Calculator KB-03
LED resistor calculator
Choose a current-limiting resistor for a single LED or a series string.
Result
In brief: Calculate a current-limiting resistor for an LED: resistance, nearest E12 and E24 standard values, actual current and power dissipation. Example: E24 series resistor: 470 Ω.
Calculation example
| Supply voltage | 12 V |
|---|---|
| LED | Blue / white, ~3.2 V |
| LED forward voltage | 3.2 V |
| LED current | 20 mA |
| LEDs in series | 1 pcs |
| E24 series resistor | 470 Ω |
|---|---|
| Exact calculation | 440 Ω |
| E12 series | 470 Ω |
| Actual current with an E24 resistor | 18.7 mA |
| Power dissipated by resistor | 164.8 mW |
| Minimum resistor power rating | 0.5 W |
| Power dissipated by LEDs | 59.9 mW |
How it is calculated
P = I² · R
The resistor drops the difference between the supply voltage and the total forward voltage of the LEDs. The resistance is rounded up, so the current will be slightly lower, not higher, than calculated. The resistor's power rating includes a 2× safety margin so it won't run at its limit.
What to look for
- The LED forward voltage depends on the current and the individual LED; the values in the list are approximate. If you have a datasheet, enter the value from it.
- Don't connect LEDs in parallel to a single resistor: variations in forward voltage will cause the current to be unevenly distributed. Each string needs its own resistor.
- The less voltage headroom across the resistor, the more the current depends on the supply voltage. For high-power LEDs, a constant-current driver is better: otherwise, the resistor will just waste power as heat.
Frequently asked questions
What resistor do I need for an LED on 12 V?
For a white or blue LED at 20 mA: (12 − 3.2) / 0.02 = 440 Ω; the next higher standard value is 470 Ω. For a red LED with a 2 V drop, you need 500 Ω; the nearest E24 value is 510 Ω.
What power rating should the resistor have?
With a 470 Ω resistor, the current will be 18.7 mA, and it will dissipate about 0.17 W. Choose a resistor with twice the required power rating, so a 0.5 W resistor will do.
Can I connect several LEDs to one resistor?
You can connect them in series if there’s enough voltage: three white LEDs at 12 V have a total voltage drop of 9.6 V. Don’t connect them in parallel to a single resistor—the current won’t be shared evenly.
