LED Resistor Value Calculator
Calculate the current-limiting resistor for an LED from supply voltage, LED forward voltage, and forward current. Returns the exact resistance, the nearest standard E12/E24 value (rounded up), power dissipation, and a safe resistor wattage rating. Supports LED colour presets and multiple LEDs in series or parallel.
Input
Battery or power-source voltage.
Picks a typical forward voltage for common LED colours. Choose Custom to enter your own.
Voltage drop across each LED. Used when the preset is Custom; check your datasheet.
Target drive current. Most small LEDs are rated 20 mA.
Series LEDs share one resistor; parallel LEDs each get their own.
Output
| Property | Value |
|---|---|
| No data yet | |
Guides
Every LED needs a current-limiting resistor in series with it. An LED is a diode, not a linear load: connect it straight across a battery and it draws whatever current the supply can push, then burns out. A resistor sets that current to a safe value. This calculator sizes that resistor from three numbers — your supply voltage, the LED's forward voltage, and the forward current you want to drive — and also tells you the nearest real resistor you can buy and how much power it will dissipate.
How it works
The math is Ohm's law applied to the voltage the resistor has to "soak up":
- Resistor voltage = Supply voltage − LED forward voltage (for series strings, the forward voltages add up)
- Resistance R = Resistor voltage ÷ Forward current
- Power P = Resistor voltage × Forward current (equivalently P = I²R)
For example, powering one 2 V LED at 20 mA from a 9 V supply needs (9 − 2) ÷ 0.02 = 350 Ω, dissipating 7 V × 0.02 A = 0.14 W (140 mW).
Using the calculator
- Enter your supply voltage (battery or regulated rail).
- Pick an LED colour preset for a typical forward voltage, or choose Custom and type the exact Vf from your LED's datasheet — red is around 1.8–2.0 V, blue and white around 3.2–3.4 V.
- Set the forward current (20 mA is standard for indicator LEDs; high-brightness parts may want more — always check the datasheet).
- Choose how many LEDs and whether they're wired in series (one shared resistor) or parallel (one resistor per LED).
The result shows the exact required resistance, the nearest standard E12 and E24 preferred value, the power the resistor dissipates, and a minimum resistor wattage rating with a 2× safety margin so it won't run hot.
Why the standard value is rounded up
Resistors only come in fixed preferred values (the E12/E24 series). This tool rounds up to the next standard value at or above the calculated resistance. A slightly larger resistor runs the LED a touch dimmer; a smaller one pushes more than your target current through the LED and shortens its life — so up is the safe direction.
Series vs. parallel
In series, the LEDs' forward voltages add together, so the supply must exceed their combined drop — the calculator warns you when it can't. In parallel, each LED gets its own resistor; sharing a single resistor across parallel LEDs is discouraged because small forward-voltage differences make them draw uneven current.
Privacy
The calculation runs entirely in your browser. Nothing you enter is uploaded or stored.