LED Resistor Calculator Widget

Add an LED resistor calculator to your website. Hobbyists enter the supply voltage, LED forward voltage, current and number of LEDs in series, and get the next standard resistor value up, the current it really gives and the wattage to buy.

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<iframe src="https://a2z.tools/embed/w/led-resistor-calculator" title="LED Resistor Calculator by A2Z Tools" width="100%" height="740" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>

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How it works

The widget uses the LED engine of the A2Z LED Series Resistor Calculator. The resistor drops the voltage left over after the LEDs, so R = (supply - n x Vf) / I. Because resistors come in preferred values, it rounds UP to the next value in the IEC 60063 E12 or E24 series - so the LED current can only come out at or below the target - and then recalculates the actual current and the power in the resistor. The suggested rating is the next common wattage at or above twice the dissipation, a design margin rather than a rule from any standard, which keeps the part well below its maximum temperature.

Calculation method

  • R = (Vsupply - n x Vf) / I
  • Standard value = smallest E12 or E24 (IEC 60063) value >= R
  • Actual I = (Vsupply - n x Vf) / Rstandard; resistor power = I^2 x R; efficiency = n x Vf / Vsupply
  • Suggested rating = next of 1/16, 1/10, 1/8, 1/4, 1/2, 1, 2, 3, 5, 10 W at or above 2 x the dissipation

Worked examples

Three LEDs on 12 V

Inputs: 12 V supply, 3 LEDs at 2.0 V, 20 mA, E12

Result: 330 ohm (E12, next up); actual current 18.18 mA; 109.1 mW in the resistor; buy at least 1/4 W; efficiency 50%

(12 - 6) / 0.02 = 300 ohm, next E12 value 330; 6 / 330 = 18.18 mA; 6^2 / 330 = 0.109 W, doubled 0.218 W -> 1/4 W.

Limitations

  • Treats the LED forward voltage as fixed; in reality it changes with current and temperature, so the real current differs slightly.
  • Does not check the LED's maximum current or the supply's tolerance; a supply that runs high raises the current.
  • Series strings only - no parallel strings or constant-current driver design.

Where publishers use it

  • Beginner electronics and Arduino tutorials wiring the first LED
  • LED strip and signage suppliers explaining current-limiting resistors
  • Model railway and cosplay lighting blogs
  • School design-and-technology project pages

Questions

What forward voltage should I use?

The datasheet value at your chosen current. It depends on the LED's colour and chemistry - red and yellow parts are usually lower than blue and white ones - and it varies between batches, so the datasheet figure beats any rule of thumb.

Why round up instead of to the nearest value?

A larger resistor gives slightly less current, which is safe for the LED. Rounding down could push the current above the target.

Why is the efficiency so low?

Everything the resistor drops is turned into heat. Putting more LEDs in series, or using a constant-current driver, wastes less.

Can I put LEDs in parallel with one resistor?

It is not recommended: LEDs' forward voltages differ slightly, so one LED can take most of the current. Give each string its own resistor.

Sources

  1. IEC 60063:2015 Preferred number series for resistors and capacitors - International Electrotechnical Commission . E12 and E24 values

Cite or recommend this tool

If you reference this tool in an article, course or documentation, these formats are ready to copy. They are optional - nothing is added to your site unless you paste it.

A2Z Tools LED Resistor Calculator
https://a2z.tools/led-series-resistor-calculator

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