Documentation sections

BLDC Motor Performance Calculator

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

Slug bldc-motor. BLDC motor calculator: enter Kv rating and voltage to get no-load RPM, stall torque, max efficiency point, and propeller thrust. Supports drone, RC, and industrial winding calculations.

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 bldc-motor
Name	Unit	Range	Default	Description
kvRatingnumber	RPM/V	at least 1	1000	Motor velocity constant — RPM per volt at no load
voltagenumber	V	at least 0.1	12	Operating Voltage
motorResistancenumber	mΩ	at least 0.1	50	Phase-to-phase winding resistance from datasheet
noLoadCurrentnumber	A	at least 0.001	0.5	No-load current (iron losses, friction) from datasheet
propDiameternumber	inch	at least 0.5	5	Propeller diameter for thrust estimate (multirotor)

Outputs

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

Outputs of bldc-motor
Field	Unit	Description
noLoadRpmheadline	RPM	RPM with no propeller attached (theoretical maximum speed)
stallCurrent	A	Theoretical locked-rotor current = V/Rm. Size your ESC well above typical operating current, not stall current.
ktMnm	mNm/A	Torque produced per amp of current. K_t = 60/(2π × K_v)
stallTorque	Nm	Maximum theoretical torque at locked rotor = K_t × I_stall
maxEfficiency	%	Maximum Efficiency
inputPower	W	Power drawn at the operating point of maximum efficiency: P = V × √(I₀ × I_stall)
thrust	g	Static thrust C_T·ρ·n²·D⁴ with C_T = 0.11, sea-level air, at the loaded speed where the motor torque equals the propeller torque (C_P = 0.047). Actual thrust varies with prop pitch, air density, and throttle.
loadedRpm	RPM	Full-throttle speed with the propeller on, where Q_m = (I − I₀)/K_v equals Q_p = C_P·ρ·n²·D⁵/(2π) (Drela 2007)
loadedCurrent	A	Motor current at the loaded speed, I = (V − Ω/K_v)/R

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": "bldc-motor",
    "inputs": {
      "kvRating": 1000,
      "voltage": 12,
      "motorResistance": 50,
      "noLoadCurrent": 0.5,
      "propDiameter": 5
    }
  }'

import rftools

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

result = client.calculate("bldc-motor", {
    "kvRating": 1000,
    "voltage": 12,
    "motorResistance": 50,
    "noLoadCurrent": 0.5,
    "propDiameter": 5,
})
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": "bldc-motor",
    "inputs": {
      "kvRating": 1000,
      "voltage": 12,
      "motorResistance": 50,
      "noLoadCurrent": 0.5,
      "propDiameter": 5
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "bldc-motor",
	  "inputs": {
	    "kvRating": 1000,
	    "voltage": 12,
	    "motorResistance": 50,
	    "noLoadCurrent": 0.5,
	    "propDiameter": 5
	  }
	}`)
	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": "bldc-motor",
  "values": {
    "noLoadRpm": 12000,
    "stallCurrent": 240,
    "ktMnm": 9.549296585513721,
    "stallTorque": 2.291831180523293,
    "maxEfficiency": 91.07962404158056,
    "inputPower": 131.45341380123986,
    "thrust": 140.90532398081277,
    "loadedRpm": 11912.515006913327,
    "loadedCurrent": 1.7496998617334896
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:bldc-motor",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Drela, First-Order DC Electric Motor Model (MIT, 2007); UIUC Propeller Data Site, Vol. 2, GWS Direct Drive 5×3 static data (gwsdd_5x3_static_0323rd.txt); Brandt & Selig, Propeller Performance Data at Low Reynolds Numbers, AIAA 2011-1255",
    "assumptions": [
      {
        "code": "static-thrust-coefficient",
        "text": "Thrust uses a static thrust coefficient C_T = 0.11, from the UIUC Propeller Data Site static data of a GWS Direct Drive 5×3 (C_T 0.102–0.113) and inside the 0.08–0.15 range published for small two-blade propellers, with sea-level air at 1.225 kg/m³."
      },
      {
        "code": "motor-propeller-balance",
        "text": "The propeller turns where the motor's torque (I − I₀)/K_v (Drela 2007) equals the propeller's C_P·ρ·n²·D⁵/(2π), with C_P = 0.047 from the same propeller's static data; every propeller is taken to have these coefficients."
      }
    ],
    "validRange": {
      "status": "inside",
      "bounds": {
        "kvRating": {
          "min": 1,
          "max": null,
          "unit": "RPM/V"
        },
        "voltage": {
          "min": 0.1,
          "max": null,
          "unit": "V"
        },
        "motorResistance": {
          "min": 0.1,
          "max": null,
          "unit": "mΩ"
        },
        "noLoadCurrent": {
          "min": 0.001,
          "max": null,
          "unit": "A"
        },
        "propDiameter": {
          "min": 0.5,
          "max": null,
          "unit": "inch"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "kvRating": 1000,
      "voltage": 12,
      "motorResistance": 50,
      "noLoadCurrent": 0.5,
      "propDiameter": 5
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "kvRating"
      ],
      "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 pageBLDC Motor Performance Calculator, at these values