LED Series Resistor Calculator
LED Series Resistor Calculator
The resistor an LED needs: exact value, the next standard value up, the current you actually get with it, the resistor’s power and wattage rating, and the circuit’s efficiency — for one LED or a string in series.
LED series resistor
A red LED (Vf 1.85 V) at 20 mA from 5 V, rounded up to E12
LED resistor formula
- Vs
- supply voltage
- n × Vf
- number of LEDs in series times each one’s forward voltage
- IF
- the LED current you want
- Rstd
- the next E12 or E24 value at or above R, so the current does not exceed IF
Worked example
A red LED (Vf 1.85 V) at 20 mA from 5 V, rounded up to E12
R = (5 − 1.85) ÷ 0.020 = 157.5 Ω
Next E12 value up: 180 Ω (E24 would give 160 Ω)
Actual current = 3.15 ÷ 180 = 17.5 mA
Resistor power = 0.0175² × 180 = 55.1 mW; twice that is 110 mW, so a ⅛ W (125 mW) part or larger
LED power = 1.85 × 0.0175 = 32.4 mW; efficiency = 1.85 ÷ 5 = 37.0%
Resistor for one LED at 20 mA (E12, rounded up)
| Supply | Red, 1.85 V | Green (GaP), 2.2 V | Blue or white, 3.3 V |
|---|---|---|---|
| 3.3 V | 82 Ω (25.6 mW) | 56 Ω (21.6 mW) | — |
| 5 V | 180 Ω (55.1 mW) | 150 Ω (52.3 mW) | 100 Ω (28.9 mW) |
| 9 V | 390 Ω (131 mW) | 390 Ω (119 mW) | 330 Ω (98.5 mW) |
| 12 V | 560 Ω (184 mW) | 560 Ω (172 mW) | 470 Ω (161 mW) |
| 24 V | 1.2 kΩ (409 mW) | 1.2 kΩ (396 mW) | 1.2 kΩ (357 mW) |
Typical forward voltages used on this page
| Colour | Kingbright part | Vf typical | Vf maximum | At |
|---|---|---|---|---|
| Red (GaAlAs) | WP7113SRD/D | 1.85 V | 2.5 V | 20 mA |
| Yellow (GaAsP) | WP7113YD | 1.95 V | 2.4 V | 10 mA |
| Orange (AlGaInP) | WP7113SED | 2 V | 2.5 V | 20 mA |
| Green (GaP) | WP7113SGD | 2.2 V | 2.5 V | 20 mA |
| Green (InGaN) | WP7113ZGC/G | 3.2 V | 4 V | 20 mA |
| Blue or white (InGaN) | WP7113QBC/D, WP7113QWC/D | 3.3 V | 4 V | 20 mA |
Choosing an LED resistor
An LED is not a resistor. Below its forward voltage it passes almost nothing; above it the current rises so steeply that a few tenths of a volt can destroy it. The series resistor fixes that: it takes whatever voltage is left over once the LED’s forward voltage is subtracted, and Ohm’s law sets the current. For a red LED at 20 mA from 5 V, the resistor sees 5 − 1.85 = 3.15 V and needs 157.5 Ω.
Rounding up. 157.5 Ω is not a value you can buy. The page picks the next standard value above it — 180 Ω in the E12 series — so the current falls slightly, to 17.5 mA, rather than rising above the target. E24 has closer steps: 160 Ω gives 19.69 mA. Rounding down would push the current over what you asked for.
Forward voltage varies. The colour list uses typical values from Kingbright’s 5 mm indicator LED datasheets, but the same datasheets give maxima 0.3 to 0.7 V higher, other makers differ, and Vf falls a little as the LED warms up. With the red LED at its 2.5 V maximum, the 180 Ω resistor passes 13.89 mA instead of 17.5 mA. That is why the resistor needs a healthy share of the supply: when only a volt or so is left for it, the current swings widely from part to part. Enter your own LED’s Vf to override the colour.
Series strings and efficiency. LEDs in series share one resistor and one current, so the string wastes less power. Three white LEDs from 12 V need 12 − 9.9 = 2.1 V across 120 Ω and run at 82.5% efficiency; one white LED from 12 V needs 470 Ω, burns 161 mW in it, and manages 27.5%. Never put LEDs in parallel on one resistor: the one with the lowest Vf takes most of the current.
Power rating. The page recommends a resistor rated at least twice its calculated dissipation, rounded to the common ratings (⅛, ¼, ½, 1, 2, 3 and 5 W), because ratings are specified at a set ambient temperature and a resistor at full rating runs hot. Above a watt or so, a switching constant-current driver wastes far less. For other resistor arithmetic, use the Ohm’s law calculator; to identify the resistor you have, the resistor colour code calculator.
Frequently asked questions
What resistor do I need for an LED on 5 V?
For a red LED at 20 mA, (5 − 1.85) ÷ 0.02 = 157.5 Ω, so 180 Ω (E12) or 160 Ω (E24). For a blue or white LED (3.3 V), (5 − 3.3) ÷ 0.02 = 85 Ω, so 100 Ω (E12).
Should I round the LED resistor up or down?
Up. A larger resistor gives slightly less current than you asked for, which is safe; a smaller one gives more.
What resistor for an LED on 12 V?
One white LED at 20 mA: (12 − 3.3) ÷ 0.02 = 435 Ω, so 470 Ω, dissipating 161 mW — use a ½ W resistor. Three white LEDs in series need only 120 Ω.
Why can’t I run an LED without a resistor?
Because its current is set by its forward voltage, which varies between parts and with temperature. On a fixed supply the current is effectively uncontrolled and can exceed the rating. Something has to limit it: a resistor or a constant-current driver.
What wattage resistor do I need for an LED?
At least twice the power it dissipates, I² × R. A 180 Ω resistor at 17.5 mA dissipates 55 mW, so an ⅛ W (125 mW) or ¼ W part is fine.
Related calculators
References
- Kingbright. Datasheets for WP7113SRD/D, WP7113SED, WP7113YD, WP7113SGD, WP7113ZGC/G, WP7113QBC/D and WP7113QWC/D, T-1¾ (5 mm) solid-state lamps: forward voltage (typical and maximum) and maximum DC forward current.
- Horowitz P, Hill W. The Art of Electronics, 3rd ed. Cambridge University Press, 2015. Chapter 1: voltage, current and resistance; Ohm’s law; power in resistors; voltage dividers and Thévenin equivalents.
- IEC 60063:2015. Preferred number series for resistors and capacitors (the E6, E12, E24, E48, E96 and E192 series). International Electrotechnical Commission.