LED Voltage & Resistor Calculator
Enter your power source and LED specs to calculate the resistor needed for safe operation.
How to use this LED voltage calculator
This tool helps you calculate a safe resistor value for an LED circuit. LEDs are current-driven components, so a resistor is often required to prevent too much current from flowing. To use this calculator:
- Enter your supply voltage (battery or power adapter output).
- Enter the forward voltage of one LED from the datasheet.
- Enter how many LEDs are wired in series.
- Enter desired LED current in milliamps (mA).
The calculator then gives resistor value, nearest standard resistor, estimated current with that resistor, and recommended wattage.
LED resistor formula
1) Total LED voltage: VLED,total = Vf × N
2) Resistor voltage: VR = Vsupply − VLED,total
3) Resistor value: R = VR ÷ I
4) Resistor power: PR = VR × I
Where I is current in amperes (A), so 20 mA = 0.02 A.
Worked example
Suppose you have a 12 V supply, three red LEDs (2.0 V each), and want 20 mA:
- VLED,total = 2.0 × 3 = 6.0 V
- VR = 12.0 − 6.0 = 6.0 V
- R = 6.0 / 0.02 = 300 Ω
- PR = 6.0 × 0.02 = 0.12 W
A practical choice is a 300 Ω or nearest standard value, with at least a 1/4 W resistor for thermal margin.
Typical LED forward voltage values
| LED Color / Type | Typical Forward Voltage (V) | Common Current |
|---|---|---|
| Red | 1.8 – 2.2 | 10–20 mA |
| Yellow / Amber | 2.0 – 2.2 | 10–20 mA |
| Green (standard) | 2.0 – 3.0 | 10–20 mA |
| Blue | 3.0 – 3.4 | 10–20 mA |
| White | 3.0 – 3.4 | 10–20 mA |
| High-power LED | Varies by model | 350 mA+ |
Series vs. parallel LED wiring
Series
- Same current through each LED.
- Forward voltages add together.
- Usually easier to control with one resistor.
Parallel
- Each branch should have its own resistor.
- Without separate resistors, one LED may hog current and fail early.
- Best for matching brightness across branches.
Tips for safer LED design
- Use datasheet values for forward voltage and maximum current.
- Pick resistor wattage above calculated dissipation (2× margin is a good rule).
- If supply voltage can vary, design for the highest expected voltage.
- For high-power LEDs, use a constant-current LED driver instead of only a resistor.
FAQ
Can I run an LED directly from a battery?
Usually no. Even if it lights briefly, current can spike and shorten LED life. A resistor or current driver is strongly recommended.
Why does my LED look dimmer than expected?
Common causes include too large a resistor, lower battery voltage under load, or LED forward voltage being higher than expected.
What if supply voltage is lower than total LED forward voltage?
Then the string will not reach target current. Reduce LEDs in series, increase supply voltage, or use a boost/current-driver solution.