LED Resistor Calculator
Calculate the current-limiting resistor for any LED from supply voltage, forward voltage, and target current.
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What is LED Resistor Calculator?
The LED Resistor Calculator is a free online tool that computes the current-limiting resistor value for LED circuits. It applies Ohmβs law, R = (Vs β N Γ Vf) / I, using your supply voltage, LED forward voltage, target current, and the number of LEDs in series, then snaps the result to the closest E24 standard resistor and reports actual current, power dissipation, recommended wattage rating, efficiency, and the 4-band color code. Everything is computed locally in your browser β nothing is uploaded.
Key Benefits
- Prevents burnt-out LEDs by sizing the series resistor correctly the first time
- Bridges theory and your parts bin: ideal value, E24 standard value, and color code in one view
- Recommends a safe wattage rating with a 2Γ power margin so resistors stay cool
- Handles multi-LED series strings up to 20 LEDs with total forward-voltage checks
- Works offline on any device β no install, no account, no data leaving your browser
Common Use Cases
- β’Hobby electronics: adding a status LED to an Arduino or Raspberry Pi project
- β’Prototyping: quickly sizing resistors for LED indicators on breadboards
- β’Education: verifying Ohmβs law and series-circuit homework with step-by-step math
- β’Automotive and DIY lighting: 12 V LED strips, panel lights, and custom indicators
- β’Repair work: replacing a burnt resistor and double-checking the original design value
How to Use the LED Resistor Calculator
- Enter the supply voltage: Type your supply voltage (e.g. 5 V for USB, 3.3 V for a microcontroller GPIO, 12 V for automotive) or tap one of the common presets.
- Set the LED forward voltage: Enter the forward voltage from the LED datasheet, or tap a color preset β red ~1.8 V, green ~2.1 V, blue/white ~3.0β3.2 V.
- Choose the target current: Pick the drive current: 20 mA is typical for a standard 5 mm LED; 2β5 mA is enough for indicator use. Add the number of LEDs wired in series if more than one.
- Read the result and pick the part: Copy the closest E24 standard resistor value, check the actual current and the recommended wattage rating, then match the 4-band color code from your parts bin.
Key Features
- Ideal resistor from supply voltage, LED forward voltage, current, and series LED count
- Closest E24 standard value with the actual current you will really get
- Power analysis: resistor dissipation, recommended wattage rating, LED power, and efficiency
- Visual 4-band color code to pick the resistor straight from your parts bin
- LED color presets (red, amber, green, blue, white, IR) and common supply rails (3.3Vβ24V)
- Copyable step-by-step calculation summary β 100% client-side, works offline
Frequently Asked Questions
What resistor do I need for a 5 mm LED on 5 V?
For a typical red LED (Vf β 1.8 V) at 20 mA on a 5 V supply: R = (5 β 1.8) / 0.02 = 160 Ξ©. The closest E24 values are 150 Ξ© (21.3 mA) and 160 Ξ© exactly, so 150β160 Ξ© works. For a blue or white LED (Vf β 3.2 V) use 100 Ξ© for about 18 mA.
Can I connect an LED without a resistor?
No β an LED is a diode with an exponential current-voltage curve. Without a current-limiting resistor (or a proper constant-current driver), even a small voltage excess causes current to spike and destroys the LED. The only safe exception is LEDs with a built-in resistor, which are sold specifically for a stated voltage.
Why does the calculator recommend a wattage rating with a 2Γ margin?
Resistors running near their rated power get very hot and drift in value or fail early. Running at no more than 50% of the rated power keeps the temperature rise safe. For example, if the resistor dissipates 0.11 W, the tool recommends a 0.25 W part, not a 0.125 W one.
How do I wire multiple LEDs β series or parallel?
Series is preferred: one resistor limits the same current through all LEDs, and they share brightness evenly. Set "LEDs in series" to the count and make sure the supply voltage exceeds the total forward voltage (N Γ Vf) plus some headroom for the resistor. For parallel, give each LED (or each series string) its own resistor β a single shared resistor causes uneven current sharing.