Documentation sections

4–20 mA Loop Transmitter

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

Slug 4-20ma-transmitter. Calculate 4-20 mA current loop voltage budget, sensor value from loop current, and maximum loop resistance for industrial transmitter design.

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 4-20ma-transmitter
Name	Unit	Range	Default	Description
supplyVoltagenumber	V	at least 12	24	Loop Supply Voltage
loopCurrentnumber	mA	4 … 20	12	Loop Current
loopResistancenumber	Ω	at least 0	250	Total cable + load resistance (250 Ω = standard 1–5 V input)
sensorRangeMinnumber	none	at least −99999	0	Sensor Range Min
sensorRangeMaxnumber	none	at least −99999	100	Sensor Range Max
loopElementsnumber	none	0 … 6	0	How many further resistances sit in series in the loop (0–6)
element1Resistancenumber	Ω	0 … 10000	0	A further resistance in series in the loop: the cable pair, an intrinsic-safety barrier, a loop-powered indicator or a HART resistor
element2Resistancenumber	Ω	0 … 10000	0	A further resistance in series in the loop: the cable pair, an intrinsic-safety barrier, a loop-powered indicator or a HART resistor
element3Resistancenumber	Ω	0 … 10000	0	A further resistance in series in the loop: the cable pair, an intrinsic-safety barrier, a loop-powered indicator or a HART resistor
element4Resistancenumber	Ω	0 … 10000	0	A further resistance in series in the loop: the cable pair, an intrinsic-safety barrier, a loop-powered indicator or a HART resistor
element5Resistancenumber	Ω	0 … 10000	0	A further resistance in series in the loop: the cable pair, an intrinsic-safety barrier, a loop-powered indicator or a HART resistor
element6Resistancenumber	Ω	0 … 10000	0	A further resistance in series in the loop: the cable pair, an intrinsic-safety barrier, a loop-powered indicator or a HART resistor

Outputs

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

Outputs of 4-20ma-transmitter
Field	Unit	Description
sensorValueheadline	eng units	Sensor Value
voltageAtLoad	V	Voltage at Load
availableVoltage	V	Transmitter Voltage
maxLoopResistance	Ω	Max Loop Resistance
totalLoopResistance	Ω	The loop resistance and every further element in series

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": "4-20ma-transmitter",
    "inputs": {
      "supplyVoltage": 24,
      "loopCurrent": 12,
      "loopResistance": 250,
      "sensorRangeMin": 0,
      "sensorRangeMax": 100,
      "loopElements": 0,
      "element1Resistance": 0,
      "element2Resistance": 0,
      "element3Resistance": 0,
      "element4Resistance": 0,
      "element5Resistance": 0,
      "element6Resistance": 0
    }
  }'

import rftools

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

result = client.calculate("4-20ma-transmitter", {
    "supplyVoltage": 24,
    "loopCurrent": 12,
    "loopResistance": 250,
    "sensorRangeMin": 0,
    "sensorRangeMax": 100,
    "loopElements": 0,
    "element1Resistance": 0,
    "element2Resistance": 0,
    "element3Resistance": 0,
    "element4Resistance": 0,
    "element5Resistance": 0,
    "element6Resistance": 0,
})
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": "4-20ma-transmitter",
    "inputs": {
      "supplyVoltage": 24,
      "loopCurrent": 12,
      "loopResistance": 250,
      "sensorRangeMin": 0,
      "sensorRangeMax": 100,
      "loopElements": 0,
      "element1Resistance": 0,
      "element2Resistance": 0,
      "element3Resistance": 0,
      "element4Resistance": 0,
      "element5Resistance": 0,
      "element6Resistance": 0
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "4-20ma-transmitter",
	  "inputs": {
	    "supplyVoltage": 24,
	    "loopCurrent": 12,
	    "loopResistance": 250,
	    "sensorRangeMin": 0,
	    "sensorRangeMax": 100,
	    "loopElements": 0,
	    "element1Resistance": 0,
	    "element2Resistance": 0,
	    "element3Resistance": 0,
	    "element4Resistance": 0,
	    "element5Resistance": 0,
	    "element6Resistance": 0
	  }
	}`)
	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": "4-20ma-transmitter",
  "values": {
    "voltageAtLoad": 3,
    "availableVoltage": 21,
    "sensorValue": 50,
    "maxLoopResistance": 600,
    "totalLoopResistance": 250
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:4-20ma-transmitter",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "supplyVoltage": {
          "min": 12,
          "max": null,
          "unit": "V"
        },
        "loopCurrent": {
          "min": 4,
          "max": 20,
          "unit": "mA"
        },
        "loopResistance": {
          "min": 0,
          "max": null,
          "unit": "Ω"
        },
        "sensorRangeMin": {
          "min": -99999,
          "max": null,
          "unit": ""
        },
        "sensorRangeMax": {
          "min": -99999,
          "max": null,
          "unit": ""
        },
        "loopElements": {
          "min": 0,
          "max": 6,
          "unit": ""
        },
        "element1Resistance": {
          "min": 0,
          "max": 10000,
          "unit": "Ω"
        },
        "element2Resistance": {
          "min": 0,
          "max": 10000,
          "unit": "Ω"
        },
        "element3Resistance": {
          "min": 0,
          "max": 10000,
          "unit": "Ω"
        },
        "element4Resistance": {
          "min": 0,
          "max": 10000,
          "unit": "Ω"
        },
        "element5Resistance": {
          "min": 0,
          "max": 10000,
          "unit": "Ω"
        },
        "element6Resistance": {
          "min": 0,
          "max": 10000,
          "unit": "Ω"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "supplyVoltage": 24,
      "loopCurrent": 12,
      "loopResistance": 250,
      "sensorRangeMin": 0,
      "sensorRangeMax": 100,
      "loopElements": 0,
      "element1Resistance": 0,
      "element2Resistance": 0,
      "element3Resistance": 0,
      "element4Resistance": 0,
      "element5Resistance": 0,
      "element6Resistance": 0
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "supplyVoltage"
      ],
      "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 page4–20 mA Loop Transmitter, at these values