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LED Resistor Calculator

Calculate the resistor value for any LED β€” with standard E12/E24 rounding, color bands, power rating, and series LED support

V
V
mA
1 Total Vf = β€”

Results

Enter values above to calculate the resistor

About This Tool

Every LED needs a current-limiting resistor or it will burn out instantly. The formula is simple β€” R = (Vin βˆ’ Vf) / If β€” but picking the right standard resistor value, checking the power rating, and reading color bands takes extra steps that this calculator handles for you. Select your supply voltage, pick an LED color (or enter custom forward voltage), set your desired current, and optionally add LEDs in series. The calculator gives you the exact resistance, snaps it to the nearest E24 standard value, shows the resistor color bands, computes power dissipation, and recommends the right wattage rating. Whether you're building an Arduino project, a custom LED strip, or learning electronics β€” this is the fastest way to pick the right resistor.

How to Use

1. Select or enter your supply voltage (3.3V, 5V, 9V, 12V, etc.) 2. Pick an LED color preset or enter a custom forward voltage 3. Set the desired current (typical: 20mA for standard LEDs, 5-10mA for indicators) 4. Optionally increase LEDs in series count 5. See the exact and standard resistor value, color bands, and power info 6. Click "Copy Link" to share your calculation

Frequently Asked Questions

What resistor do I need for an LED?
Use the formula R = (Vin βˆ’ Vf) / If. For example, a red LED (Vf = 2.0V) on a 5V supply at 20mA needs (5 βˆ’ 2) / 0.02 = 150Ξ©. Always round up to the next standard resistor value β€” in this case, 150Ξ© is already a standard E24 value.
What is the typical forward voltage of an LED?
It depends on color: Red β‰ˆ 1.8–2.2V, Orange β‰ˆ 2.0–2.2V, Yellow β‰ˆ 2.0–2.2V, Green β‰ˆ 2.0–3.2V, Blue β‰ˆ 3.0–3.4V, White β‰ˆ 3.0–3.4V, UV β‰ˆ 3.2–3.6V, Infrared β‰ˆ 1.1–1.5V. Always check your LED's datasheet for the exact value.
What current should I use for an LED?
Standard 5mm LEDs are typically rated at 20mA maximum. For indicators and low-power applications, 5–10mA is usually bright enough and extends LED life. High-power LEDs may need 350mA or more β€” always check the datasheet.
What are E12 and E24 resistor values?
E12 and E24 are standardized series of preferred resistor values. E12 has 12 values per decade (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82), and E24 has 24. Resistors are only manufactured in these standard values, so you need to pick the nearest one. This calculator does that automatically.
Can I put multiple LEDs in series with one resistor?
Yes. For LEDs in series, the total forward voltage is the sum of all LEDs' Vf. The formula becomes R = (Vin βˆ’ (Vf Γ— n)) / If. Make sure Vin > (Vf Γ— n), otherwise the supply voltage is too low to drive that many LEDs in series.
How do I read resistor color bands?
A 4-band resistor has: 1st band = first digit, 2nd band = second digit, 3rd band = multiplier, 4th band = tolerance. For example, brown-green-brown-gold = 15 Γ— 10 = 150Ξ© Β±5%. This calculator shows the color bands visually for any standard value.
What wattage resistor should I use?
The resistor must be rated above the power it dissipates: P = IΒ² Γ— R. For most LED circuits at 20mA, power is under 0.1W, so a standard 1/4W resistor is fine. The calculator recommends the minimum wattage rating for you.

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