Duct Size Calculator — Free Online Ductulator
This duct size calculator — a free online ductulator — converts airflow in CFM into the round duct diameter you need, for smooth sheet metal or flex duct, using the standard residential friction rate, with an air-velocity check so the run won't whistle. Use it to size a new branch, verify a trunk, or diagnose a room that never gets enough air.
Don't know the CFM yet? Get it first from the CFM / Airflow Calculator — room CFM by air changes, or system CFM by tonnage — then size the duct here.
Enter your airflow
Flex duct carries less air at the same diameter — the calculator sizes up automatically.
Recommended round duct
Air velocity
733 fpm
Size capacity
425 CFM
Same airflow in flex duct
12" flex duct
Velocity is under the 900 fpm supply-trunk comfort threshold — this run should be quiet.
Sized at 0.1 in. w.c./100 ft friction rate. Final whole-system duct design should follow ACCA Manual D.
Round duct sizing chart (quick reference)
Approximate CFM capacity per diameter at the 0.1 in/100 ft residential friction rate. Flex values assume properly stretched duct.
| Duct diameter | Sheet metal (CFM) | Flex duct (CFM) |
|---|---|---|
| 4" | 35 | ~25 |
| 5" | 60 | ~45 |
| 6" | 100 | ~75 |
| 7" | 160 | ~120 |
| 8" | 230 | ~170 |
| 9" | 320 | ~240 |
| 10" | 425 | ~320 |
| 12" | 700 | ~525 |
| 14" | 1,050 | ~790 |
| 16" | 1,500 | ~1,125 |
| 18" | 2,050 | ~1,540 |
| 20" | 2,750 | ~2,060 |
Capacities are planning values for straight runs. Long runs, elbows, and fittings reduce real-world capacity — Manual D accounts for total effective length.
The method, explained in plain English
Duct sizing balances two limits: friction (how hard the blower works) and velocity (how loud the air is). The residential shortcut fixes the friction rate and reads sizes off a chart — exactly what this calculator automates.
Why fix the friction rate?
A residential blower has a limited pressure budget. Sizing every run at 0.1 in/100 ft keeps the whole system inside what the blower can push.
Why the velocity check?
Past ~900 fpm, supply air whistles at registers and rumbles in trunks. The calculator shows velocity so you can size up if a run gets loud.
The flex duct penalty
Flex duct's ribbed liner adds ~25% friction, so it carries less air per inch of diameter — and far less if it sags or gets compressed. Pull it tight.
Branches vs. trunks
Size each branch for its room's CFM and the trunk for the total it carries at that point — trunks taper as branches peel off airflow.
Worked examples
Three runs sized end to end — the same rooms used in the CFM calculator's examples.
Bedroom branch — 100 CFM
flex: 6" only carries ~75 → 7" flex
Result: the classic bedroom run — 6-inch metal or 7-inch flex. Quiet at ~509 fpm, with capacity to spare.
Kitchen branch — 213 CFM
flex: 8" carries ~173 → 9" flex
Result: 8-inch metal or 9-inch flex. The kitchen needs double the bedroom's air — and two duct sizes more to deliver it quietly.
3-ton system trunk — 1,200 CFM
flex: 16" carries ~1,125 → 18" flex
Result: a 16-inch metal trunk, right at the comfortable edge of the 900 fpm velocity limit — exactly why 3-ton systems get 16-inch (not 14-inch) main trunks.
Frequently asked questions
Common questions about duct sizing and airflow.
What size duct do I need for 400 CFM?
At the standard residential friction rate, 400 CFM needs a 10-inch round sheet-metal duct (carries ~425 CFM). In flex duct — about 25% more friction — the same airflow needs a 12-inch run. Velocity at 10 inches is ~733 fpm, comfortably under the 900 fpm noise threshold.
How is duct size calculated?
Residential ducts are sized so friction loss is about 0.1 inches of water column per 100 feet, while keeping velocity below ~900 fpm on supply trunks. Each round diameter has a known CFM capacity at that friction rate — you pick the smallest size whose capacity meets your airflow.
Why does flex duct need to be bigger than metal?
The ribbed inner liner of flex creates roughly 25% more friction than smooth sheet metal, so it carries less air at the same diameter. Sizing up one diameter compensates — and flex must be pulled tight and supported, because sagging or compressed flex can cut airflow drastically.
What's a good air velocity in residential ducts?
Keep supply trunks at or below ~900 feet per minute and branch runs near 600 fpm; returns are typically sized for ~700 fpm or less. Higher velocities whistle at registers and waste blower energy; much lower usually means the duct is oversized.
What size duct does each ton of AC need?
Each ton moves about 400 CFM, so one ton fits a 10-inch round metal duct, two tons (800 CFM) need 14 inches, and three tons (1,200 CFM) a 16-inch metal trunk. These are trunk sizes — branches are sized for each room's share of the airflow.
How do I size rectangular duct from this?
Rectangular duct is matched to round by equivalent diameter, not equal area, because corners add friction — a 10-inch round is roughly an 8×11 rectangular. Contractors convert with the equivalent-diameter tables in ACCA Manual D or a duct calculator wheel.
Is this a substitute for Manual D?
No. This sizes individual runs with the standard friction-rate shortcut — reliable for single branches and simple trunks. ACCA Manual D designs the whole system: balancing every branch, fittings, total effective length, and the blower's available static pressure. Permits and warranties expect it.
What is a ductulator?
A ductulator is the classic circular slide-rule HVAC techs use to size ducts — line up the airflow (CFM) against the friction rate and read off the duct diameter. This tool is an online ductulator: it runs the same friction-rate math instantly, adds an air-velocity check, and covers both sheet-metal and flex duct — no wheel required.
Ducts sized? Quote the whole run in seconds.
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