LED Resistor: formula
Enter supply voltage, forward voltage at the intended operating current, and target current from the specific LED’s datasheet. Color alone does not specify its electrical limits. The resistor limits the current: it equals the voltage left after the LED, divided by the current.
The tool also computes the power the resistor dissipates and rounds to the next standard E12 value.
LED Resistor: worked example
| Supply voltage (V) | 12 |
|---|---|
| LED forward voltage (V) | 2 |
| LED current (mA) | 20 |
| Resistor needed (ohms) | 500 |
| Resistor power (watts) | 0.2 |
| Next standard E12 value (ohms) | 560 |
Forward voltage varies between LEDs and with current and temperature, so real current differs slightly. Batteries drop in voltage as they discharge. Constant-current drivers are better for high-power LEDs.
Reverse polarity can damage an LED. The E12 series has a limited set of values, so pick the next higher one.
LED Resistor: changing Supply voltage (V)
| Supply voltage (V) | Resistor needed (ohms) | Resistor power (watts) | Next standard E12 value (ohms) |
|---|---|---|---|
| 10 | 400 | 0.16 | 470 |
| 11 | 450 | 0.18 | 470 |
| 12 (example) | 500 | 0.2 | 560 |
| 13 | 550 | 0.22 | 560 |
| 14 | 600 | 0.24 | 680 |
LED Resistor: compare input ranges
| Input | Tested from | To | Resistor needed (ohms): low input | Resistor needed (ohms): high input | Swing |
|---|---|---|---|---|---|
| Supply voltage (V) | 11 | 13 | 450 | 550 | 100 (20%) |
| LED current (mA) | 18 | 22 | 555.6 | 454.5 | 101 (20%) |
| LED forward voltage (V) | 1.8 | 2.2 | 510 | 490 | 20 (4.0%) |
LED Resistor: individual input comparisons
| Input | Value | Resistor needed (ohms) | Resistor power (watts) | Next standard E12 value (ohms) |
|---|---|---|---|---|
| Supply voltage (V) | 10 | 400 | 0.16 | 470 |
| 11 | 450 | 0.18 | 470 | |
| 12 | 500 | 0.2 | 560 | |
| 13 | 550 | 0.22 | 560 | |
| 14 | 600 | 0.24 | 680 | |
| LED forward voltage (V) | 1.6 | 520 | 0.208 | 560 |
| 1.8 | 510 | 0.204 | 560 | |
| 2 | 500 | 0.2 | 560 | |
| 2.2 | 490 | 0.196 | 560 | |
| 2.4 | 480 | 0.192 | 560 | |
| LED current (mA) | 16 | 625 | 0.16 | 680 |
| 18 | 555.6 | 0.18 | 560 | |
| 20 | 500 | 0.2 | 560 | |
| 22 | 454.5 | 0.22 | 470 | |
| 24 | 416.7 | 0.24 | 470 |
LED Resistor: combined input changes
| Supply voltage (V) / LED forward voltage (V) | 1.6 | 2 | 2.4 |
|---|---|---|---|
| 10 | 420 | 400 | 380 |
| 11 | 470 | 450 | 430 |
| 12 | 520 | 500 | 480 |
| 13 | 570 | 550 | 530 |
| 14 | 620 | 600 | 580 |
LED Resistor: which assumptions move the result?
LED Resistor: values and definitions
| Quantity and definition | Value used |
|---|---|
| Supply voltage (V) | 12 |
| LED forward voltage (V) — Red is about 2 V and white or blue about 3 V. Check the LED's datasheet. | 2 |
| LED current (mA) | 20 |
| Resistor needed (ohms) | 500 |
| Resistor power (watts) | 0.2 |
| Next standard E12 value (ohms) | 560 |
LED Resistor: half and double Supply voltage (V)
| Supply voltage (V) | Resistor needed (ohms) |
|---|---|
| 6 | 200 |
| 12 | 500 |
| 24 | 1,100 |
Common error: Rounding down to the nearest standard value. That pushes more current through the LED than planned and shortens its life.
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An LED has almost no resistance of its own once it turns on, so a resistor must set its current. Subtract the LED's forward voltage from the supply and divide by the current you want. On 12 volts with a 2 volt LED at 20 milliamps that is 500 ohms.
Resistors come in standard values, so you round up to the next one, 560 ohms, which runs the LED a little below its full current.
Look up the forward voltage on the LED's datasheet. Colors differ, and the wrong value shifts the current you get.
See resistance combinations on the resistor calculator, power on the Ohm's law page, a divider on the voltage divider calculator and color bands on the resistor color code page. The watts to amps and voltage drop pages cover wiring, and the RC time constant calculator handles timing circuits.
Why does an LED need a resistor?
An LED has very low resistance once it conducts, so without a limiting resistor the current rises until the LED is destroyed.
What resistor value should I use?
Subtract the LED's forward voltage from the supply voltage and divide by the desired current, then round up to a standard value.
What is the E12 series?
A set of 12 preferred resistor values per decade, such as 100, 120, 150, 180, 220, 270, 330, 390, 470, 560, 680 and 820 ohms.