Technical utilities
LED Resistor Calculator
LED series resistance equals the voltage remaining after the LED string divided by target current. Resistor power equals current squared multiplied by resistance.
Calculate a series resistor and resistor dissipation for a simple string of LEDs.
Calculated result
LED series resistor
Calculating…
Calculated locally in your browser
Planning math only. Verify equipment specifications and installation requirements separately.
What this calculator returns
Choose a real resistor value and rating that meet component tolerances and thermal conditions.
Formula and variables
The calculation runs entirely in your browser. Static formulas, definitions, examples, and tables remain readable without JavaScript.
R = (Vsupply - count × Vf)/I; Presistor = I²R.
- V
- Voltage across the modeled component or network.
- I
- Current through the modeled component or network.
- R
- Resistance in ohms; subscripts identify individual resistors.
- P
- Power dissipated or delivered, in watts.
Vf is the user-entered forward voltage and I is target current in amperes.
Worked example
| Input | Value |
|---|---|
| Source voltage | 12 V |
| LED forward voltage | 2 V each |
| LEDs in series | 1 |
| Target current | 20 mA |
12 V, one 2 V LED, and 20 mA requires 500 Ω and dissipates 0.2 W before power headroom.
Reference table
| Solve for | Formula variants |
|---|---|
| Voltage | V = I × R; V = P / I; V = √(P × R) |
| Current | I = V / R; I = P / V; I = √(P / R) |
| Resistance | R = V / I; R = V² / P; R = P / I² |
| Power | P = V × I; P = I² × R; P = V² / R |
| Networks | Series: Req = ΣR; parallel: 1/Req = Σ(1/R) |
Frequently asked questions
Why subtract LED forward voltage from the supply?
The series resistor carries the remaining voltage after the LED forward drop, which sets the ideal resistor value for the target current.
How is resistor power calculated?
The tool uses the resistor voltage drop multiplied by LED current and then shows a planning wattage margin.
Assumptions and limitations
- Simple DC string only.
- LED forward voltage varies with current, temperature, and part.
- Parallel LED strings need separate current-control analysis.
Method and sources
Review the electrical formulas, calculation methodology, and reference tables for the references most relevant to this calculation. The broader technical sources index records source scope and verification. Last reviewed .