Duct Size Calculator — Free Online Ductulator
Who this is for: For HVAC installers and sheet-metal fitters who left the ductulator in the truck — and homeowners sanity-checking a quote that calls for 5-inch runs to a 300 CFM room.
Enter CFM and friction rate — get the round duct diameter, the standard size to order, velocity, and the full size chart.
Quick answer: Duct sizing by equal friction: 400 CFM at 0.08 in/100ft wants an 8-inch round duct (~1,150 fpm). The rule of thumb baked into every ductulator: diameter ≈ 8 × (CFM/400)^0.37 × (0.08/FR)^0.2. Enter CFM below for the exact diameter, standard size to order, velocity, and the full 50–2000 CFM chart.
| CFM | Diameter | Standard size | Velocity |
|---|---|---|---|
| 50 | 3.7" | 4" | 573 fpm |
| 80 | 4.4" | 5" | 587 fpm |
| 100 | 4.8" | 5" | 733 fpm |
| 150 | 5.6" | 6" | 764 fpm |
| 200 | 6.2" | 7" | 748 fpm |
| 250 | 6.7" | 7" | 935 fpm |
| 300 | 7.2" | 8" | 859 fpm |
| 400 | 8" | 8" | 1146 fpm |
| 500 | 8.7" | 9" | 1132 fpm |
| 600 | 9.3" | 10" | 1100 fpm |
| 800 | 10.3" | 12" | 1019 fpm |
| 1000 | 11.2" | 12" | 1273 fpm |
| 1200 | 12" | 12" | 1528 fpm |
| 1600 | 13.3" | 14" | 1497 fpm |
| 2000 | 14.5" | 16" | 1432 fpm |
| Method | Equal friction (residential standard) |
|---|---|
| Friction rate | 0.05–0.1 in. w.c./100 ft (default 0.08) |
| Fitted formula | D ≈ 8 × (CFM/400)^0.37 × (0.08/FR)^0.2 |
| Standard sizes | 4–20 in round, snap-to-order list |
| Example | 400 CFM @ 0.08 → 8.0" round ≈ 1,150 fpm |
| Status | Estimating tool — formal design per ACCA Manual D. Compiled September 2026. |
How does this online ductulator work?
It applies the equal-friction method — the standard residential duct design approach. You supply airflow in CFM and a design friction rate (inches of water column per 100 ft; residential work runs 0.05–0.1, with 0.06–0.08 the usual target), and the calculator returns the round diameter that keeps friction at that rate, snapped to the standard sizes that actually exist (4–20 inch), plus the resulting air velocity. Below the calculator sits the full chart from 50 to 2,000 CFM at your chosen friction rate — the same table the metal slide rule prints, without the wheel. Rectangular equivalents use the equal-area method: a 10-inch round becomes a 12×6.5 rectangle. For engineered systems — long runs, high static, commercial — this is a starting point, and ACCA Manual D is the formal answer.
Common uses
- Sizing branch runs room-by-room before a rough-in
- Checking a contractor's quote: is a 6-inch run enough for a 250 CFM bedroom?
- Converting round duct calls to rectangular trunks in a tight joist bay
- Explaining to a client why the return needs 14-inch, not 10-inch
Why the ductulator's wheel matches this math
The metal ductulator is a logarithmic-scale calculator for exactly the equal-friction equation used here. The fitted formula reproduces the wheel's printed values within about half an inch across the residential range — at 400 CFM/0.08 both say 8 inches, at 1,200 CFM/0.08 both say 12. Where they can differ: the wheel assumes a specific roughness (typical galvanized), while flex duct has significantly more resistance — a flex run should be sized at a lower friction rate or one size up, which is why flex runs to a far register so often underperform.