---
title: LED Resistor Calculator
description: Calculate a standard LED series resistor, compare current and heat loss, and check the required resistor power rating.
canonical: https://ledyi.com/tools/led-resistor-calculator
updatedAt: 2026-10-08T13:00:00Z
---

# LED Resistor Calculator

Calculate a standard LED series resistor, compare current and heat loss, and check the required resistor power rating.

## 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 — Full tool instructions (Markdown)](https://ledyi.com/tools/led-resistor-calculator.md)
- [OpenAPI contract](https://ledyi.com/openapi.json)
- [Agent site index](https://ledyi.com/llms.txt)

Your agent needs an HTTP tool permitted to send POST requests. Reading this page or importing OpenAPI does not automatically grant execution access.

### LED Resistor Calculator

```
POST /api/tools/v1/led-resistor
Content-Type: application/json
```

One 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 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.

| Field | Type and constraints | Meaning |
| --- | --- | --- |
| supplyVoltageV (Required) | {"type":"number","minimum":0.1,"maximum":60} | Nominal supply voltage in volts DC. |
| ledForwardVoltageV (Required) | {"type":"number","minimum":0.1,"maximum":60} | Forward voltage of the complete LED package at intended current and temperature. Package size does not determine Vf. |
| ledCount (Required) | {"type":"integer","minimum":-9007199254740991,"maximum":9007199254740991} | Whole LED packages in one series branch. A multi-chip package counts as one. |
| targetCurrentMa (Required) | {"type":"number","minimum":0.01,"maximum":5000} | Target branch current in milliamps, from the LED specifications. |
| maxSupplyVoltageV (Optional) | {"default":null,"anyOf":[{"type":"number","minimum":0.1,"maximum":60},{"type":"null"}]} | Maximum supply voltage, at least nominal; supply together with minLedForwardVoltageV or omit/null both. |
| minLedForwardVoltageV (Optional) | {"default":null,"anyOf":[{"type":"number","minimum":0.1,"maximum":60},{"type":"null"}]} | Minimum package forward voltage, at most nominal; supply together with maxSupplyVoltageV or omit/null both. |

```
{
  "supplyVoltageV": 24,
  "ledForwardVoltageV": 22,
  "ledCount": 1,
  "targetCurrentMa": 20
}
```

### Reading the result

Nominal drop = supplyVoltageV − ledCount × ledForwardVoltageV. With limits, upper drop = maxSupplyVoltageV − ledCount × minLedForwardVoltageV; otherwise upper drop equals nominal. Choose the smallest E24 value at least upper drop / ((targetCurrentMa / 1000) × (1 − 5/100)). Nominal current is nominal drop / resistance; upper current uses upper drop and minimum resistance. Loss = drop × current; requiredPowerRatingW = 2 × upper loss. Exact decimal input comparisons determine the selection; displayed/JSON numbers are rounded floating-point projections.

result contains resistanceOhms, nominalCurrentMa, upperCurrentMa, nominalResistorPowerW, upperResistorPowerW, requiredPowerRatingW and powerRatingW. The latter is the next listed wattage class, or null above the tool’s list: use requiredPowerRatingW and verify a suitable part. smdExample is a standard-power example, not stock availability or a universal footprint rating. alternatives contains the selected and next two E24 values, each recalculated. Loss fractions are 0–1. hasVoltageLimits states whether supply/Vf variation was included.

Missing values, numeric strings, unknown fields, non-integer LED counts, unpaired/reversed voltage limits and supply not above the total LED Vf return 422. They never become a default circuit. The constant-Vf model is not a complete LED current–voltage or thermal simulation.

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

Every success carries apiVersion, a server-generated requestId, operation, normalized inputs, result, warnings and links (tool, markdown, openapi, share). requestId also appears in X-Request-Id. The domain result is deterministic; the request ID changes. Preserve warnings and special statuses when explaining results.

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.

Source errors use application/problem+json with status, stable code, requestId and at most one invalidParams JSON pointer. 400: malformed JSON/query; 405: method; 408: body timeout; 413: body limit; 415: media/encoding; 422: parameter/domain error; 500/503: service failure. Fix invalid inputs rather than retrying or changing engineering facts. For 429 or transient failures, use at most two retries with backoff and jitter, honoring Retry-After when present.

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.

Calculations do not create inquiries, send emails, call a language model or store project results. Operational logs exclude fixture labels, groups and request bodies. Existing network/access logs still apply. Submit a quotation request only when the user asks, using the separate inquiry contract and buyer confirmation flow.

### Warning codes

| Field | Meaning |
| --- | --- |
| RESISTOR_HIGH_LOSS | At least half the branch electrical power is lost in the resistor at nominal or supplied voltage limits. Review the circuit design. |
| RESISTOR_HIGH_POWER | The required rating exceeds the common standard-power SMD examples. Check a suitable power resistor and cooling. |
| RESISTOR_NO_SMD_EXAMPLE | No example meets the selected resistance, power and voltage requirements. Verify an appropriate real component. |

### Error codes

```
TOOLS_INVALID_JSON
TOOLS_QUERY_UNSUPPORTED
TOOLS_METHOD_NOT_ALLOWED
TOOLS_BODY_TIMEOUT
TOOLS_BODY_TOO_LARGE
TOOLS_MEDIA_TYPE_UNSUPPORTED
TOOLS_CONTENT_ENCODING_UNSUPPORTED
TOOLS_INPUT_INVALID
TOOLS_NUMERIC_DOMAIN_UNSUPPORTED
TOOLS_INTERNAL_ERROR
TOOLS_SERVICE_UNAVAILABLE
DMX_INPUT_INVALID
DMX_ADDRESS_NOT_ENCODABLE
DMX_CHANNEL_RANGE_EXCEEDED
DMX_SPACING_OVERLAP
DMX_UNIVERSE_RANGE_EXCEEDED
```

## 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.