LED Series Resistor Calculator

Frequently Asked Questions

How do I size an LED series resistor?

R = (V_supply − n × V_forward) ÷ I, where n is the number of LEDs in series and I is the desired current. Example: (12 − 2×2.0) ÷ 0.02 = 400 Ω; pick the next standard value.

How is this different from a resistor color-code calculator?

A color-code tool decodes the bands of an existing resistor. This computes the resistance you need to safely drive an LED, plus the power the resistor must dissipate.

What resistor power rating do I need?

P = I² × R. A typical 20 mA LED resistor dissipates well under ¼ W, but always pick a rating at least 2× the calculated dissipation for margin.

Can I put LEDs in series or parallel on one resistor?

Series LEDs share one resistor (sum their forward voltages). Parallel LEDs should each have their own resistor - shared resistors cause unequal current and current hogging.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

An LED is a current-driven device with a roughly fixed forward voltage drop; connecting it straight across a supply destroys it because nothing limits the current. A series resistor sets the operating current. Kirchhoff's voltage law gives the resistor value: R = (V_S − n × V_F) ÷ I, where V_S is the supply voltage, n is the number of LEDs wired in series, V_F is each LED's forward voltage, and I is the desired LED current. The resistor must drop the leftover voltage at that current, dissipating power P = I²R. This calculator also snaps the result to the nearest standard E12 value and recommends a resistor wattage with safety margin. Note this is not a resistor color-code decoder - it sizes the current-limiting resistor itself.

Worked Numeric Example

Drive one red LED (V_F = 2.0 V) at 20 mA from a 12 V supply. Voltage the resistor must drop = 12 − (1 × 2.0) = 10 V. R = 10 V ÷ 0.020 A = 500 Ω. The nearest E12 value is 470 Ω. With 470 Ω the actual current becomes 10 ÷ 470 = 21.3 mA - safely close to target. Resistor power = I²R = 0.0213² × 470 = 0.213 W, so a ½ W resistor (more than 2× the dissipation) is appropriate; a ¼ W part would run hot. For three of these LEDs in series you would need 3 × 2.0 = 6 V of drop, leaving only 6 V for the resistor.

Real-World Use

This is the single most common hobby and product electronics calculation: indicator LEDs on PCBs, panel pilot lights, Arduino and Raspberry Pi status LEDs, LED strips wired from a regulated supply, and automotive 12 V LED replacements all need a correctly sized series resistor. Designers also use it to trade brightness against power and to confirm the chosen resistor's wattage will not overheat in an enclosed housing.

Common Mistakes

Entering current in amps instead of milliamps is the top error - 20 mA is 0.020 A, not 20 A. Forgetting to multiply V_F by the number of series LEDs produces far too small a resistor and burns out the string. Using a typical V_F of 2 V for a blue or white LED is wrong - those drop 3.0–3.4 V. Ignoring resistor power rating is a fire risk: always pick a wattage at least twice the calculated dissipation. Finally, putting LEDs in parallel behind one resistor is unreliable because slight V_F differences cause uneven current; give each branch its own resistor.

Limitations & Related Tools

This calculator assumes a stiff, regulated DC supply and a constant forward voltage; in reality V_F varies with current and temperature, so consult the LED datasheet I-V curve for precision designs. For battery operation, V_S sags as the cell discharges, shifting the current. High-power LEDs (1 W and up) should use a constant-current driver rather than a series resistor, which wastes energy as heat. Use the E12 result as a starting point and verify with an inline current measurement. Related tools: resistor color-code decoder (to read an existing resistor), Ohm's law calculator, and a battery-life estimator for portable builds.

LED Series Resistor Calculator

Frequently Asked Questions

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