How do I calculate the resistor value for an LED?

Use R = (VS − VF) / IF, where VS is supply voltage, VF is LED forward voltage, and IF is desired forward current in amperes. Example: 5V supply, red LED (VF = 2.0V), IF = 20mA → R = (5−2.0)/0.020 = 150Ω.

What resistor do I need for an LED with a 5V supply?

For a red LED (VF ≈ 2.0V) at 20mA: R = (5−2.0)/0.020 = 150Ω. For blue/white (VF ≈ 3.2V) at 20mA: R = (5−3.2)/0.020 = 90Ω (use 100Ω). Always round up to the nearest E24 standard value.

What resistor do I need for an LED with a 12V supply?

For a red LED (VF ≈ 2.0V) at 20mA: R = (12−2.0)/0.020 = 500Ω (use 510Ω). For blue/white (VF ≈ 3.2V): R = (12−3.2)/0.020 = 440Ω (use 470Ω). The higher voltage means more power in the resistor — check the wattage rating.

What is LED forward voltage (VF)?

Forward voltage is the voltage drop across the LED when it is conducting. Typical values: IR ≈ 1.3V, red ≈ 2.0V, orange ≈ 2.1V, yellow/green ≈ 2.2V, blue/white ≈ 3.2V, UV ≈ 3.6V. Always check the datasheet for the exact value.

What wattage resistor do I need for an LED?

Calculate PR = (VS − VF) × IF. Then use a resistor rated at least 2× that value. For most LED circuits at 5V with 20mA, PR ≈ 40–80mW, so a ¼W (250mW) resistor is fine. For 12V supplies, PR can reach 200mW — use a ½W resistor to be safe.

Can I use the same resistor for multiple LEDs in series?

Yes. For LEDs in series, subtract all forward voltages from the supply: R = (VS − VF1 − VF2 − …) / IF. The same current flows through all LEDs in the chain, so one resistor controls them all.

Why should I round up to the nearest E24 resistor?

Rounding up gives a slightly higher resistance, which slightly reduces the current. This keeps the LED within safe operating limits. Rounding down would push more current through the LED, risking damage.

What is a typical LED forward current?

Most standard 5mm through-hole LEDs operate at 10–20mA. High-brightness and high-power LEDs may require 50mA to several amps. Always check the datasheet maximum forward current rating and stay well below it.

LED Resistor Calculator – R = (VS−VF)/IF, Resistor Value, Power Rating & E24 Standard

R = V_S − V_F I_F

Series resistor formula for LED circuits

🎨 Select LED Color (sets V_F)

⚡Supply Voltage (V_S)

1.5V24V

💡Forward Voltage (V_F)

1.0V (IR)4.5V (UV)

🔵Forward Current (I_F)

1 mA (dim)50 mA (max)

💡 LED Brightness 20 mA — Normal

🌑 Off🌙 Dim💡 Normal☀️ Bright🌟 Max

Required Resistor (R)

140 Ω

Exact calculated value

Nearest Standard (E24)

150 Ω

Closest E24 value

Resistor Power (P_R)

56 mW

Use ≥ ¼W resistor

LED Power (P_LED)

44 mW

Power in LED

E24 Nearest Standard Values

ℹ️

Always use a resistor equal to or above the calculated value. Choose a power rating at least 2× the calculated resistor power dissipation for safe operation.

⚙️ Calculation Steps

Identify supply voltage (V_S) and LED forward voltage (V_F) from datasheet.

Choose desired forward current (I_F) — typically 10–20 mA for standard LEDs.

Calculate voltage across resistor: V_R = V_S − V_F

Apply Ohm's Law: R = V_R / I_F

Calculate resistor power: P_R = V_R × I_F = I_F² × R

Round up to nearest standard E24 resistor value and verify power rating.

