Quick Duct Sizing Calculator
Get accurate duct sizing on-site in seconds instead of fumbling with a ductulator
Why people pay for this
It replaces a $25 plastic slide rule with a faster, more accurate digital tool that prevents costly callbacks from undersized ductwork.
Target Audience
HVAC residential installers, heating and cooling technicians, ductwork contractors
Suggested Price
$49 - $79/year
Est. Build Time
3-5 days
What This Is
A fast, focused duct sizing tool that replaces the physical "ductulator" slide rule. Enter CFM (airflow), friction loss rate, and duct material — get the recommended round duct diameter and equivalent rectangular sizes instantly. Includes velocity warnings for noise and a troubleshooting mode for existing systems. Three inputs, one result, no complexity.
HVAC technicians either carry a physical ductulator (slide rule from the 1980s), guess duct sizes from experience, or use expensive Manual D software. Undersized ducts cause airflow problems, callbacks, and unhappy customers. Oversized ducts waste money. A simple digital tool that gives the right answer in 5 seconds — with big buttons for gloved hands — is something techs will use on every install.
How to Make Yours Different
Extreme simplicity — three inputs, one answer. No feature bloat, no accounts, no tutorials needed
A "velocity warning" that flags if air speed will exceed 700-900 FPM (causing noise complaints) — this is the mistake that generates the most callbacks
Supply vs. return air toggle, since return ducts are sized differently and many calculators only handle supply
"Troubleshooting mode" for existing systems: enter the installed duct size and airflow to see if it's undersized — useful for diagnosing comfort complaints
The Starter Prompt
Copy this into your vibe coding tool. Customize the [bracketed sections] with your industry knowledge.
Build me a "Quick Duct Sizing Calculator" as a single-page web app for HVAC technicians. Here's what I need: **Core Features:** - Inputs: CFM (airflow required), Friction Loss Rate (default 0.08 for residential, adjustable), and Duct Material (dropdown: Sheet Metal, Flex Duct, Fiberglass Duct Board — each has different friction characteristics) - Toggle: Supply Air vs. Return Air (return ducts typically need to be larger) - Calculated outputs: - Recommended round duct diameter (in inches) - Equivalent rectangular duct dimensions (2-3 common options) - Air velocity (FPM) with a warning if it exceeds the noise threshold - A "fits" indicator showing which common stock sizes work - "Quick Reference" table showing CFM ranges and recommended duct sizes for common residential scenarios (e.g., "Bedroom: 100-150 CFM → 6-7 inch round") - "Troubleshoot" mode: enter existing duct size and measured/desired CFM to see if the duct is adequate - Settings: default friction rate, preferred stock duct sizes (so the app rounds up to what you actually carry), noise threshold preference **Design:** - "Industrial" aesthetic — dark background, bold white/green numbers, high contrast - LARGE buttons and inputs designed for use with work gloves - No unnecessary navigation — everything on one screen - The answer should be visible without scrolling on a phone - Fast, instant calculation as values change **Technical:** - Single HTML file with embedded CSS and JavaScript - Math based on standard ASHRAE friction loss formulas or equivalent lookup tables - All data stored locally - No external APIs — must work in basements and crawl spaces with no signal **HVAC-specific:** - [Set your preferred default friction rate for residential systems] - [List the duct sizes you actually stock in your truck so the app recommends available sizes] - [Add any local code requirements, like minimum return air sizing ratios]
Got the prompt? You're halfway there.
Build it, test it on a real scenario, then come back to list it.
Pricing Guidance
$49 - $79/year
Annual subscription. HVAC standards and duct sizing references evolve, and you can add new equipment types and calculation methods.
One prevented callback ($100-300+) pays for years of access. Annual updates let you incorporate new ASHRAE data, add new equipment categories, and refine noise prediction algorithms.
AppGild takes a flat 20% commission. Monthly or annual subscriptions give your buyers ongoing value and give you recurring revenue, a win for everyone.
Getting Started
Gather the ASHRAE friction loss data or duct sizing tables you trust (or the formulas from your Manual D training)
List the stock duct sizes you carry and want the tool to recommend from
Build the first version and test against your ductulator — do they agree?
Use it on a real job and time yourself vs. the slide rule. It should be noticeably faster
Show it to other techs at supply house visits or trade events
List on AppGild targeting HVAC installers
Ready to build this?
Copy the prompt, open your building tool, and start. When it's ready, list it on AppGild and start reaching buyers.