LED Resistor Calculator
Size the current-limiting resistor for an LED from your supply voltage, the LED forward voltage and the current you want, with the nearest standard value and the wattage to buy.
Use this LED series resistor calculator to work out the current-limiting resistor an LED needs so it lights safely instead of burning out. Enter the supply voltage, pick the LED colour or type its forward voltage, set the current in milliamps and the number of LEDs in series, and read the resistor value straight away. It gives the exact ohms, the nearest standard E12 or E24 value to actually buy, and the power the resistor dissipates so you can choose the right wattage. Everything runs in your browser.
LED & supply
Resistor (nearest E24)
160 Ω
Exact value 160 Ω
- Exact resistance
- 160 Ω
- Power dissipated
- 0.06 W
- Min. wattage rating
- 0.25 W
- Resistor voltage
- 3.2 V
Put one resistor in series with the LED string. A higher forward current is brighter but shortens LED life; 20 mA is a safe default for standard 5 mm LEDs.
Forward voltage varies part to part and with temperature, so read the actual Vf from your LED datasheet. Pick a resistor rated above the dissipated power, and round up rather than down to keep the current within the LED's limit.
How it works
- 1
Enter the supply voltage
Type the source voltage: 5 V from USB, a 9 V battery, a 12 V rail or whatever powers the circuit.
- 2
Set the LED and current
Pick the LED colour for a typical forward voltage, or type the exact Vf from the datasheet, then set the forward current (20 mA is a safe default).
- 3
Read the resistor value
See the exact ohms, the nearest standard resistor to buy, the power it dissipates and the minimum wattage rating.
Instant & 100% private — nothing is uploaded
Every calculation runs locally in your browser. The income, balances and goals you enter stay on your own device and are never sent to a server — nothing is stored, logged or shared.
Frequently asked questions
- How do I calculate the resistor value for an LED?
- Subtract the LED forward voltage from the supply voltage, then divide by the current in amps: R = (Vsupply − Vf) / I. For a 5 V supply, a 2 V LED and 20 mA that is (5 − 2) / 0.02 = 150 Ω. The calculator does this and rounds to the nearest standard part.
- What resistor do I need for a 5 V supply?
- It depends on the LED. A red LED (about 1.8 V) at 20 mA needs roughly 160 Ω, while a blue or white LED (about 3.2 V) needs about 90 Ω. Enter your LED colour and current and the tool gives the exact value and the nearest standard resistor.
- What wattage resistor should I use?
- Look at the power the resistor dissipates, P = I² × R. Most LED resistors dissipate well under a tenth of a watt, so a common 1/8 W or 1/4 W resistor is fine. The calculator flags the dissipation and suggests a rating with a safety margin.
- How do I wire several LEDs with one resistor?
- Put the LEDs in series and add up their forward voltages, then size one resistor for the whole string. Set the number of LEDs in series and the tool subtracts the combined forward voltage before working out the current-limiting resistor.
- Why does the tool say the supply is too low?
- If the combined LED forward voltage is higher than the supply voltage, there is no headroom left for a resistor and the LEDs will not light. Raise the supply voltage or reduce how many LEDs you place in series.
- What if I do not know the LED forward voltage?
- The colour presets give typical figures: about 1.8 V for red, 2.1 V for yellow and green, and 3.2 V for blue and white. For an accurate resistor value, read the real Vf from the LED datasheet and type it in the custom field.
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