Documentation sections

Strain Gauge Bridge Calculator

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

Slug strain-gauge-bridge. Calculate bridge output voltage for strain gauges in quarter, half, and full bridge configs. Determine mV output and resistance change for load cells.

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 strain-gauge-bridge
Name	Unit	Range	Default	Description
vinnumber	V	0.1 … 30	5	Bridge excitation (supply) voltage
gaugeFactornumber	none	0.1 … 200	2.1	Gauge factor (sensitivity) of the strain gauge. Metal foil: ~2, semiconductor: 50–200.
strainnumber	με	0 … 50000	1000	Strain in microstrain (με). 1000 με = 0.1% elongation.
bridgeConfignumber	active arms	1 … 4	1	Number of active gauge armsOne of: 1 Quarter bridge (1 active arm) · 2 Half bridge (2 active arms) · 4 Full bridge (4 active arms)
frontGainnumber	V/V	0.01 … 100000	400	The gain between the bridge output and the ADC. A gain of 400 brings a quarter bridge at 3000 με (7.9 mV at 5 V, GF 2.1) to about 3.2 V.
adcRefnumber	V	0.1 … 50	3.3	The ADC full scale
adcBitsnumber	bit	1 … 24	12	ADC resolution in bits

Outputs

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

Outputs of strain-gauge-bridge
Field	Unit	Description
voutheadline	mV	Bridge differential output voltage in millivolts
resistance_change	%	Fractional change in gauge resistance as percentage (ΔR/R = GF × ε)
strain_SI	m/m	Strain in SI units (dimensionless, m/m)
amplifiedVoltage	V	The bridge output times the amplifier gain
adcCode	none	The code an ideal ADC reads: floor(V_amp / V_ref × 2^N)
resolution	με/LSB	The strain one ADC step spans

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": "strain-gauge-bridge",
    "inputs": {
      "vin": 5,
      "gaugeFactor": 2.1,
      "strain": 1000,
      "bridgeConfig": 1,
      "frontGain": 400,
      "adcRef": 3.3,
      "adcBits": 12
    }
  }'

import rftools

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

result = client.calculate("strain-gauge-bridge", {
    "vin": 5,
    "gaugeFactor": 2.1,
    "strain": 1000,
    "bridgeConfig": 1,
    "frontGain": 400,
    "adcRef": 3.3,
    "adcBits": 12,
})
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": "strain-gauge-bridge",
    "inputs": {
      "vin": 5,
      "gaugeFactor": 2.1,
      "strain": 1000,
      "bridgeConfig": 1,
      "frontGain": 400,
      "adcRef": 3.3,
      "adcBits": 12
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "strain-gauge-bridge",
	  "inputs": {
	    "vin": 5,
	    "gaugeFactor": 2.1,
	    "strain": 1000,
	    "bridgeConfig": 1,
	    "frontGain": 400,
	    "adcRef": 3.3,
	    "adcBits": 12
	  }
	}`)
	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": "strain-gauge-bridge",
  "values": {
    "vout": 2.6250000000000004,
    "resistance_change": 0.21000000000000002,
    "strain_SI": 0.001,
    "amplifiedVoltage": 1.0500000000000003,
    "adcCode": 1303,
    "resolution": 0.7672991071428571
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:strain-gauge-bridge",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "vin": {
          "min": 0.1,
          "max": 30,
          "unit": "V"
        },
        "gaugeFactor": {
          "min": 0.1,
          "max": 200,
          "unit": ""
        },
        "strain": {
          "min": 0,
          "max": 50000,
          "unit": "με"
        },
        "bridgeConfig": {
          "min": 1,
          "max": 4,
          "unit": "active arms"
        },
        "frontGain": {
          "min": 0.01,
          "max": 100000,
          "unit": "V/V"
        },
        "adcRef": {
          "min": 0.1,
          "max": 50,
          "unit": "V"
        },
        "adcBits": {
          "min": 1,
          "max": 24,
          "unit": "bit"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "vin": 5,
      "gaugeFactor": 2.1,
      "strain": 1000,
      "bridgeConfig": 1,
      "frontGain": 400,
      "adcRef": 3.3,
      "adcBits": 12
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "vin"
      ],
      "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 pageStrain Gauge Bridge Calculator, at these values