor — results update as you type.
Formula & step-by-step
Session history
| Vs | Vf×n | I | E12 | Watt |
|---|---|---|---|---|
| No calculations yet this session. | ||||
Formula & worked example
The math the calculator runs, explained step by step.
Resistance
R = (VS − (Vf × N)) ÷ I
Power
P = I² × R
What each variable means
| Symbol | Meaning | Unit |
|---|---|---|
| VS | Supply voltage from your power source | Volts (V) |
| Vf | Forward voltage of a single LED (from its datasheet) | Volts (V) |
| N | Number of LEDs wired in series | count |
| I | Target LED current (converted from mA to A in the math) | Amps (A) |
| R | Required series resistance | Ohms (Ω) |
| P | Power the resistor dissipates as heat | Watts (W) |
Worked example
Goal: drive one red LED (Vf = 2 V, 20 mA) from a 12 V supply.
- Total LED voltage = 2 V × 1 = 2 V
- Voltage across resistor = 12 − 2 = 10 V
- Current in amps = 20 mA ÷ 1000 = 0.02 A
- R = 10 ÷ 0.02 = 500 Ω
- Nearest E12 value (rounded up) = 560 Ω
- P = 0.02² × 560 = 0.224 W → with 2× margin, use a 1/2 W resistor
Standard LED forward voltages
Typical datasheet values. Click a row to load it into the calculator.
| LED color | Typical forward voltage | Typical current |
|---|---|---|
| Red | 1.8–2.2 V | 20 mA |
| Yellow | 2.0–2.2 V | 20 mA |
| Green | 2.0–3.2 V | 20 mA |
| Blue | 3.0–3.4 V | 20 mA |
| White | 3.0–3.4 V | 20 mA |
| Infrared | 1.2–1.5 V | 20 mA |
Standard resistor wattages
Pick a rating above your calculated dissipation, with a 2× safety margin.
| Power dissipation | Recommended resistor |
|---|---|
| ≤ 0.125 W | 1/4 W |
| ≤ 0.25 W | 1/2 W |
| ≤ 0.5 W | 1 W |
| ≤ 1 W | 2 W |
| > 1 W | Next higher standard rating |
Common supply voltages
Click a row to load it into the calculator.
| Device | Voltage |
|---|---|
| USB | 5 V |
| Arduino | 5 V |
| 9V Battery | 9 V |
| 12V Adapter | 12 V |
| 24V Supply | 24 V |