liminfoLED Resistor Calculator
Free web tool: LED Resistor Calculator
Resistor
140.0 \u2126
Nearest E24
130 \u2126
Power
56.0 mW
Recommended
1/8W
About LED Resistor Calculator
The LED Resistor Calculator computes the current-limiting resistor needed to safely drive an LED from a given supply voltage. Enter the supply voltage (Vs), LED forward voltage (Vf), and desired forward current (If in mA), and the tool instantly applies Ohm's law in the form R = (Vs − Vf) / If. It also calculates the power dissipated in the resistor as P = If² × R so you can select an appropriately rated component and avoid overheating.
Electronics hobbyists wiring LEDs to microcontrollers such as Arduino and Raspberry Pi, students learning basic circuit design, and engineers prototyping indicator lights will all find this tool useful. Quick-select buttons for common supply voltages (3.3 V, 5 V, 9 V, 12 V, 24 V) and LED color presets (Red 1.8 V, Green 2.2 V, Blue/White 3.2 V, IR 1.2 V) let you populate the inputs in one click, covering the most common LED driving scenarios.
After computing the exact resistor value, the calculator finds the nearest E24 series standard resistor value — the closest value from the 24-resistor-per-decade IEC standard series. It also recommends the minimum resistor wattage rating (choosing from standard values 1/8 W, 1/4 W, 1/2 W, 1 W, 2 W, 5 W) by applying a safety factor of 2× the calculated dissipation, which is a standard engineering practice for thermal reliability. All calculations run entirely in the browser with no server required.
Key Features
- Calculates current-limiting resistor R = (Vs − Vf) / If using Ohm's law
- Computes resistor power dissipation P = If² × R in milliwatts
- Finds nearest E24 standard series resistor value for real-world component selection
- Recommends minimum wattage rating with 2× safety factor from standard values (1/8 W to 5 W)
- One-click supply voltage presets: 3.3 V, 5 V, 9 V, 12 V, 24 V
- LED forward voltage presets: Red (1.8 V), Green (2.2 V), Blue (3.2 V), White (3.2 V), IR (1.2 V)
- Input validation — returns no result when Vs ≤ Vf or If ≤ 0 to prevent invalid calculations
- Dark mode support and responsive layout for desktop, tablet, and mobile
Frequently Asked Questions
What is a current-limiting resistor and why does an LED need one?
An LED has a very low dynamic resistance, which means without a current-limiting resistor, the current through it is controlled only by the small forward voltage drop (typically 1.8–3.2 V). Connecting an LED directly to a voltage source that exceeds Vf will cause excessive current and instantly destroy the LED. The resistor limits current to the rated forward current, typically 20 mA for standard 5 mm LEDs.
What formula does the LED Resistor Calculator use?
The calculator uses R = (Vs − Vf) / If, derived from Kirchhoff's voltage law. Vs is the supply voltage, Vf is the LED forward voltage, and If is the desired forward current in amperes (converted from the mA input). Power dissipation is computed as P = If² × R.
What is the E24 resistor series?
E24 is an IEC standard series of 24 preferred resistor values per decade (e.g. 1.0, 1.1, 1.2, 1.3, 1.5 ... 9.1). Real resistors are manufactured in these values with ±5% tolerance. The calculator finds the E24 value closest to the exact computed resistance so you can order a standard component.
Why does the calculator recommend 2× the calculated power?
Operating a resistor near its rated power limit causes it to run hot, which reduces reliability and lifespan. The standard engineering practice is to derate by 50%, meaning you should select a resistor with a wattage rating of at least twice the calculated dissipation. The calculator applies this rule automatically.
What is the typical forward voltage for common LED colors?
Typical forward voltages are: infrared (IR) ≈ 1.2 V, red ≈ 1.8 V, yellow/green ≈ 2.0–2.2 V, blue ≈ 3.2 V, white ≈ 3.2 V. These vary by manufacturer and brightness, so check your LED datasheet for the exact Vf at your target current.
How do I use this calculator for a 5 V Arduino and a red LED?
Set supply voltage to 5 V, LED forward voltage to 1.8 V (use the Red preset), and forward current to 20 mA. The calculator computes R = (5 − 1.8) / 0.02 = 160 Ω. The nearest E24 value is 160 Ω. Power = 0.02² × 160 = 64 mW, so a 1/4 W or larger resistor is recommended.
Can I use this calculator for multiple LEDs in series?
For LEDs in series, add up all the forward voltages (e.g. 3 red LEDs = 3 × 1.8 = 5.4 V total Vf) and enter that sum as Vf. You will need a supply voltage higher than the total Vf. The same R = (Vs − Vf_total) / If formula applies.
What happens if the supply voltage is equal to or less than the LED forward voltage?
The calculator returns no result and shows nothing. This is by design — if Vs ≤ Vf, the LED cannot turn on and the resistor formula produces zero or negative resistance, which is physically meaningless.