Circuit Design
LED Series Resistor Calculator
Work out the current-limiting resistor for a single LED or a series string — with the real E-series part you would actually fit, the current you will actually get, and the power both parts have to survive.
Circuit
ideal resistance
How the calculation works
An LED is a diode, not a resistor. Above its forward voltage the current rises almost vertically, so connecting one straight across a supply destroys it. The series resistor exists to absorb the difference between the supply and the LED's forward voltage, and it is that difference — not the supply voltage — that sets the current.
R = (Vsupply − n × Vf) / If
where n is the number of LEDs wired in series. The resistor then dissipates P = (Vsupply − n × Vf) × If.
Worked example
A red LED with Vf = 2.0 V at 20 mA, on a 5 V rail: the resistor drops 3.0 V, so R = 3.0 / 0.02 = 150 Ω. That is already an E24 value, so fit 150 Ω. It dissipates 3.0 × 0.02 = 60 mW, comfortable for an 0805 or a quarter-watt through-hole part.
Why forward voltage is a range, not a number
Datasheets quote Vf at a specific test current, with a spread of several hundred millivolts across a production batch — and it falls as the die heats up. This matters more than most people expect. If your supply is 5 V and Vf is nominally 3.4 V for a white LED, the resistor only has 1.6 V to work with. A part at the top of its Vf distribution might leave 1.2 V, and your current drops 25% below target. A part at the bottom leaves 2.0 V and runs 25% over.
Rule of thumb: keep at least 20% of the supply voltage across the resistor, and more if brightness matching between channels matters. If you cannot, use a constant-current driver instead — that is what they are for.
Series versus parallel LEDs
LEDs in series share one resistor and carry identical current, so they match well. The limit is supply headroom: three white LEDs at 3.2 V need 9.6 V before the resistor gets anything at all, so a 12 V rail is about the minimum.
LEDs in parallel on a single shared resistor is a beginner trap. Forward voltages never match exactly, so the LED with the lowest Vf hogs the current, gets hotter, drops its Vf further, and takes even more. The result is one bright LED, several dim ones, and eventually a failure. Give every parallel branch its own resistor.
Choosing the resistor package
Multiply the calculated dissipation by two and pick a part rated above that. Typical maxima at 70 °C: 0603 = 100 mW, 0805 = 125 mW, 1206 = 250 mW, through-hole quarter-watt = 250 mW. High-current indicator LEDs on a 24 V industrial rail routinely need a half-watt or one-watt part, and it will run hot enough to discolour the board over time — plan the layout accordingly.
When not to use a resistor at all
- Power LEDs above about 100 mA. The resistor wastes more power than the LED consumes and the current varies wildly with temperature. Use a constant-current buck driver.
- Wide input voltage ranges. A resistor sized for 12 V passes roughly twice the current at 24 V. Automotive and industrial supplies swing far enough to matter.
- Battery-powered devices where brightness must not fade. As the cell voltage falls, resistor-limited current falls with it.
- PWM dimming with precise ratios. A resistor still works, but the current changes with Vf drift, so absolute brightness is not repeatable.
Frequently asked questions
What resistor do I need for a 5 V LED?
For a typical red LED (Vf ≈ 2.0 V) at 20 mA on 5 V, the resistor drops 3.0 V, so R = 3.0 / 0.02 = 150 Ω. For a white or blue LED (Vf ≈ 3.2 V) on 5 V it is 1.8 / 0.02 = 90 Ω, so fit 91 Ω or 100 Ω.
How do I know my LED's forward voltage?
Check the datasheet — it is specified at a stated test current. As a guide: red and yellow 1.8–2.2 V, green 2.0–3.2 V, blue and white 2.8–3.6 V, infrared 1.2–1.6 V. Forward voltage falls as the LED warms up, so the datasheet figure is a starting point, not a constant.
Can I put several LEDs on one resistor?
In series, yes — they carry identical current and share one resistor. In parallel, no. Forward voltages never match exactly, so one LED takes most of the current, heats up, takes more, and eventually fails. Give each parallel branch its own resistor.
What wattage resistor does an LED need?
Multiply the voltage across the resistor by the LED current, then double it for margin. A 150 Ω resistor dropping 3 V at 20 mA dissipates 60 mW, so a 125 mW 0805 or a quarter-watt through-hole part is fine. On 24 V rails you often need half a watt or more.
Why is my LED dimmer than expected?
Usually the forward voltage is higher than you assumed, leaving less across the resistor and reducing current. It can also be a resistor that is too large, a supply sagging under load, or an LED being driven well below its rated current. Measure the actual voltage across the resistor and divide by its value to find the true current.
Do I need a resistor for a 3 V coin cell LED?
Strictly yes, though many throwie-style circuits omit it because the cell's own internal resistance — often tens of ohms — limits the current. That works but is not a design; the LED runs at an uncontrolled current that falls as the cell depletes.