Documentation sections

I2C Pull-Up Resistor Calculator

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

Slug i2c-pullup. Calculate I2C pull-up resistor values for Standard (100 kHz), Fast (400 kHz), and Fast-Plus (1 MHz) modes. Derives minimum, maximum, and recommended resistance from supply voltage and bus capacitance per NXP UM10204.

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 i2c-pullup
Name	Unit	Range	Default	Description
vddnumber	V	1.8 … 5.5	3.3	I2C bus supply voltage (pull-up rail)
speedModenumber	none	0 … 2	1	Speed ModeOne of: 0 Standard (100 kHz) · 1 Fast (400 kHz) · 2 Fast-Plus (1 MHz)
busCapnumber	pF	10 … 550	100	Total bus capacitance including PCB traces and device pins
pullupnumber	Ω	100 … 100000	4700	The pull-up fitted on SDA and SCL; its rise time with the bus capacitance is checked against UM10204

Retired, still accepted and ignored with a warning: numDevices (Number of Devices, 2026-10-12).

Outputs

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

Outputs of i2c-pullup
Field	Unit	Description
R_max_ohm	Ω	Maximum pull-up for required rise time (t_r / (0.8473 × C_bus))
R_min_ohm	Ω	Minimum pull-up from sink current limit ((V_DD − V_OL) / I_OL; V_OL 0.4 V at 3 mA, or 0.2·V_DD at 2 mA at or below 2 V)
R_recommended_ohmheadline	Ω	Geometric mean of R_min and R_max; choose nearest standard E24/E96 value
current_ma	mA	Static current drawn per line when the bus is at V_DD
powerPerLine_mw	mW	Approximate power dissipated in pull-up when line is held low
riseTimeTarget_ns	ns	Maximum permitted rise time for the selected speed mode (NXP UM10204)
riseTime	ns	Rise time of SDA and SCL with the fitted pull-up, 0.3·V_DD to 0.7·V_DD: t_r = 0.8473 × R_p × C_bus (UM10204 §7.1)
riseTimePercent	%	The rise time as a percentage of the speed mode’s UM10204 maximum; above 100 % does not comply
maxBusCap	pF	The largest bus capacitance at which the fitted pull-up meets the rise-time limit, and no more than UM10204 Table 11 allows (400 pF; 550 pF in Fast-mode Plus)

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": "i2c-pullup",
    "inputs": {
      "vdd": 3.3,
      "speedMode": 1,
      "busCap": 100,
      "pullup": 4700
    }
  }'

import rftools

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

result = client.calculate("i2c-pullup", {"vdd": 3.3, "speedMode": 1, "busCap": 100, "pullup": 4700})
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": "i2c-pullup",
    "inputs": {
      "vdd": 3.3,
      "speedMode": 1,
      "busCap": 100,
      "pullup": 4700
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "i2c-pullup",
	  "inputs": {
	    "vdd": 3.3,
	    "speedMode": 1,
	    "busCap": 100,
	    "pullup": 4700
	  }
	}`)
	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": "i2c-pullup",
  "values": {
    "R_max_ohm": 3540.6585624926242,
    "R_min_ohm": 966.6666666666666,
    "R_recommended_ohm": 1850.0369213638783,
    "current_ma": 1.7837481846401122,
    "powerPerLine_mw": 5.88636900931237,
    "riseTimeTarget_ns": 300,
    "riseTime": 398.23100000000005,
    "riseTimePercent": 132.74366666666668,
    "maxBusCap": 75.33316090409836
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:i2c-pullup",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "NXP I2C-bus specification and user manual, Rev. 7.0 (UM10204), §7.1",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "vdd": {
          "min": 1.8,
          "max": 5.5,
          "unit": "V"
        },
        "speedMode": {
          "min": 0,
          "max": 2,
          "unit": ""
        },
        "busCap": {
          "min": 10,
          "max": 550,
          "unit": "pF"
        },
        "pullup": {
          "min": 100,
          "max": 100000,
          "unit": "Ω"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "vdd": 3.3,
      "speedMode": 1,
      "busCap": 100,
      "pullup": 4700
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "vdd"
      ],
      "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 pageI2C Pull-Up Resistor Calculator, at these values