LED Resistor Calculator

Calculate the correct series resistor and wattage for an LED string. Includes unit support, power dissipation, and Show Work.

How to Use

  1. Enter your supply voltage (Vs).
  2. Enter LED forward voltage (Vf) and target current (If).
  3. Set how many LEDs are in series (N).
  4. Review resistor value, power, and recommended wattage.
Circuit Lab View
Visual feedback: LED string drop + resistor heat (power dissipation).
Vs
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Vf×N
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I
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R
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Risk: —
Supply Vs Resistor (R) LED(s) Vf × N Vf Drag: Vs (battery) or Vf (LED). Results update instantly. Resistor Wattage Visual heat scales with resistor power dissipation (Pr). Use Show Work for exact math.
Inputs & Settings
Compute a series resistor for a single LED string: R = (Vs − Vf×N) / I.
Examples: 3.3V, 5V, 12V, 24V
Typical: red ~2.0V, white/blue ~3.0–3.4V (varies by LED)
N = number of LEDs in a single series string
Common: 5–10mA indicator, 20mA standard LED, higher for power LEDs
Headroom adds safety margin (e.g., 100% = choose ≥2× calculated watts)
Optional: rounds to common preferred values (rounds up for safety)
Show Work (step-by-step)
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Work is shown in base units (V, A, Ω, W). The tool assumes a simple series resistor model (single string).

Reference

Core formula: R = (Vs − Vf×N) / I

  • Resistor voltage: Vr = Vs − Vf×N
  • Resistor power: Pr = I²×R (also Pr = Vr×I)
  • LED string voltage: Vled = Vf×N
Tip: If Vs ≤ Vf×N, a simple resistor can’t regulate current. Use fewer series LEDs, a higher supply, or a proper LED driver.

FAQ

Should I round the resistor up or down?

Round up to reduce current (safer for the LED). This tool’s rounding modes round up.

What wattage resistor should I use?

Choose a resistor rated above Pr. A common rule is 2× headroom (100% headroom).

Can I run LEDs in parallel with one resistor?

Not recommended for most LEDs due to current sharing issues. Prefer one resistor per string, or use a constant-current LED driver.

Tool Info

Last updated:

Updates may include UI improvements, preferred-value rounding tables, and edge-case handling.