📊 Live Calculation

Step 1 — Voltages

V_S = 5.0V V_F = 2.2V

Step 2 — Voltage across resistor

V_R = 5.0 − 2.2 = 2.8V

Step 3 — Required Resistance

R = 2.8V / 20mA = 140 Ω

Step 4 — Power Dissipation

P_R = 2.8 × 20mA = 56 mW (use ¼W)

📋 Quick Reference — LED Types & Specifications

LED Color	V_F Typical	V_F Range	Typical I_F	Wavelength	R @ 5V, 20mA
Infrared (IR)	1.3V	1.2–1.6V	20–100 mA	850–950 nm	185 Ω
Red	2.0V	1.8–2.2V	10–30 mA	620–750 nm	150 Ω
Orange	2.1V	2.0–2.2V	10–20 mA	590–620 nm	145 Ω
Yellow	2.2V	2.0–2.4V	10–20 mA	570–590 nm	140 Ω
Green (standard)	2.2V	2.0–2.5V	10–20 mA	520–560 nm	140 Ω
Green (bright)	3.3V	3.0–3.5V	20–30 mA	515–525 nm	85 Ω
Blue	3.2V	3.0–3.5V	20–30 mA	460–500 nm	90 Ω
White	3.2V	3.0–3.6V	20–30 mA	Broadband	90 Ω
UV / Violet	3.6V	3.4–4.0V	10–20 mA	380–420 nm	70 Ω

ℹ️

Resistor values shown are for 5V supply at 20 mA. Always verify with the LED datasheet. Forward voltage varies with temperature — LEDs get brighter when cold and dimmer when hot. Add ~20% safety margin to resistor power rating.

LED Resistor Calculator — Complete Guide to LED Series Resistor Calculation

This LED resistor calculator finds the correct current-limiting series resistor for any LED using the fundamental formula R = (V_S − V_F) / I_F. Enter your supply voltage (V_S), LED forward voltage (V_F), and desired forward current (I_F) and the calculator instantly shows the exact resistance, the nearest E24 standard value, resistor power dissipation, recommended wattage rating, and a live LED brightness simulation. Use the colour presets to set typical V_F values automatically for red, orange, yellow, green, blue, white, UV, or infrared LEDs.

Quick Reference: LED Resistor Formula

Quantity	Formula	Unit
Series Resistor	R = (V_S − V_F) / I_F	Ω (ohms)
Voltage across R	V_R = V_S − V_F	V (volts)
Resistor Power	P_R = V_R × I_F = I_F² × R	W (watts)
LED Power	P_LED = V_F × I_F	W (watts)
Total Power	P_total = V_S × I_F	W (watts)

Why Every LED Needs a Series Resistor

An LED is not an ohmic device — it does not obey Ohm's law like a resistor. Instead, it has a steep current–voltage curve: below the forward voltage V_F it barely conducts; above it, current rises almost vertically. A supply voltage just slightly above V_F drives enormous, destructive current. The series resistor absorbs the excess voltage (V_S − V_F) and converts it to heat, holding the current to the safe level you choose. Without it, the LED burns out in milliseconds.

R = (V_S − V_F) / I_F

• V_S = Supply voltage (V) — e.g., 3.3V, 5V, 9V, 12V
• V_F = LED forward voltage (V) — from datasheet; typically 2.0–3.6V
• I_F = Desired forward current (A) — typically 0.010–0.020 A (10–20 mA)

P_R = (V_S − V_F) × I_F — power the resistor must dissipate
Use a resistor rated ≥ 2 × P_R for a safe thermal margin.

