Documentation sections

Resistor Color Code Calculator

POSThttps://rftools.io/api/py/v1/calculate

Slug resistor-color-code. Decode resistor color bands to resistance value and tolerance. Supports 4-band and 5-band resistors; the 6th (temperature-coefficient) band of a 6-band resistor is not decoded. Instant color band to ohms conversion.

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of resistor-color-code
Name	Unit	Range	Default	Description
numBandsnumber	none	4 … 6	4	Number of Bands
band1number	none	0 … 9	1	Band 1 (1st digit)One of: 0 Black (0) · 1 Brown (1) · 2 Red (2) · 3 Orange (3) · 4 Yellow (4) · 5 Green (5) · 6 Blue (6) · 7 Violet (7) · 8 Grey (8) · 9 White (9)
band2number	none	0 … 9	0	Band 2 (2nd digit)One of: 0 Black (0) · 1 Brown (1) · 2 Red (2) · 3 Orange (3) · 4 Yellow (4) · 5 Green (5) · 6 Blue (6) · 7 Violet (7) · 8 Grey (8) · 9 White (9)
band3number	none	0 … 11	4	The third digit of a 5- or 6-band resistor. Ignored for 4-band resistors, whose third band is the multiplier band.One of: 0 Black (0) · 1 Brown (1) · 2 Red (2) · 3 Orange (3) · 4 Yellow (4) · 5 Green (5) · 6 Blue (6) · 7 Violet (7) · 8 Grey (8) · 9 White (9)
multipliernumber	none	0 … 11	2	Multiplier BandOne of: 0 Black (×1) · 1 Brown (×10) · 2 Red (×100) · 3 Orange (×1k) · 4 Yellow (×10k) · 5 Green (×100k) · 6 Blue (×1M) · 7 Violet (×10M) · 8 Grey (×100M) · 9 White (×1G) · 10 Gold (×0.1) · 11 Silver (×0.01)
tolerancenumber	none	1 … 11	10	Tolerance BandOne of: 1 Brown (±1%) · 2 Red (±2%) · 3 Orange (±0.05%) · 4 Yellow (±0.02%) · 5 Green (±0.5%) · 6 Blue (±0.25%) · 7 Violet (±0.1%) · 8 Grey (±0.05%) · 10 Gold (±5%) · 11 Silver (±10%)

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of resistor-color-code
Field	Unit	Description
resistanceheadline	Ω	Resistance
tolerance	%	Tolerance
minResistance	Ω	Min Value
maxResistance	Ω	Max Value

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "resistor-color-code",
    "inputs": {
      "numBands": 4,
      "band1": 1,
      "band2": 0,
      "band3": 4,
      "multiplier": 2,
      "tolerance": 10
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("resistor-color-code", {
    "numBands": 4,
    "band1": 1,
    "band2": 0,
    "band3": 4,
    "multiplier": 2,
    "tolerance": 10,
})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "resistor-color-code",
    "inputs": {
      "numBands": 4,
      "band1": 1,
      "band2": 0,
      "band3": 4,
      "multiplier": 2,
      "tolerance": 10
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "resistor-color-code",
	  "inputs": {
	    "numBands": 4,
	    "band1": 1,
	    "band2": 0,
	    "band3": 4,
	    "multiplier": 2,
	    "tolerance": 10
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "resistor-color-code",
  "values": {
    "resistance": 1000,
    "tolerance": 5,
    "minResistance": 950,
    "maxResistance": 1050
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:resistor-color-code",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "numBands": {
          "min": 4,
          "max": 6,
          "unit": ""
        },
        "band1": {
          "min": 0,
          "max": 9,
          "unit": ""
        },
        "band2": {
          "min": 0,
          "max": 9,
          "unit": ""
        },
        "band3": {
          "min": 0,
          "max": 11,
          "unit": ""
        },
        "multiplier": {
          "min": 0,
          "max": 11,
          "unit": ""
        },
        "tolerance": {
          "min": 1,
          "max": 11,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "numBands": 4,
      "band1": 1,
      "band2": 0,
      "band3": 4,
      "multiplier": 2,
      "tolerance": 10
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "numBands"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator pageResistor Color Code Calculator, at these values