LED Resistor Calculator
The right resistor for your LEDs, its wattage, the real current and the power wasted.
The same circuit in each E-series
| Series | Resistor | Current | Resistor power |
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How it was calculated
About the LED Resistor Calculator
An LED needs something to limit its current, and for small LEDs that is a series resistor: whatever voltage the LEDs do not drop appears across it, so R = (Vs − n × Vf) / If. Enter the supply voltage, the LED forward voltage (pick a colour preset or type the value from your datasheet), the current and how many LEDs you have in series and in parallel strings.
The calculator rounds the resistor to a standard E-series value — up by default, so the current stays at or below what you asked for — and shows the real current, the power in the resistor with a recommended wattage, the power in the LEDs, the total current from the supply and the efficiency. It also warns when the supply leaves too little voltage for the resistor, which makes the current depend on the LEDs’ exact forward voltage.
How to use it
- Enter the supply voltage — for example 5 V USB, 9 V battery or 12 V.
- Choose the LED colour (the forward voltage fills in from a datasheet) or type the forward voltage of your LED, and enter the current: 20 mA suits a standard 5 mm LED; modern bright LEDs often look fine at 2–5 mA.
- Enter the number of LEDs in series in each string and the number of strings in parallel (each string gets its own resistor), and choose the E-series and rounding.
- Read the resistor value, its wattage and the real current. To check a resistor you already have, switch to Check a resistor and type its value.
Examples
Vs 5 V, Vf 2.0 V, 20 mA
150 Ω (E12), 20 mA, 60 mW → 1/8 W resistor, 40% efficient
Vf 3.3 V × 3, 20 mA
exact 105 Ω → 120 Ω (E12), 17.5 mA, 82.5% efficient
12 V, 4 strings of 3 white LEDs
four 120 Ω resistors, 70 mA from the supply
5 V, two 3.3 V LEDs in series
Error: 6.6 V needed — at most 1 LED per string
9 V, red LED (1.9 V), 330 Ω
21.5 mA, 152 mW in the resistor → 1/2 W
How it is calculated
- Resistor (Kirchhoff’s voltage law): R = (Vs − n × Vf) / If, for n LEDs in series.
- Real current with the standard value: I = (Vs − n × Vf) / R.
- Resistor power: P = I² × R; the recommended rating is the next standard size of at least twice that, a common margin that keeps the resistor cool.
- LED power: Vf × I per LED. Supply current: I × number of strings.
- Efficiency: the share of the supply power that reaches the LEDs, n × Vf / Vs. The rest is heat in the resistor — more LEDs in series per string, or a lower supply voltage, waste less.
Forward voltage presets
The colour presets are the typical forward voltages of 5 mm indicator LEDs from Kingbright’s WP7113 datasheets, measured at 10 or 20 mA (dominant wavelengths; peak for the infrared): infrared (940 nm) 1.2 V, red (617 nm) 1.9 V, orange 2.0 V, yellow 1.95 V, yellow-green (570 nm) 2.1 V, and InGaN green (525 nm), blue and white 3.3 V. The same datasheets give maximum forward voltages 0.4–0.8 V higher, and maximum continuous currents of 25–30 mA for the visible colours (50 mA for the infrared one). Forward voltage varies with the LED type, the current and temperature, so use your own LED’s datasheet when you have it.
Leave enough voltage for the resistor
Forward voltage varies from LED to LED. If the resistor only gets a few tenths of a volt, a 0.1 V difference in Vf changes the current a lot — the calculator shows by how much and warns above 20%. A larger margin (a higher supply or fewer LEDs per string) makes the current predictable. For high-power LEDs or long strings, a constant-current driver is the better choice.
Parallel strings
Give each parallel string its own resistor. LEDs wired straight in parallel to one resistor do not share the current evenly: the one with the lowest forward voltage takes the most, gets hotter, and its forward voltage falls further.
Sources
- Kirchhoff’s voltage law: R = (Vs − n·Vf) / If
- IEC 60063:2015, Preferred number series for resistors and capacitors (E-series)
- Kingbright WP7113-series 5 mm LED datasheets (WP7113F3C, ID, SED, YD, CGCK, ZGCK, QBC/D, QWC/D): typical and maximum forward voltage, maximum current
Limitations
- Forward voltage is taken as fixed; in reality it rises a little with current and falls with temperature, so the real current differs slightly. Measure it if it matters.
- For power LEDs (hundreds of milliamps), a resistor wastes a lot of power and the LED needs a heatsink — use a constant-current driver instead.
- LEDs survive only a few volts in reverse (5 V in the Kingbright datasheets). On AC or a reversible supply, add a reverse diode.
- The wattage rule of thumb assumes normal room temperature and free air; resistors in a closed box or on a hot board need more margin (see the resistor datasheet’s derating curve).
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Frequently asked questions
What resistor do I need for an LED on 5 V?
For a red LED (about 2 V) at 20 mA: (5 − 2) / 0.02 = 150 Ω. For a white or blue LED (about 3.3 V): (5 − 3.3) / 0.02 = 85 Ω, so use 100 Ω (17 mA). A 1/4 W resistor is plenty for either.
What resistor for an LED on 12 V?
One red LED at 20 mA needs (12 − 2) / 0.02 = 500 Ω → 560 Ω, dissipating about 0.18 W, so use a 1/2 W part. Putting three or four LEDs in series on 12 V wastes much less power in the resistor.
Should I round the resistor up or down?
Up. A slightly larger resistor gives slightly less current, which the LED will not notice, while a smaller one pushes it above the current you planned.
Can several LEDs share one resistor?
In series, yes — one resistor per string. In parallel, no: each parallel LED or string needs its own resistor, or the current crowds into the LED with the lowest forward voltage.
What wattage resistor do I need?
Work out P = I² × R and choose a rating at least twice as large. 20 mA through 150 Ω is 60 mW, so a 1/8 W or 1/4 W resistor is fine.