LED Resistor Calculator

LED Component

Calculate the series resistor value, power dissipation, and the nearest standard resistor (E12 / E24 / E96). Inputs start blank — use Load example for a quick demo set.

Offline Units: V · mA · Ω · W Strict mode: requires S and P

Inputs

Assumes a resistor per series string (recommended). For LEDs in parallel, build identical strings and give each string its own resistor.

V

Example: 5V USB, 12V rail, 3.0V coin cell.

mA

Typical indicator LEDs: 2–20mA (check datasheet). Total current = per-string × P

⚠ Outside typical range (2–20mA)—check datasheet

×

Required (strict mode). Number of LEDs in each series string.

×

Required (strict mode). Number of parallel strings.

V

Vf depends on colour, current, temperature, and LED type.

  • Custom
  • IR (≈1.3V)
  • Red (≈1.8V)
  • Red (≈2.0V)
  • Amber (≈2.1V)
  • Green (≈2.1V)
  • Blue (≈3.0V)
  • White (≈3.1V)
  • UV (≈3.2V)

Choosing a preset updates the Vf field and LED colour.

  • None (ideal only)
  • E12 (±10%)
  • E24 (±5%)
  • E96 (±1%)

Nearest standard value is computed only when a series is selected.

For high‑power LEDs, wide input variation, or efficiency‑sensitive designs, use a constant‑current LED driver rather than a resistor.

Results

Calculated once all required inputs are present. Always validate thermals and tolerances for production.

Resistor (ideal)
—
Ω
Nearest series value
—
Select E12/E24/E96
Headroom
—
V = Vs − N·Vf
Resistor power
—
W (total)
Suggested wattage
—
W (2× margin)
Total current
—
mA = I · P
Std wattage (SMD)
—
W (nearest standard)
Min SMD package
—
Typical guidance
Std wattage (TH)
—
W (nearest standard)
Offline

Diagram (SVG output)

Live schematic with P parallel strings of S series LEDs. Labels update with calculated values.

A battery in series with a resistor and P parallel strings of S series LEDs. LED Resistor Calculator S = 1 (single LED per branch) · P = 1 string + − Vs = — Itotal = — Vdrop = — Rideal = — Rsel = — PR = — S = — P = — Vf = — V per LED Parallel LED strings assume ideal matching. Real designs may require per-string balancing.Download SVG

Tip: choose resistor wattage with margin (often 2× or more) to keep temperatures reasonable.

How the calculation works

LEDs are current-driven devices. A small voltage change can cause a large current change, so a current limiter is needed. The simplest limiter is a series resistor.

Core formulas

Vdrop = Vs − (N · Vf)
R = Vdrop / I
PR = Vdrop · I
Itotal = I · P

Why it's required

  • Protects the LED: limits current to a safe value, preventing damage.
  • Improves consistency: Vf varies part-to-part and with temperature — current limiting stabilises brightness.
  • Avoids thermal runaway: as LEDs warm, Vf tends to drop, which can increase current without a limiter.

Practical tip: choose a resistor power rating with margin (often 2× or more) to keep temperatures reasonable.