LED Colour Reference — Forward Voltage & Typical Resistor Values

Forward voltage varies significantly with LED colour because different semiconductor materials are used. Use this table as a starting point — always verify with the manufacturer's datasheet:

LED Colour	V_F Typical	V_F Range	Typical I_F	Wavelength	R at 5V, 20mA	R at 12V, 20mA
🟣 Infrared (IR)	1.3 V	1.2–1.6 V	20–100 mA	850–950 nm	185 Ω → 180 Ω	535 Ω → 560 Ω
🔴 Red	2.0 V	1.8–2.2 V	10–30 mA	620–750 nm	150 Ω	500 Ω → 510 Ω
🟠 Orange	2.1 V	2.0–2.2 V	10–20 mA	590–620 nm	145 Ω → 150 Ω	495 Ω → 510 Ω
🟡 Yellow	2.2 V	2.0–2.4 V	10–20 mA	570–590 nm	140 Ω	490 Ω → 510 Ω
🟢 Green (standard)	2.2 V	2.0–2.5 V	10–20 mA	520–560 nm	140 Ω	490 Ω → 510 Ω
🟢 Green (bright)	3.3 V	3.0–3.5 V	20–30 mA	515–525 nm	85 Ω	435 Ω → 470 Ω
🔵 Blue	3.2 V	3.0–3.5 V	20–30 mA	460–500 nm	90 Ω	440 Ω → 470 Ω
⚪ White	3.2 V	3.0–3.6 V	20–30 mA	Broadband	90 Ω	440 Ω → 470 Ω
🟣 UV / Violet	3.6 V	3.4–4.0 V	10–20 mA	380–420 nm	70 Ω	420 Ω → 430 Ω

Worked Examples

💡 Example 1 — Red LED on a 5V Arduino Pin

GivenV_S = 5V | Red LED: V_F = 2.0V | I_F = 20 mA (0.020 A)

Step 1V_R = V_S − V_F = 5.0 − 2.0 = 3.0 V

Step 2R = V_R / I_F = 3.0 / 0.020 = 150 Ω (exact E24 match ✓)

Step 3P_R = 3.0 × 0.020 = 60 mW → use ¼W (250 mW) resistor ✓

ResultR = 150 Ω, ¼W | P_LED = 2.0 × 0.020 = 40 mW

🔵 Example 2 — Blue LED on a 12V Automotive Supply

GivenV_S = 12V | Blue LED: V_F = 3.2V | I_F = 20 mA (0.020 A)

Step 1V_R = 12.0 − 3.2 = 8.8 V

Step 2R = 8.8 / 0.020 = 440 Ω → round up to nearest E24 = 470 Ω

Step 3P_R = 8.8 × 0.020 = 176 mW → use ½W (500 mW) resistor ✓ (2× margin)

ResultR = 470 Ω, ½W | At 12V always check wattage!

⚪ Example 3 — White LED on a 3.3V Microcontroller

GivenV_S = 3.3V | White LED: V_F = 3.0V | I_F = 10 mA (0.010 A)

Step 1V_R = 3.3 − 3.0 = 0.3 V (very small margin — check V_F carefully!)

Step 2R = 0.3 / 0.010 = 30 Ω → nearest E24 = 33 Ω

Step 3P_R = 0.3 × 0.010 = 3 mW → ⅛W resistor is more than sufficient

ResultR = 33 Ω, ⅛W | ⚠️ Verify V_F ≤ 3.0V — tight supply margin

Standard Resistor Power Ratings

Rating	Max Power	Physical Size	Use Case
⅛W	125 mW	Very small	Low-current LEDs (≤10 mA at 5V)
¼W	250 mW	Standard through-hole	Most LED circuits at 5V–9V, 20mA
½W	500 mW	Slightly larger	12V supplies or currents >20mA
1W	1,000 mW	Metal body, heatsink	High-power LEDs, 24V supplies
2W–5W	2,000–5,000 mW	Large, needs heatsink	High-power LED modules

Always use a resistor rated at at least 2× the calculated dissipation. Running a resistor near its limit raises its temperature, shifts its resistance value, and shortens its life.

