Enter your supply voltage, the LED’s forward voltage and current to get the series resistor value — and the wattage it must handle.
What Is an LED Resistor Calculator?
An LED resistor calculator sizes the series resistor: (9 − 2) V ÷ 20 mA = 350.00 Ω, dissipating 0.14 W — so a ½ W resistor is the safe choice (at least double the 0.14 W dissipated). LEDs are current devices: without a resistor they draw until they burn out. The resistor drops the excess voltage at the chosen current.
How to Use
- Enter the supply voltage.
- Enter the LED forward voltage (from its datasheet — ~2 V red, ~3.2 V white/blue).
- Enter the LED current in mA.
- Click “Calculate Resistor”.
Formula
R = (supply − forward V) ÷ current. P = I² × R.
Worked Example
9 V supply, 2 V LED, 20 mA: (9 − 2) ÷ 0.02 = 350.00 Ω; power 0.14 W → use a ½ W resistor.
How It Works, In Plain Words
The maths is Ohm’s law applied to whatever voltage the LED does not use. An LED holds roughly its forward voltage across itself — about 2 V for the red LED in the example — and the resistor’s job, in the introduction’s words, is to drop the excess voltage at the chosen current. So step one subtracts: 9 V supply − 2 V forward = 7 V left for the resistor. Step two converts the current: the box takes milliamps, and 20 mA is 0.02 A in the formula. Step three divides: R = (supply − forward V) ÷ current = 7 ÷ 0.02 = 350.00 Ω. Nothing else is hidden in the tool: change any one of the three inputs and only that subtraction or division moves.
The second half sizes the resistor’s body, not just its value. Power is P = I² × R, here 0.02² × 350 = 0.14 W turned into heat inside the resistor. A resistor run at its full rating runs hot and ages fast, so the page’s rule is to use at least double the dissipated power — 0.14 W → a ½ W resistor. The tool also refuses nonsense: current must be above zero, and the supply must exceed the forward voltage, because with nothing left to drop there is no resistor value that makes the circuit work as intended.
Real-World Uses
A hobbyist wiring a single indicator LED from a 9 V battery — the exact setup in the worked example — uses the tool to learn they need 350.00 Ω at ½ W before opening the parts drawer, instead of guessing and watching the LED flash once and die, the burnout the introduction warns about. A student building a first breadboard circuit uses the colour guidance in the steps: swap the red LED for a white one at ~3.2 V forward and the same supply needs a smaller resistor, which the tool shows instantly when the forward-voltage box changes. A maker adding LEDs to a model or costume checks the wattage line as carefully as the ohms: the value says what to buy, and the “use at least” line says the job is not finished until the resistor’s rating clears the doubled-power rule.
Benefits
- Resistor value and wattage together
- The wattage recommendation included
- Forward-voltage guidance by LED colour
- Useful for hobby electronics
Features of Our Tool
- Instant results in your browser
- No sign-up, no download, 100% free
- Works on mobile, tablet and desktop
- Clear step-by-step explanation below the tool
- Private — your entries never leave your device
Common Mistakes
- Skipping the resistor — the LED will burn out
- Using the wrong forward voltage (colours differ)
- Undersizing wattage — the resistor overheats
Common Mistakes to Avoid
The mistakes list covers skipping the resistor, using the wrong forward voltage and undersizing the wattage. In prose, three more. Do not copy a resistor value from someone else’s circuit without checking their supply and LED: 350.00 Ω is right for 9 V, 2 V and 20 mA together, and wrong the moment any one of the three changes. Do not treat the calculated value as the only purchasable one — the bigger-resistor FAQ gives the safe direction: a little larger merely dims the LED slightly, while smaller pushes current up towards burnout. And do not forget the current box is milliamps: entering amps by mistake shrinks the answer a thousandfold. A safety note: this tool is for low-voltage hobby circuits like the 9 V example; anything involving mains electricity belongs to a qualified electrician, not a calculator.
FAQ
What resistor for a 9 V LED circuit?
(9 − 2) ÷ 0.02 = 350.00 Ω with a ½ W rating.
What is LED forward voltage?
~2 V for red, ~2.1 V green, ~3.2 V white/blue — check the datasheet.
Can I use a bigger resistor?
Yes — the LED just gets dimmer; smaller risks burnout.
Why does the LED need a resistor?
LEDs are current devices — without one they draw unlimited current and burn out.
What wattage resistor?
At least double the dissipated power — here 0.14 W → ½ W.
My resistor value is not one I can buy. What now?
Round up to the nearest value you can get. The bigger-resistor FAQ gives the direction: a larger resistor only makes the LED a little dimmer, while a smaller one raises the current towards the burnout this page warns about. Keep the wattage rule from the worked example as well — at least double the dissipated power, 0.14 W → ½ W here.
Why does the tool ask for current in mA but divide by 0.02?
Milliamps are how LED currents are usually written — 20 mA in the example — but the formula R = (supply − forward V) ÷ current works in amps, and 20 mA is 0.02 A. The tool converts for you behind the scenes; just type the milliamps figure from the LED’s datasheet, exactly as the steps describe.