HVAC Professional Tools

Overview

A free ductulator API for HVAC duct sizing calculations. Submit airflow (CFM) and friction rate or velocity, get back round and rectangular duct dimensions. All requests are GET. No authentication required. Returns XML by default for Excel compatibility, or JSON for use in code.

Endpoint: https://rysercad.com/api.php

Parameters

Parameter	Type		Description
cfm	number	required	Airflow in cubic feet per minute
friction	number	optional	Friction rate in in.w.g./100ft. Default: 0.1. Used when mode=friction.
velocity	number	optional	Air velocity in FPM. Required when mode=velocity.
mode	string	optional	friction or velocity. Default: friction.
height	number	optional	Fix rectangular duct height in inches and solve for width. If omitted, height is calculated automatically.
format	string	optional	xml (default) or json. Use json for Python, Google Apps Script, or any HTTP client.

Response Fields

Field	Unit	Description
cfm	CFM	Airflow (echoed from input)
roundDiameter	in	Round duct diameter
roundArea	sq ft	Round duct cross-sectional area
roundVelocity	FPM	Air velocity in round duct
roundFriction	in.w.g./100ft	Friction rate in round duct
rectWidth	in	Rectangular duct width
rectHeight	in	Rectangular duct height
rectArea	sq ft	Rectangular duct cross-sectional area
rectVelocity	FPM	Air velocity in rectangular duct
rectEquivDia	in	Equivalent diameter of rectangular duct
rectFriction	in.w.g./100ft	Friction rate in rectangular duct
aspectRatio	—	Width-to-height ratio

Excel Usage

Retrieve the full XML response in one cell, then extract individual fields with FILTERXML().

Basic — friction mode:

=WEBSERVICE("https://rysercad.com/api.php?cfm="&A1&"&friction="&B1)

Velocity mode:

=WEBSERVICE("https://rysercad.com/api.php?cfm="&A1&"&mode=velocity&velocity="&B1)

Fixed height:

=WEBSERVICE("https://rysercad.com/api.php?cfm="&A1&"&friction="&B1&"&height="&C1)

Extract round diameter from result in D1:

=FILTERXML(D1,"//roundDiameter")

Extract and round up rect width:

=ROUNDUP(FILTERXML(D1,"//rectWidth"),0)

Suggested column layout:

A: CFM	B: Friction	C: Height Override	D: WEBSERVICE (raw XML)	E: Round Dia	F: Rect Width
400	0.1	12	=WEBSERVICE(...)	=FILTERXML(D1,"//roundDiameter")	=ROUNDUP(FILTERXML(D1,"//rectWidth"),0)

Rate Limiting

60 requests per minute per IP address. Exceeding this returns an error in the response.

Google Sheets Usage

Use IMPORTXML() to pull individual fields directly — no intermediate cell needed.

Round diameter:

=IMPORTXML("https://rysercad.com/api.php?cfm="&A1&"&friction="&B1,"//roundDiameter")

Rect width with fixed height:

=IMPORTXML("https://rysercad.com/api.php?cfm="&A1&"&friction="&B1&"&height="&C1,"//rectWidth")

JSON Usage

Add &format=json to get a JSON response for use in Python, JavaScript, or any HTTP client.

Example request:

https://rysercad.com/api.php?cfm=400&friction=0.1&format=json

Example response:

{
  "success": true,
  "data": {
    "cfm": "400",
    "roundDiameter": "9.8",
    "roundArea": "0.52",
    "roundVelocity": "764",
    "roundFriction": "0.100",
    "rectWidth": "16.2",
    "rectHeight": "8",
    "rectArea": "0.90",
    "rectVelocity": "445",
    "rectEquivDia": "9.8",
    "rectFriction": "0.100",
    "aspectRatio": "2.0"
  }
}

Python example:

import requests

r = requests.get("https://rysercad.com/api.php", params={
    "cfm": 400,
    "friction": 0.1,
    "format": "json"
})
data = r.json()["data"]
print(data["roundDiameter"], data["rectWidth"], data["rectHeight"])

Error Response

XML error:

<duct><error>CFM must be greater than 0.</error></duct>

JSON error:

{"success": false, "error": "CFM must be greater than 0."}

Check for XML errors in Excel with: =FILTERXML(D1,"//error")