LED Resistor Calculator
Calculate the series resistor needed to drive an LED at its rated current.
About LED Resistor Calculator
The resistor drops the difference between supply and the total LED forward voltage. Adequate headroom matters because LED forward voltage varies with temperature — with too little, current changes sharply. Driving LEDs with a resistor is only acceptable at low power; above about a watt a constant-current driver is needed for efficiency and stability.
Formula
LED Resistor
R = (V_supply − n V_f) ÷ I_LED
- V_f
- LED forward voltage drop, roughly constant once conducting
- headroom
- Voltage across the resistor, which stabilises the current
Worked example
With the default values below, LED Resistor Calculator returns Required Resistor of 495.00. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Supply voltageminimum 0 V
- 12 V
- LED forward voltageminimum 0 V
- 2.1 V
- Desired LED currentminimum 0.1 mA
- 20 mA
- LEDs in seriesaccepts 1–20
- 1
Results
- Required Resistor
- 495.00 ohm
- Nearest Standard Value
- 500 ohm
- Power Dissipated in the Resistor
- 0.19800 W
- Voltage Across the Resistor
- 9.900 V
- Verdict
- Workable design
Step by step
- 1Values used
- 2LED Resistor
- 3Required Resistor= 495.00 ohm
- 4Nearest Standard Value= 500 ohm
- 5Power Dissipated in the Resistor= 0.19800 W
- 6Voltage Across the Resistor= 9.900 V
- 7Verdict= Workable design
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is LED Resistor calculated?
R = (V_supply − n V_f) ÷ I_LED The resistor drops the difference between supply and the total LED forward voltage. Adequate headroom matters because LED forward voltage varies with temperature — with too little, current changes sharply.
Why does LED Resistor matter?
Driving LEDs with a resistor is only acceptable at low power; above about a watt a constant-current driver is needed for efficiency and stability.
What values do I need to enter?
This calculator takes 4 inputs: Supply voltage, LED forward voltage, Desired LED current, LEDs in series. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.