LED Resistor Calculator

Enter your supply voltage, LED forward voltage, LED forward current, and number of LEDs to find the correct series resistor value for your LED circuit. The LED Resistor Calculator computes the required resistance in ohms, the nearest standard resistor value, and the power dissipated — so you can pick a safe, properly rated component every time.

V

The voltage of your power source (battery or PSU).

V

Typical values: ~2.0V for red/yellow, ~3.2V for blue/white/green LEDs.

mA

Desired operating current. Most standard LEDs run at 20 mA.

Number of LEDs connected in series on a single string.

Results

Required Resistance

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Nearest Standard Resistor (E24)

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Power Dissipated by Resistor

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Recommended Resistor Rating (2× safety)

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Voltage Across Resistor

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Voltage Distribution

Frequently Asked Questions

What is a series resistor and why do LEDs need one?

A series resistor limits the current flowing through an LED. LEDs are current-driven devices — without a resistor, even a small increase in voltage causes a large surge in current that quickly destroys the LED. The resistor drops the excess voltage so the LED receives its rated current.

What formula does this calculator use?

The resistor value is calculated as R = (Vs − n × Vf) / If, where Vs is the supply voltage, Vf is the LED forward voltage, n is the number of series LEDs, and If is the forward current in amps. Power dissipated is then P = If² × R.

What forward voltage should I enter for my LED?

Forward voltage depends on the LED color and type. Common values are approximately 1.8–2.2 V for red, orange, and yellow LEDs; 2.0–2.4 V for green; and 3.0–3.5 V for blue, white, and UV LEDs. Check your LED's datasheet for the exact Vf at your desired current.

What power rating should I choose for the resistor?

Choose a resistor with a power rating at least 2× the calculated dissipation, and ideally up to 10× for cooler operation and longer life. Common standard ratings are 1/8 W, 1/4 W, 1/2 W, 1 W, and 2 W.

What is an E24 standard resistor value?

The E24 series is a standard set of 24 preferred resistor values per decade (e.g. 10, 11, 12, 13, 15, 16, 18 … 91, 100 Ω). Most resistors sold off-the-shelf follow this series. The calculator shows the nearest E24 value so you can pick a real component easily.

Can I use this calculator for multiple LEDs in series?

Yes. Enter the number of LEDs in the 'Number of LEDs (in series)' field. The calculator multiplies the forward voltage by that count before computing the resistor, since each LED in series drops its own Vf. Make sure your supply voltage is higher than n × Vf or the circuit won't work.

What happens if my supply voltage is less than or equal to the total LED forward voltage?

If Vs ≤ n × Vf, the LEDs cannot be driven — there is no remaining voltage for the resistor to drop. You would need a higher supply voltage or fewer LEDs in series. The calculator will warn you in this scenario.

Does this calculator work for parallel LED arrays?

This calculator handles LEDs wired in series on a single string. For parallel configurations, treat each parallel branch independently — run the calculator for one branch and use one resistor per branch. Sharing a single resistor across parallel LEDs is not recommended because slight Vf differences cause unequal current sharing.

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