LED Resistor Calculator
Series resistor R = (Vs - Vf) / I for one LED on DC. Informational, not a driver design or wiring approval.
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How to size an LED series resistor
- Enter the DC supply voltage Vs.
- Enter the LED forward voltage Vf from the datasheet.
- Enter the desired LED current in milliamps.
- Read ohms, resistor power, and the next E12 size at or above the exact value. Nothing is uploaded.
LED series resistor
R = (Vs - Vf) / I for one LED
| Vs | DC supply voltage |
|---|---|
| Vf | LED forward voltage at the chosen current |
| I | LED current. The form takes milliamps and divides by 1000. |
| Formula | R = (Vs - Vf) / I |
| E12 hint | Next standard size at or above the exact ohms |
| Approval | Informational. Not a driver design or wiring approval. |
What the LED resistor formula is
The LED drops Vf. The resistor drops the rest: Vs minus Vf. Current I is milliamps divided by 1000. R = (Vs - Vf) / I. The default 5 V, 2.0 V, and 20 mA gives 150 ohms. Resistor power is I squared times R.
Forward voltage, current, and E12
Vf depends on color and current. Red is often near 2 V. Blue and white sit higher. 20 mA is a common indicator current, not a rule. E12 is a 12-step preferred number series. The page suggests the next size at or above the exact ohms so you can pick a stock part.
How this page computes
Supply voltage must exceed Vf. Milliamps must be greater than 0. The result is one LED on DC with a single series resistor. It is not a constant-current driver and not a string of LEDs.
Limits
One LED, DC, no PWM model, no thermal derating. Informational, not a driver design or wiring approval. CZNull does not receive the values. See the disclaimer.