LED Series Resistor Calculator

Enter your supply voltage, the LED's forward voltage, and your target current — get the exact current-limiting resistor value, the nearest standard (E12/E24/E96) value, the resistor's power dissipation, and its matching 4-band color code, instantly.

Supply voltage

Battery or power supply voltage, e.g. 5V (USB), 9V (battery), 12V.

LED forward voltage (per LED)

Check the LED's datasheet, or use a color preset below.

Target current

20mA is a common safe default for standard 5mm LEDs.

Number of LEDs in series

Quick answer LED resistor value is calculated with Ohm's law: R = (Supply voltage − LED forward voltage) ÷ LED current. For example, a 9V supply with a 2V red LED at 20mA needs (9−2)/0.02 = 350 Ω — round up to the nearest standard value like 360 Ω or 390 Ω so the current stays at or below your target. Enter your own values above to calculate instantly.

R = (Vs−Vf)/IOhm's law formula

20 mATypical 5mm LED current

Round UPNever round down the resistor

2× marginRecommended power headroom

⚙️ Accuracy note: Calculations use Ohm's law (R = V/I) and IEC 60063 standard value series (E12/E24/E96) for rounding. LED forward voltage varies by manufacturer, batch, and drive current — always check the specific LED's datasheet for critical designs. All operations run locally in your browser; nothing you enter is sent to a server.

💡 LED Series Resistor Calculator — ToolPico ↗

How do you calculate the right resistor value for an LED?

A complete guide — with formulas and examples — to sizing an LED current-limiting resistor, rounding to a standard value, and checking the resistor's power rating.

An LED (light-emitting diode) has almost no internal resistance of its own — without something limiting the current, it will draw as much as the power supply can deliver and burn out almost instantly. A series resistor (also called a current-limiting resistor) is placed in the circuit specifically to hold the current at a safe level. This tool calculates the exact value from your supply voltage, the LED's forward voltage, and your target current, then rounds it to a standard resistor value and shows you the matching color code.

What is the LED resistor formula?

Quick answerR = (Supply voltage − LED forward voltage) ÷ LED current, derived directly from Ohm's law (V = I × R). The "extra" voltage the LED doesn't use (Vs − Vf) has to be dropped across the resistor, and dividing that voltage by your target current gives the resistance needed to do it.

Why do you round the resistor value up, not down?

Quick answerResistance and current are inversely related — less resistance means more current flows. Standard resistor values rarely match your calculated value exactly, so you must round up to the next available standard value, which results in slightly less current than your target (safe). Rounding down would push more current through the LED than planned, risking overheating or a shortened lifespan.

What LED forward voltage should I use?

Quick answerForward voltage depends on the LED's color and semiconductor material: red ≈ 1.8-2.2V, yellow/orange ≈ 2.0-2.2V, green ≈ 2.0-3.2V, blue ≈ 2.8-3.4V, white ≈ 2.8-3.4V, UV ≈ 3.0-3.4V, infrared ≈ 1.2-1.6V. Use the color presets above for a quick estimate, but check the specific LED's datasheet whenever precision matters.

How do you calculate multiple LEDs in series?

Quick answerWhen LEDs are chained in series (sharing one current path), their forward voltages add together: R = (Supply voltage − (LED forward voltage × number of LEDs)) ÷ LED current. Three 2V red LEDs on a 9V supply at 20mA need (9 − 2×3) / 0.02 = 150 Ω. Set the "Number of LEDs in series" field above to calculate this automatically — note the supply voltage must exceed the combined LED forward voltage.

What power rating resistor do I need?

Quick answerPower dissipated by the resistor is P = I² × R (equivalently P = V_resistor × I). For safety margin, pick a resistor rated for at least 2× the calculated power — most common 1/4W (0.25W) through-hole resistors are more than enough for typical low-power indicator LED circuits, but the calculator always shows you the exact figure so you can check against what you have on hand.

How does this relate to the resistor color code?

Quick answerOnce you know the standard resistor value you need (e.g. 330 Ω), you still need to identify or verify the physical resistor by its color bands. This calculator shows the 4-band color code for the recommended value directly in the result — or use the Resistor Color Code Calculator's "Ohm → Color" tab for the full breakdown.

Common LED + supply voltage combinations

Frequently used supply voltage / LED color combinations at a 20mA target current, with the resulting resistor value already rounded up to the nearest standard (±5%) value.

Common LED series resistor combinations — single LED, 20mA target
Supply	LED color (Vf)	Calculated	Standard value
5V (USB)	Red (2.0V)	150 Ω	150 Ω
5V (USB)	Blue/White (3.2V)	90 Ω	91 Ω / 100 Ω
9V (battery)	Red (2.0V)	350 Ω	360 Ω
9V (battery)	Green (2.1V)	345 Ω	360 Ω
12V	Red (2.0V)	500 Ω	510 Ω
12V	Blue/White (3.2V)	440 Ω	470 Ω

ΩResistor Color Code CalculatorVerify the standard value's color bands, or decode an SMD code 🔲SMD Size → Power RatingChoosing an SMD package for your calculated power dissipation

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LED resistor terms glossary

Short definitions of the core terms used in LED current-limiting resistor sizing.

Forward voltage (Vf)The voltage drop across an LED when it's conducting current in the forward direction; depends on color/material.

Forward current (If)The current flowing through the LED; determines brightness and, if too high, can damage the LED.

Current-limiting resistorA series resistor sized to keep LED current at a safe level, since LEDs have negligible internal resistance.

Ohm's law (V = I × R)The relationship between voltage, current, and resistance used to derive the LED resistor formula.

