Tools
LED Resistor Calculator
Choose a standard resistor. Check current, heat loss and the power rating you need.
Your circuit
One resistor and one series branch. Example LED: SMD 2835.
A finished constant-voltage strip normally already has current regulation. This tool is for a known LED branch.
Your result
At entered voltagesCheck your circuit
Calculated E24 resistor
Your circuit at a glance
At nominal valuesOne series branch. Each LED symbol represents one complete package.
12 volt DC supply connected in a closed series loop through 3 LED packages at 3 volts each, then a 160 Ω resistor and back to the negative supply terminal. LEDs use 9 volts in total. The resistor drops 3 volts. Estimated current is 18.8 milliamps throughout the branch.
Heat in the resistor
Actual loss, separate from the part’s wattage rating.
Electrical input power split, not light-output efficiency.
Upper calculated resistor loss: 0.0592 W at −5% resistance.
Temperature depends on the part, PCB and cooling. This calculator does not predict surface temperature.
Resistor specification
160 Ω · 0.125 W or higher · ±5% or tighter
Resistor footprint example: 0805 (2012 metric), 0.125 W.
Footprint alone does not specify power. Check the exact part’s rating and temperature derating.
Nearby values & resistor sizes
The selected value and two larger E24 options. More resistance gives less current.
| Resistance | Current | Rating ≥ | SMD example |
|---|---|---|---|
| 160 Ω | 18.8 mA | 0.125 W | 0805 |
| 180 Ω | 16.7 mA | 0.125 W | 0805 |
| 200 Ω | 15 mA | 0.125 W | 0805 |
Inch package codes; metric sizes in brackets. These standard-power examples are rated at 70°C. Higher-power variants also exist. Confirm your supplier’s exact part.
A single HTML file with your current settings. Open it in a browser.
Calculation & assumptions
- Nominal resistor voltage: 12 − 3 × 3 = 3 V.
- Resistance with ±5% allowance: 3 V ÷ (0.02 A × 0.95) = 157.895 Ω. Round up to E24: 160 Ω.
- Nominal current: 3 V ÷ 160 Ω = 18.75 mA. Upper calculated current: 19.74 mA.
- Nominal resistor loss: 3² ÷ 160 = 0.0563 W. Upper loss: 3² ÷ (160 × 0.95) = 0.0592 W. Apply 2×: at least 0.118 W; choose 0.125 W or higher.
Supply and LED Vf are held at the nominal values entered. Only resistor resistance tolerance is included. Use Check voltage limits to include maximum supply and minimum Vf.
The LED Vf is treated as a specified voltage, not a full current–voltage curve. Use the datasheet at the intended current and temperature; verify the finished circuit.
A 2× power allowance is a starting point. PCB temperature, enclosure and the part’s derating curve may require more.
Supply and LED Vf variation are not included. Add voltage limits before selecting parts for a real circuit.
High-Vf LEDs & shorter cutting unitsHow one package can replace several LEDs in a branch.
A higher-Vf LED package can contain several chips connected in series. A suitable strip design can then use fewer packages between cutting points, including one package per cut.
12 V · three packages
3 × 3 V = 9 V across the LEDs
12 V · one package
1 × 9 V = 9 V across the LED
Both examples leave 3 V across the resistor. At the same current, their resistor loss is the same. The benefit here is fewer LED packages in the cutting unit.
A 22 V LED on a 24 V supply
One package leaves 2 V across the resistor. Only 8.3% of nominal branch power is lost in that resistor, but small supply or Vf changes now have a larger effect on current. Use the optional voltage-limit check with your actual specifications.
These are illustrative circuits, not a finished product specification. Actual cut length depends on the PCB pitch and circuit layout. A “22 V LED” describes the component’s forward voltage, not the supply voltage.
Good to know
Why is the resistor losing so much power?
A series resistor drops the voltage that the LEDs do not use. Its power loss is voltage drop × current. For one 3 V LED on 24 V, the resistor drops 21 V and takes 87.5% of the branch’s electrical input power.
