DuctDuckHVAC duct sizing
The whole residential design chain in the order ACCA actually specifies — on your phone, in the basement, with no signal.
$14.99/month or $119/year · card required, cancel any time
A field estimating tool. Sizes a system and its ductwork in about two minutes at the truck, using the standard equal-friction method and the real ASHRAE equations. Good enough to quote from, order from, and sanity-check someone else's numbers with.
It is not ACCA-approved and not a substitute for stamped Manual J software. If a permit office or a builder needs certified paperwork, run full Manual J/S/D and use this to check that the answer is sane.
Plenty of guys size the box off square footage and work backward to the ducts. That gets you an oversized unit on undersized duct. This walks the chain the way Manual J through T lays it out.
95–115% sizing window, with an honest warning when you're outside it. Over a single 5-ton residential system it tells you to split the job into two units instead of quietly rounding up.
400 default, drop it for humid climates.
Installs to your home screen and runs with the radio off. Not "cached pages" — the whole calculator. Crawlspaces and mechanical rooms don't have bars, and that's exactly where you need it.
Change a window and the load, tonnage, CFM, trunk size and register all move together. No retyping numbers between four separate calculators.
Assign rooms to System 1 or 2 on a multi-unit job and size each one's CFM, equipment, trunk, branches, registers and friction separately.
Tag rooms 1st / 2nd / basement. Get load broken out per level and above versus below grade — the number you need when you're deciding how to split a job.
Paste or import an .xlsx or .csv with names in column A and square footage in column B. Beats thumbing in twelve rooms on a phone.
Start a job on the phone at the walkthrough, finish it at the desk. Export to JSON or print a PDF for the file.
No black box and nothing invented. These are the standard forms — the same ones behind a paper ductduck wheel and the ASHRAE friction chart.
Sanity check you can run yourself: 400 CFM at 0.10 in. wg/100 ft comes out around a 9″ round, and 20×8 rectangular has an equivalent diameter of about 13.5″. Both land where the chart puts them.
Everything is included on either plan — there's no crippled version. Fourteen days free, card up front, cancel from your account page whenever you like.
Billed monthly. Cancel any time.
Works out to $9.92 a month.
Full ACCA-approved suites do things this doesn't, and cost accordingly. Here's the actual trade-off rather than a rigged table.
| DuctDuck | Stamped Manual J suite | $4 ductduck app | |
|---|---|---|---|
| Typical cost | $119/yr | $239–$900/yr | one-time |
| Load calculation | J-lite, room by room | Full certified MJ8 | None |
| Equipment sizing | Manual S window | Yes | None |
| Duct sizing | Equal friction + step-downs | Yes | Yes |
| Registers / grilles | Manual T | Often extra | None |
| Accepted for permits | No | Yes | No |
| Works with no signal | Fully | Desktop or online | Yes |
| Time to a duct size | ~2 minutes | 30+ minutes | Seconds, one duct |
No, and it doesn't claim to be. ACCA approval means a vendor has submitted software for third-party verification against Manual J8, which costs real money and takes time. This is a field estimating tool built by a working tech. If you need certified output for a permit or a builder contract, use approved software — Elite's online Rhvac and Wrightsoft are the usual choices — and use this to check the answer looks right before you trust it.
No. Permit offices that require a Manual J want output from approved software with the calculation sheets attached. What this is good for is the ninety percent of work that never sees a permit desk: change-outs, quoting, checking whether the duct on an existing house can carry the airflow, and catching an oversized proposal before it gets installed.
The duct side is exact — the friction and equivalent-diameter equations are the standard published forms, and equivalent diameters land within a few percent of the ASHRAE tables. The Manual T grille sizing is straight face-velocity math.
The load side is where honesty matters. Quick mode is a square-foot rule of thumb; treat it as a rule of thumb. Detailed mode is a genuine component calc but simplified — it does not model every Manual J8 correction factor, duct losses in unconditioned space, or a full latent split. Expect it to land in the right neighbourhood, not to match a stamped report line for line.
Don't default to 0.10 out of habit — that number assumes a total effective length that a lot of real houses don't have. Manual D wants you to divide available static pressure by total effective length and multiply by 100. Take the blower's rated external static, subtract the coil and filter and anything else in the air path, and what's left is what the duct gets. The app has a blower-table helper and a fittings list so you can build a TEL rather than guess one, and it'll show you the resulting rate.
400 is the usual starting point and it's the default. Drop toward 350 where you need more latent removal in a humid climate, and you can push toward 450 in a dry one. It's an editable field because the right answer depends on the equipment and the climate, not on a constant.
Yes. Install it to your home screen and the whole calculator runs locally with no network at all. Cloud sync and your subscription check need a connection, but the app keeps working for two weeks between check-ins, so a stretch of basements and crawlspaces won't lock you out mid-job.
No, and that's deliberate. It's a progressive web app, so it installs from the browser to your home screen and looks like any other app. That means no store cut, no update queue, and one version everywhere — Android, iPhone and desktop. On Android, tap the menu and "Add to Home screen". On iPhone, the Share button then "Add to Home Screen".
You keep them. You can still open and export everything you've saved after a subscription ends — you just can't save new work to the cloud. Your job records are yours, and locking them up would be a lousy way to treat someone who paid.
An HVAC tech in Youngstown, Ohio who got tired of sizing duct off a paper wheel and a rule of thumb, and wanted the whole chain in the right order on one screen. It was built to be used on actual jobs before it was ever built to be sold.