This LED resistor calculator helps you choose a current-limiting resistor for LEDs in series. Enter the supply voltage, LED forward voltage (Vf), number of LEDs, and target current to calculate the required resistance, a standard value, actual current, and resistor power.
Try the calculator below, then use the worked example and design notes to interpret the results. It assumes a constant-voltage DC supply and LEDs connected in series.
LED resistor calculator
Use the Vf from your LED’s datasheet at approximately the intended current. Current is entered in mA. The recommended standard resistor is selected at or above the calculated value, so it will not exceed the target current if Vf is treated as constant.
Inputs
Check the datasheet for your specific LED’s Vf.
P＝I²R
Results
+Vs → resistor → LED anode (A) → cathode (K) → 0 V. Simplified series-circuit diagram.
Design checks
Resistor options
The recommended value is at or above the required resistance. A smaller value may exceed the target current.
| Option | Resistance | LED current | Resistor power |
|---|
How to use the calculator
- Enter the supply voltage Vs. You can also use the 3.3 V, 5 V, or 12 V presets.
- Enter the forward voltage Vf of one LED, the number of series LEDs, and the target current If.
- Choose E12, E24, or E96 and review the recommended resistor and resulting current.
- Check resistor power and the suggested power rating against the component datasheet and temperature conditions.
LED series resistor formula
The voltage across the resistor is Vs − (N × Vf), where N is the number of series LEDs. Convert the target current to amperes, then calculate R = (Vs − N × Vf) / If. The resistor dissipates P = I²R. If Vs is less than or equal to N × Vf, this simple series-resistor circuit cannot deliver the target current.
Example: one 2 V LED at 15 mA from a 5 V supply
The resistor drops 5 V − 2 V = 3 V, so R = 3 V / 0.015 A = 200 Ω. An E24 200 Ω resistor gives an idealized current of 15 mA and dissipates 3 V × 0.015 A = 0.045 W. With a target margin of at least 2×, the suggested nominal rating is 0.1 W. Check the resistor’s temperature derating in its datasheet.
Choosing a standard resistor value
When the calculated value is not a standard nominal value, the tool recommends the next E-series value at or above it. A larger resistor reduces current. Actual current also depends on resistor tolerance, LED Vf, and supply voltage. Check the high-supply, low-Vf worst case to make sure the LED’s maximum rating is not exceeded.
Frequently asked questions
Can I use this calculator for LEDs in parallel?
No. This calculator is for LEDs in series. Parallel branches can have different LED characteristics, so assess current limiting for each branch. Do not enter the total number of parallel LEDs as the series LED count.
Is the suggested resistor power rating always safe?
No. The displayed rating is a candidate based on at least twice the calculated dissipation. Check the actual resistor’s rating, permitted temperature, mounting, cooling, and supply variation in its datasheet.
What Vf should I enter?
Use the datasheet for the specific LED, not its color alone. Vf varies with current, temperature, and part-to-part differences. Check the current separately under worst-case conditions.
Related: our voltage divider calculator illustrates the relationship between resistor values and voltage.
Disclaimer
This article and calculator are for learning and approximate estimates for DC circuits with constant LED Vf. They do not account for every LED or resistor tolerance, temperature effect, or supply variation. For product or safety-critical use, consult each component’s datasheet, measure the actual circuit, and apply suitable safety margins. The site accepts no liability for losses arising from use of this article or tool.
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