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 |
| Result | 12 V, one 2 V LED, and 20 mA requires 500 Ω and dissipates 0.2 W before power headroom. |
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
Is this LED resistor result code compliant?
No. PowerTotals performs transparent electrical math and planning estimates. Installation decisions must be checked against equipment documentation, the adopted code, and the authority having jurisdiction.
Are my inputs uploaded?
No. Calculations run locally in your browser, and analytics never receives raw electrical values.
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
Read the calculation methodology, review the technical sources, or browse the reference table index. Last reviewed .