HVAC Duct Sizing Calculator.
Enter airflow (CFM) and application. Get the required round-duct diameter and equivalent rectangular sizes using the ACCA Manual D equal-friction method.
How do you size an HVAC duct from CFM?
Divide required airflow (CFM) by the target velocity (fpm) to get the duct's cross-sectional area in square feet, then convert to a round-duct diameter with D = 2 · √(A / π). Typical residential targets are 900 fpm for supply trunks, 600 fpm for branch runouts, and 700 fpm for returns. Convert to a rectangular duct with the Huebscher equivalent-diameter formula.
Inputs
Rule of thumb: ~1 CFM per sq. ft. for a typical residential room. Use Manual J for real numbers.
Recommended duct
Round duct diameter
11.1 in
Cross-sectional area ≈ 96.0 sq. in. at 600 fpm.
Equivalent rectangular sizes
| Width (in) | Height (in) |
|---|---|
| 6 | 18.5 |
| 8 | 13.0 |
| 10 | 10.0 |
| 12 | 8.5 |
| 14 | 7.5 |
| 16 | 6.5 |
| 18 | 6.0 |
| 20 | 5.5 |
| 22 | 5.0 |
| 24 | 5.0 |
How this calculator works
This is the equal-friction method that HVAC designers and installers use every day. You pick a target air velocity for the run type (supply trunk, branch runout, or return), then solve Area = CFM / Velocity to get the duct's cross-sectional area. That converts to a round-duct diameter with D = 2 · √(A / π). For rectangular ducts, we solve the Huebscher equivalent-diameter equation for the height that matches each stock width.
For engineered systems, always back these numbers with an ACCA Manual D design that accounts for equivalent length, fitting losses, and total external static pressure — this calculator is meant for quick sizing, sanity checks, and estimates.
Duct sizing starts from room-by-room airflow, and that comes out of the load calculation. If you have not run one yet, read the Manual J HVAC load calculation guide first — it covers the design conditions, envelope inputs and room-by-room math that produce the CFM targets this calculator needs.
CFM to duct size chart (round metal, rectangular and flex)
Use this chart for a quick look-up when you already know the airflow, and use the calculator above when you need the exact number for a CFM value that is not on this list, or for a different design velocity. The chart uses fixed assumptions; the calculator lets you change the application and solves for your specific airflow.
| Airflow (CFM) | Round metal — exact (in) | Round metal — stock size (in) | Rectangular equivalent (in) | Flex duct — stock size (in) |
|---|---|---|---|---|
| 50 | 3.6 | 4 | 6 × 4 | 5 |
| 75 | 4.4 | 5 | 6 × 4 | 5 |
| 100 | 5.1 | 6 | 6 × 4 | 6 |
| 150 | 6.3 | 7 | 8 × 4 | 8 |
| 200 | 7.2 | 8 | 8 × 6 | 9 |
| 250 | 8.1 | 9 | 10 × 6 | 10 |
| 300 | 8.9 | 9 | 10 × 7 | 10 |
| 400 | 10.2 | 12 | 12 × 7 | 12 |
| 500 | 11.4 | 12 | 12 × 9 | 14 |
| 600 | 12.5 | 14 | 14 × 9 | 16 |
| 800 | 14.5 | 16 | 16 × 11 | 18 |
| 1000 | 16.2 | 18 | 18 × 12 | 20 |
| 1200 | 17.7 | 18 | 18 × 15 | 20 |
| 1400 | 19.1 | 20 | 20 × 15 | 22 |
| 1600 | 20.5 | 22 | 22 × 16 | 24 |
| 2000 | 22.9 | 24 | 24 × 18 | 26 |
Assumptions behind this chart
- Residential comfort systems, equal-friction method (ACCA Manual D).
- Friction rate of roughly 0.08–0.10 in. w.c. per 100 ft of duct — the usual residential design range.
- Smooth galvanized round metal sized at a 700 fpm design velocity.
- Insulated flex sized at a lower 550 fpm design velocity, because corrugated flex has a materially higher friction rate than smooth metal at the same airflow. Flex sizes assume the duct is fully stretched and supported; compressed or sagging flex behaves far worse than this chart shows.
- Rectangular equivalents come from the Huebscher equivalent-diameter equation against the exact round metal diameter, rounded to a practical stock height.
- Not included: fitting and elbow losses, equivalent length, filter and coil drop, or total external static pressure.
This chart is a starting reference for quick sizing and estimating. It is not universally correct and does not replace a full Manual D design on engineered systems, permit work, long duct runs, or high external static pressure equipment.
Run the rest of the job on URBLD
Once the duct package is scoped, URBLD handles estimating, contracts and e-sign, scheduling and dispatch, materials, invoicing, and the customer follow-up — every touchpoint tied to the same job.
Frequently Asked Questions
Straight answers about how URBLD runs the business end-to-end.