LED Resistor Calculator with Power Rating
Find the series resistor for one LED or a string of them, rounded to a standard value you can buy. It also shows the current that resistor really sets, how hot it runs and the smallest chip package that carries it with margin, how much of the power reaches the LEDs, and how far the current moves when the LED's forward voltage does. Every input lives in the URL, so a copied link reproduces the calculation.
- LED current
- 19 mA
- -5.0% vs target
- Resistor power
- 36.1 mW
- 0603 or larger (100 mW rated)
- Efficiency
- 62.0%
- 58.9 mW in the LEDs
- Current over the Vf spread
- 18 mA to 20 mA
- ±0.1 V per LED
- Headroom
- 1.9 V
- 38% of the supply, across the resistor
- Max LEDs in series
- 1
- with at least 0.3 V for the resistor
Next value down: 91 Ω gives 20.9 mA (+4.4%), above your target.
The link reproduces these exact inputs - paste it in a design review or a pull request.
Bring the LEDs, the supply and the brightness you need. We will look at the drive scheme with you (resistor, transistor, constant-current or PWM) before the board goes to fab. If you need the whole board done, see PCB design and layout services.
Chip resistor power ratings
Typical thick-film ratings at 70 °C, and the most each package should dissipate with the 50% derating this calculator applies. Check your resistor's datasheet: anti-surge and high-power variants carry more.
| Package | Rated | Use up to |
|---|---|---|
| 0402 | 62.5 mW | 31.3 mW |
| 0603 | 100 mW | 50 mW |
| 0805 | 125 mW | 62.5 mW |
| 1206 | 250 mW | 125 mW |
| 1210 | 500 mW | 250 mW |
| 2010 | 750 mW | 375 mW |
| 2512 | 1 W | 500 mW |
The equation
An LED drops a roughly fixed forward voltage once it conducts, so the series resistor sees whatever is left: the supply minus the forward voltages of every LED in the string. Ohm's law then gives R = (Vs - n × Vf) / I. The calculator rounds up to the next standard value, so the current lands at or just below your target, and reports the current that value really sets. Power in the resistor is I² × R; the LEDs take n × Vf × I; efficiency is their share of the total, which works out to n × Vf / Vs whatever the resistor.
Headroom decides whether a resistor is good enough
Forward voltage is not a constant. It differs between parts in the same reel, falls as the LED warms up, and rises with current. The resistor only controls the current through the headroom, so a shift in Vf becomes a shift in current in proportion to how little headroom there is. With 3 V across the resistor, a 0.1 V shift moves the current about 3%; with 0.3 V across it, about 33%. That is why a white LED straight off a 3.3 V rail is unreliable, and why this page shows the current over the Vf spread rather than one number.
Never share one resistor between parallel LEDs
LEDs in parallel on one resistor do not share current evenly: the one with the lowest forward voltage takes most of it, warms up, drops its forward voltage further and takes more. Give every LED or every series string its own resistor. For more than a few LEDs, or for anything that has to look uniform, use a constant-current driver.
Frequently asked questions
What resistor do I need for an LED on 5 V?
Subtract the LED's forward voltage from the supply and divide by the current. A red LED at about 2.0 V and 20 mA needs (5 - 2.0) / 0.020 = 150 Ω, which is a standard value, and the resistor dissipates 60 mW, so use an 0805 or larger rather than an 0603. A white LED at 3.1 V needs 95 Ω; the next standard value up is 100 Ω, which gives 19 mA and 36 mW. Round up, not down, so the LED never runs above the current you chose.
What resistor do I need for an LED on a 3.3 V GPIO pin?
For a red LED at 5 mA, (3.3 - 2.0) / 0.005 = 260 Ω, so use 270 Ω for 4.8 mA. White, blue and true green LEDs are a poor fit for 3.3 V: a 3.0 V green at 5 mA needs 62 Ω, and a ±0.1 V spread in forward voltage moves its current anywhere from 3.2 to 6.5 mA. The pin's own output voltage also sags under load. Use a red or amber LED, or switch the LED with a transistor from a higher rail.
How many LEDs can I put in series on 12 V?
Three white LEDs at 3.1 V each leave 2.7 V for the resistor. At 20 mA that is 135 Ω, rounded up to 150 Ω for 18 mA, and 77.5% of the supply power reaches the LEDs. A ±0.1 V spread per LED still keeps the current between 16 and 20 mA. A fourth LED would need 12.4 V, more than the supply, so it would barely light.
What power rating does an LED resistor need?
At least twice what it dissipates, the usual 50% derating that keeps the part cool enough to hold its value. Dissipation is the current squared times the resistance, and it grows fast with supply voltage: one red LED at 20 mA from 24 V needs 1.1 kΩ dissipating 440 mW, which calls for a 2512 rated 1 W, while only 8.3% of the power reaches the LED. Put more LEDs in series or use a constant-current driver instead.
When should I use a constant-current driver instead of a resistor?
When the resistor has little voltage left across it, when the LED current is high, or when brightness must match between units. A resistor sets the current only through the headroom, the supply minus the LED voltage, so the smaller the headroom, the more a forward-voltage shift or a supply change moves the current. If the calculator shows a current range wider than about ±20%, or a resistor burning a large share of the power, a constant-current driver is the better part.
If the board is the product
An indicator resistor is a five-minute decision. Lighting, backlights and status LEDs that must match from unit to unit are not, and that is our day job: PCB design and layout explains how we work and what a board costs in engineering hours. For the rest of the power path, the LDO thermal calculator and the PCB trace width calculator are next door. If a number here decides something real, a free 30-minute technical call is the fastest way to check it against someone who has shipped the board.