Documentation sections

UART Baud Rate & Frame Timing Calculator

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

Slug uart-baud-rate. Calculate UART frame timing, throughput, and USART BRR register divisor from baud rate, data format, and MCU clock frequency. Identify baud rate error for reliable serial communication.

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 uart-baud-rate
Name	Unit	Range	Default	Description
baudRatenumber	bps	300 … 4000000	115200	Serial baud rate in bits per second
dataBitsnumber	bits	5 … 9	8	Number of data bits per frame (typically 8)
stopBitsnumber	bits	1 … 2	1	1 stop bit is standard; 2 may be used for slow receivers
paritynumber	bits	0 … 2	0	Parity BitsOne of: 0 None · 1 Even or odd (1 bit)
clockFreqnumber	MHz	1 … 1000	16	MCU peripheral (APB) clock frequency used for BRR divisor calculation
oversamplingnumber	none	8 … 16	16	Samples per bit: 16× is the usual mode; 8× (STM32 OVER8, AVR U2X) doubles the fastest rate at the cost of noise immunityOne of: 16 16× · 8 8×
divisorFractionnumber	none	0 … 1	0	A whole divisor (AVR UBRR, the 16550) or one with a fraction of 1/OS (STM32 USARTDIV), which comes far closer to most ratesOne of: 0 Whole number · 1 With fraction (1/OS)
errorBudgetnumber	%	0.1 … 10	2	The baud-rate error the link can take, ± both ends together; about ±2 % for 16× oversampling and 8N1 frames

Outputs

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

Outputs of uart-baud-rate
Field	Unit	Description
frameBits	bits	1 start + data bits + parity + stop bits
bitPeriod_us	µs	Bit Period
framePeriod_us	µs	Frame Period
framesPerSecond	fps	Frames per Second
throughputKbps	kbps	Effective data rate (data bits only, not overhead)
brrDivisor	none	The clock over the oversampling rate times the baud rate, before rounding
actualBaudRate	bps	Baud rate achieved with the divisor as set
baudRateError_pctheadline	%	Deviation of actual from target baud rate; keep below 2% for reliable operation
divisorSet	none	The divisor the register holds: whole, or in steps of 1/OS
baudError_pct	%	Actual over target, signed: negative when the actual rate is slower
stopSampleOffset_bits	bits	How far from the stop bit's centre a receiver at the actual rate samples a frame sent at the target rate; positive is late

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": "uart-baud-rate",
    "inputs": {
      "baudRate": 115200,
      "dataBits": 8,
      "stopBits": 1,
      "parity": 0,
      "clockFreq": 16,
      "oversampling": 16,
      "divisorFraction": 0,
      "errorBudget": 2
    }
  }'

import rftools

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

result = client.calculate("uart-baud-rate", {
    "baudRate": 115200,
    "dataBits": 8,
    "stopBits": 1,
    "parity": 0,
    "clockFreq": 16,
    "oversampling": 16,
    "divisorFraction": 0,
    "errorBudget": 2,
})
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": "uart-baud-rate",
    "inputs": {
      "baudRate": 115200,
      "dataBits": 8,
      "stopBits": 1,
      "parity": 0,
      "clockFreq": 16,
      "oversampling": 16,
      "divisorFraction": 0,
      "errorBudget": 2
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "uart-baud-rate",
	  "inputs": {
	    "baudRate": 115200,
	    "dataBits": 8,
	    "stopBits": 1,
	    "parity": 0,
	    "clockFreq": 16,
	    "oversampling": 16,
	    "divisorFraction": 0,
	    "errorBudget": 2
	  }
	}`)
	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": "uart-baud-rate",
  "values": {
    "frameBits": 10,
    "bitPeriod_us": 8.680555555555555,
    "framePeriod_us": 86.80555555555556,
    "framesPerSecond": 11520,
    "throughputKbps": 92.16,
    "brrDivisor": 8.680555555555555,
    "actualBaudRate": 111111.11111111111,
    "baudRateError_pct": 3.549382716049384,
    "divisorSet": 9,
    "baudError_pct": -3.549382716049384,
    "stopSampleOffset_bits": 0.34959999999999947
  },
  "warnings": [
    "Baud rate error exceeds 2% — may cause framing errors"
  ],
  "errors": [],
  "provenance": {
    "method": "calculator:uart-baud-rate",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Microchip ATmega328P datasheet, USART baud rate generator (UBRRn, the whole divisor); STM32 Reference Manual RM0008 §27.3.4 (USARTDIV, the fraction); ST AN2908",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "baudRate": {
          "min": 300,
          "max": 4000000,
          "unit": "bps"
        },
        "dataBits": {
          "min": 5,
          "max": 9,
          "unit": "bits"
        },
        "stopBits": {
          "min": 1,
          "max": 2,
          "unit": "bits"
        },
        "parity": {
          "min": 0,
          "max": 2,
          "unit": "bits"
        },
        "clockFreq": {
          "min": 1,
          "max": 1000,
          "unit": "MHz"
        },
        "oversampling": {
          "min": 8,
          "max": 16,
          "unit": ""
        },
        "divisorFraction": {
          "min": 0,
          "max": 1,
          "unit": ""
        },
        "errorBudget": {
          "min": 0.1,
          "max": 10,
          "unit": "%"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "baudRate": 115200,
      "dataBits": 8,
      "stopBits": 1,
      "parity": 0,
      "clockFreq": 16,
      "oversampling": 16,
      "divisorFraction": 0,
      "errorBudget": 2
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "baudRate"
      ],
      "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 pageUART Baud Rate & Frame Timing Calculator, at these values