Power dissipation (P = I²R)The heat energy the resistor must safely handle; the chosen resistor's wattage rating must exceed this.

E-series (E12/E24/E96)Standardized preferred resistance value ranges defined by IEC 60063, based on tolerance.

Series LEDsMultiple LEDs chained so the same current flows through all of them; their forward voltages add up.

Resistor color codeThe colored bands on a resistor's body that indicate its ohm value and tolerance (see the dedicated calculator).

In-depth guides

Step-by-step walkthroughs of the most commonly confused LED resistor questions.

How is a single LED resistor value calculated? (step by step)

Step 1: Find the "extra" voltage the resistor must absorb → 9V supply − 2V LED = 7V.
Step 2: Convert your target current to amps → 20mA = 0.02A.
Step 3: Apply Ohm's law → 7V ÷ 0.02A = 350 Ω.
Step 4: Round up to the nearest standard value → 360 Ω (E24) or 390 Ω (E12).

You can see this instantly by entering 9, 2.0, and 20 into the calculator above.

Sizing the resistor for LEDs in series

When LEDs share the same current path (series), their forward voltages stack. Three 2V red LEDs need 6V just to turn on, so a 9V supply only has 3V left for the resistor: (9 − 6) / 0.02 = 150 Ω. If the combined LED forward voltage is close to or exceeds the supply voltage, there won't be enough headroom to limit the current safely — in that case, either reduce the number of LEDs in series or increase the supply voltage.

Set "Number of LEDs in series" in the calculator above to handle this automatically.

Checking the resistor's power rating

Step 1: Take the calculated (or standard, rounded) resistor value and the target current.
Step 2: Apply P = I² × R — for 20mA through 360 Ω: 0.02² × 360 = 0.144 W.
Step 3: Apply a safety margin (2× is common) → look for a resistor rated 0.29 W or higher.
Step 4: A standard 1/4W (0.25W) resistor is borderline here — a 1/2W part would be the safer pick.

The calculator above shows this power figure directly in the result card.

Frequently asked questions

What resistor value do I need for an LED?

Use Ohm's law: R = (Supply voltage − LED forward voltage) ÷ LED current. For a common 5mm red LED (forward voltage ≈ 2.0V, current ≈ 20mA) on a 9V supply: (9 − 2) / 0.02 = 350 Ω. Since 350 Ω isn't a standard value, round up to the nearest standard resistor, such as 360 Ω or 390 Ω, to keep the current safely at or under 20mA.

Why do you round the resistor value up, not down?

A higher resistance means less current, and a lower resistance means more current. Rounding down would push more current through the LED than intended, which can overheat it and shorten its lifespan or cause immediate failure. Rounding up gives a current slightly under your target, which is the safe direction.

What is a typical LED forward voltage?

Forward voltage depends on the LED's color/material: red ≈ 1.8-2.2V, yellow/orange ≈ 2.0-2.2V, green ≈ 2.0-3.2V, blue ≈ 2.8-3.4V, white ≈ 2.8-3.4V, UV ≈ 3.0-3.4V, infrared ≈ 1.2-1.6V. Always check the specific LED's datasheet for an exact value where possible; the calculator provides common presets for a quick estimate.

What current should I use for a standard 5mm LED?

20 mA (0.02 A) is the widely used safe default for standard 5mm indicator LEDs at full brightness. For a dimmer, longer-lasting LED, 10-15 mA is common. High-brightness or high-power LEDs may be rated for much higher currents (30mA-1A+) — always check the datasheet's maximum forward current rating.

How do I calculate the resistor for multiple LEDs in series?

When LEDs are wired in series (one after another, sharing the same current path), their forward voltages add up. The formula becomes R = (Supply voltage − (LED forward voltage × number of LEDs)) ÷ LED current. For example, three 2V red LEDs on a 9V supply at 20mA: (9 − 2×3) / 0.02 = 150 Ω. This calculator supports entering the number of series LEDs directly.

What power rating resistor do I need for an LED circuit?

Power (watts) dissipated by the resistor is P = I² × R (or equivalently P = V_resistor × I). For safety, choose a resistor rated for at least 2× the calculated power — most through-hole 1/4W (0.25W) resistors comfortably handle typical low-power LED circuits, but always verify against the calculated value shown by this tool.

Can I use this calculator's result with the resistor color code calculator?

Yes — once you have the recommended standard resistor value here, use the Resistor Color Code Calculator's "Ohm → Color" tab to see the exact color bands to look for, or verify a resistor you already have on hand.

Methodology & sources

ToolPico's LED Series Resistor Calculator is a free, independent tool. The calculation engine applies Ohm's law (R = V/I) directly to the supply voltage, LED forward voltage, and target current you enter, then rounds the result to the nearest standard value from the IEC 60063 E12/E24/E96 preferred-value series. Power dissipation is calculated as P = I² × R. All calculations are deterministic; no external API or database is queried.

Sources: Ohm's law · IEC 60063 (E-series preferred values) · typical LED forward-voltage ranges from common manufacturer datasheets. Last updated: July 28, 2026. Results are for informational purposes only; for critical or precision applications, refer to the LED's official datasheet.

How do you calculate the right resistor value for an LED?

What is the LED resistor formula?

Why do you round the resistor value up, not down?

What LED forward voltage should I use?

How do you calculate multiple LEDs in series?

What power rating resistor do I need?

How does this relate to the resistor color code?

Common LED + supply voltage combinations

LED resistor terms glossary

In-depth guides

Frequently asked questions

Methodology & sources

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