CALCULATOR · INSTANT · FREE
LED Resistor CalculatorLED RESISTOR
Enter supply voltage, LED forward voltage, current and the number of LEDs in series to get the current-limiting resistor, the nearest E12 / E24 standard value, actual current, resistor power and recommended wattage. Free online.
- SeriesE12 / E24
- FormulaOhm's law
- Power margin2×
- PriceFree
RESULT
Live
Calculated resistanceR
150.0Ω
Use a 150Ω resistor rated 1/8 W or more.
Details
- Recommended resistor (E24, next higher)E24
- 150Ω
- Recommended resistor (E12, next higher)E12
- 150Ω
- Actual current with E24 resistorI
- 20.00mA
- Resistor power dissipationP
- 60.0mW
- Recommended resistor ratingPrated
- 1/8 W
- Voltage across resistorVR
- 3.00V
- Total LED powerPLED
- 40.0mW
- Efficiency (LED power / total)η
- 40.0%
HOW-TO
How to use the LED Resistor Calculator
- 1Enter the supply voltage.
- 2Enter the forward voltage Vf for your LED colour or from the datasheet.
- 3Enter the desired LED current (5–20 mA is bright enough for indicators).
- 4Enter how many LEDs are in series.
- 5Fit the recommended E24 (or E12) resistor with the suggested power rating, and check the actual current.
Worked example (default inputs)
| Item | Value |
|---|---|
| Supply voltage Vs | 5 V |
| LED forward voltage Vf | 2 V |
| LED current | 20 mA |
| LEDs in series | 1 |
| Calculated resistance(R) | 150.0 Ω |
| Recommended resistor (E24, next higher)(E24) | 150 Ω |
| Recommended resistor (E12, next higher)(E12) | 150 Ω |
| Actual current with E24 resistor(I) | 20.00 mA |
| Resistor power dissipation(P) | 60.0 mW |
| Recommended resistor rating(Prated) | 1/8 W |
| Voltage across resistor(VR) | 3.00 V |
| Total LED power(PLED) | 40.0 mW |
| Efficiency (LED power / total)(η) | 40.0 % |
How the LED resistor is calculated
The resistor sees the supply voltage minus the forward voltage of every LED in the string: VR = Vs − n·Vf. Ohm's law gives the required resistance R = VR / I. The result is rarely a stock value, so the calculator finds the next E24 and E12 value at or above it, recomputes the actual current and dissipation with that resistor, and recommends a 1/8, 1/4, 1/2 W… rating with a 2× margin.
Efficiency and design tips
All power in the resistor becomes heat, so efficiency is η = n·Vf / Vs. The closer the supply is to the total forward voltage, the higher the efficiency — but the more sensitive the current is to voltage changes; keep at least 0.5–1 V across the resistor. For LED strips or LEDs of 1 W and more, use a constant-current driver IC. Need an enclosure or LED holder? Design one with the box generator here and send it to Orinovate for 3D printing.
Formulas
VR = Vs − n · Vf R = VR / I R → next E24 / E12 value ≥ R I = VR / Rstd P = I² · Rstd ; rating ≥ 2 · P η = n · Vf / Vs
Results are for design reference only; confirm against supplier catalogues and the applicable standards before manufacturing.
FAQ
Why does an LED need a resistor?
An LED is a diode: just above its forward voltage the current rises steeply, and Vf drops as it heats up, which can cause thermal runaway. A series resistor makes the current roughly (Vs − Vf) / R — the simplest current limiter. For high-power LEDs use a constant-current driver.
Should I wire several LEDs in series or parallel?
In series every LED carries the same current and has the same brightness, and one resistor is enough — but the total Vf must stay below the supply. In parallel each string should have its own resistor; never share one resistor between LEDs in parallel, because Vf variation makes one LED hog the current.
What are E12 and E24?
The E series (IEC 60063) are standard preferred resistor values. E12 has 12 values per decade (10, 12, 15, 18, 22…), typical for ±10% parts; E24 has 24 per decade, typical for ±5% parts. This tool picks the next standard value at or above the calculated one, so the actual current never exceeds your target.
How do I choose the resistor wattage?
Compute the actual dissipation P = I²R and choose a rating at least twice that. 60 mW is fine on a 1/8 W resistor, but 200 mW calls for 1/2 W. Resistors near their rating run very hot; margin extends life and keeps nearby parts cool.
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Standards
- IEC 60063 E12 / E24
- Ohm's law