The high-loss notice appears when at least half of that power is dissipated in the resistor, at nominal values or your entered voltage limits. This is a design prompt, not a temperature limit. Using a larger-wattage resistor does not lower the loss.
Does my finished strip need an extra resistor?
Finished constant-voltage strips normally already include resistors or other current regulation. Match the power supply to the strip’s rated voltage.
This calculator checks one known LED branch. It is not intended to reduce the voltage of an entire finished strip. Each parallel LED branch normally needs its own current regulation.
Which LED voltage and current should I enter?
Use the exact LED datasheet at the intended current and temperature. Names such as 2835 and 5050 describe package dimensions, not one fixed voltage or current.
The Vf dropdown contains examples. For a multi-chip package, use the whole package’s Vf and count it once. For power LEDs and COBs, check whether a constant-current driver is required.
Why is the current below my target?
The tool rounds resistance up to an E24 standard value and includes a ±5% resistor tolerance. This keeps the calculated upper current at or below your target within the entered voltage assumptions.
With optional voltage limits, the resistor is sized for maximum supply voltage and minimum Vf, so current at nominal values can be lower. A resistor cannot regulate current as closely as a suitable constant-current driver.
What resistor should I buy?
Match the resistance, rated power, tolerance and PCB footprint. A 1% resistor of the calculated value also satisfies the tool’s 5% resistance-tolerance allowance.
The SMD examples are common standard-power classes. The same footprint can have different ratings. Check the actual series and derating curve, especially inside a hot or sealed strip.
Can I save this calculation for later?
Copy the result, share a link, or use Save results to print a PDF or download an offline HTML copy. The copy keeps your current settings and calculation logic.
On a phone, check that your browser can open local HTML files before relying on the offline copy.
For AI agents: instructions and calculation APIs
Read the tool instructions in Markdown and the request/response schemas in OpenAPI. These public calculation APIs run the same models as the web and offline calculators. No account, API key or Turnstile is required.
LED Resistor Calculator
POST /api/tools/v1/led-resistor
Content-Type: application/jsonOne resistor in one series LED branch. Enter forward voltage for the complete LED package and target current in milliamps. SMD 2835 describes the LED dimensions, not voltage/current; resistor footprints are separate. This is not a voltage-reduction tool for a finished strip.
led-resistor
LED Resistor Calculator
Supply the four circuit values. Optionally provide both maxSupplyVoltageV and minLedForwardVoltageV; omit/null both to use nominal voltages. The result includes the selected E24 resistance, current, loss, rating, nearby values and design warnings.
{
"supplyVoltageV": 24,
"ledForwardVoltageV": 22,
"ledCount": 1,
"targetCurrentMa": 20
}Open links.share to restore the inputs in the web calculator. Preserve high-loss/high-power warnings; a larger wattage rating does not reduce loss. Electrical power split is not optical efficiency. Verify exact LED/resistor datasheets, PCB temperature and derating. The diagram shows electrical connections, not a guaranteed cut length. Copying, QR, printing and offline HTML are browser exports using the same model.
Request limits and errors
Send uncompressed application/json with strict types and no query parameters. Voltage bodies are limited to 16 KiB; resistor bodies to 16 KiB; DMX bodies to 1024 KiB and 1000 fixtures. Body reading expires after 5 seconds. Complete success JSON is bounded to 4 MiB. GET/HEAD return 405; OPTIONS returns 204. Source responses are no-store. Anonymous browser CORS permits POST/OPTIONS and Content-Type with credentials omitted.
Tool POST requests and ordinary page requests share 120 requests per 60 seconds per IP/Cloudflare location; mitigation lasts 60 seconds. Heavy tool use can temporarily block pages from the same source. This is not an independent or globally exact API quota. Inquiry/search use their existing separate rule. Tool POST requests do not inherit the company-page or verified-bot exemption.
Cloudflare can return 403/429/5xx before the application, sometimes as HTML without CORS or Retry-After. Check HTTP status and Content-Type before parsing JSON. A browser may see only a network error. Do not present an error or an older result as a successful new calculation.
Need a strip with a shorter cutting unit?
Tell us your voltage, installation length and cutting requirements.