Multiple LEDs in Series — One Resistor

For LEDs wired in series (same current through all), subtract all V_F values from V_S and use one shared resistor:

R = (V_S − V_F1 − V_F2 − … − V_Fn) / I_F

Example: 3× red LEDs (V_F = 2.0V each) in series on 12V at 20mA:
R = (12 − 2.0 − 2.0 − 2.0) / 0.020 = 6.0 / 0.020 = 300 Ω (use 330 Ω)

⚠️ Ensure V_S > ΣV_F — the supply must exceed the total forward voltage.

Multiple LEDs in Parallel — Individual Resistors

Never connect LEDs in parallel without individual resistors. Even LEDs of the same part number have slightly different V_F values. The one with the lowest V_F will hog all the current and fail. Always give each parallel LED its own resistor calculated from R = (V_S − V_F) / I_F.

LED Resistor for Common Supply Voltages

Supply	Red (V_F=2.0V)	Green (V_F=2.2V)	Blue/White (V_F=3.2V)	UV (V_F=3.6V)
3.3V	65Ω→68Ω	55Ω→56Ω	5Ω→10Ω ⚠️	Not recommended
5V	150Ω ¼W	140Ω ¼W	90Ω→100Ω ¼W	70Ω ¼W
9V	350Ω→360Ω ¼W	340Ω ¼W	290Ω→300Ω ¼W	270Ω ¼W
12V	500Ω→510Ω ½W	490Ω→510Ω ½W	440Ω→470Ω ½W	420Ω→430Ω ½W

Values shown are exact calculated → rounded to nearest E24 above. Power ratings assume I_F = 20 mA. ⚠️ 3.3V + blue/white LED leaves very little headroom — verify V_F from datasheet.

E24 Standard Resistor Series

Standard resistors are made in preferred E-series values. The E24 series (24 values per decade, ±5% tolerance) covers: 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91 Ω — then ×10, ×100, etc. This calculator automatically shows the nearest E24 values above and below your exact calculated resistance. Always pick the value equal to or above the calculated result.

LED Brightness and PWM Dimming

LED brightness is approximately proportional to forward current. The simplest dimming method is reducing I_F by increasing the series resistance, but this also changes the LED's colour temperature slightly at low currents. The preferred method for smooth, colour-accurate dimming is PWM (Pulse-Width Modulation) — rapidly switching the LED on and off at full current. The eye perceives average brightness. PWM frequencies above 100 Hz appear flicker-free to most people.

Frequently Asked Questions

What happens if I don't use a resistor with an LED?

The LED draws uncontrolled current, overheats in milliseconds, and burns out permanently. An LED's forward voltage changes only a fraction of a volt between 1 mA and 100 mA — without a resistor, even a small excess supply voltage drives destructive current.

What resistor for a red LED at 5V?

R = (5 − 2.0) / 0.020 = 150 Ω, ¼W. This is an exact E24 value and the most common LED resistor in Arduino/Raspberry Pi projects.

What resistor for a white LED at 5V?

R = (5 − 3.2) / 0.020 = 90 Ω → use 100 Ω (nearest E24 above), ¼W.

What resistor for an LED at 12V?

For a red LED at 20mA: R = (12 − 2.0) / 0.020 = 500 Ω → use 510 Ω, ½W. For blue/white: R = (12 − 3.2) / 0.020 = 440 Ω → use 470 Ω, ½W. At 12V always use at least a ½W resistor.

Can I use the same resistor for different coloured LEDs?

Only if they have the same V_F and I_F. Red and green standard LEDs have slightly different V_F (2.0V vs 2.2V) so a shared 150 Ω resistor will give slightly different brightness. For colour-critical applications, use individual resistors per colour.

Why round up to the nearest E24 value?

Rounding up gives a slightly higher resistance, which reduces the current slightly, keeping the LED within safe limits. Rounding down would push more current through the LED than intended, risking damage and shortened life.

Related Calculators

Ohm's Law Calculator — V = I × R fundamentals
Resistor Color Code Calculator — decode your resistor's value
Resistor Wattage Calculator — power dissipation and rating
DC Power Calculator — P = V × I
Series Resistor Calculator