HVAC Duct Size Calculator
Size round ducts from airflow and velocity, in inches or millimeters.
What it does
A duct is a compromise between air and noise. Push air fast through a small pipe and it hisses; crawl it through a huge pipe and the budget groans. Manual D practice threads the middle with target velocities per run type, then converts each airflow into sheet metal.
The tool above does the conversion step. Enter airflow and target velocity in imperial or metric. It returns duct area, round diameter, and the next nominal stock size. Defaults describe a 300 CFM trunk at 900 fpm, so the page opens solved at 48.0 sq in, 7.82 in, and an 8 in pick.
Geometry is the whole trick here. Area equals flow over velocity, and diameter inverts the circle formula. The metric mirror (141.6 L/s at 4.57 m/s) lands at 0.0310 m2 and 198.6 mm, so a 200 mm pick. Every row follows the active system, so compare options only inside one mode.
Velocity targets come from field habit backed by ACCA tables: trunks near 800 to 1100 fpm (4 to 6 m/s), branches near 500 to 800 fpm (2.5 to 4 m/s). Bedrooms forgive slower runs; long throws and filters punish fast ones. Nominal always rounds up because a slightly large duct beats a whistling small one.
Use the table for common trunk flows at 900 fpm, then run your branch at its own speed. Length, elbows, boots, and filter grilles set pressure loss that area alone cannot clear, so carry these sizes into a Manual D sheet before ordering metal.
How to use
- Pick imperial (CFM, fpm) or metric (L/s, m/s).
- Enter the airflow the run must carry.
- Enter the target velocity for the run type.
- Read duct area first.
- Read round diameter plus nominal stock size.
- Check fittings and total length in Manual D next.
Formula
- Flow
- Air moved: CFM imperial, L/s metric.
- Velocity
- Target air speed: fpm imperial, m/s metric.
- Area
- Free cross section of the duct.
- Diameter
- Circle diameter matching that area.
Imperial divides CFM by fpm for ft2 and multiplies by 144 for in2; metric divides m3/s (L/s over 1000) by m/s for m2. Nominal picks the next whole inch (or 10 mm) up. The page sizes area only; elbows, boots, filters, and long runs set the static pressure a Manual D check must clear.
Worked example
Example: 300 CFM trunk at 900 fpm
A supply trunk carries 300 CFM at 900 fpm. Find the duct area and round diameter.
- Area = flow / velocity = 300 / 900 = 0.3333 ft2.
- Square inches = 0.3333 x 144 = 48.0 sq in.
- Diameter = sqrt(4 x 48 / pi) = sqrt(61.11) = 7.82 in.
- Pick 8 in nominal from the 7.82 in result.
- Metric mirror: 141.6 L/s at 4.57 m/s gives 198.6 mm, so 200 mm.
- Hold trunks near 800 to 1100 fpm for quiet flow.
Reference table
| Flow | Area | Diameter | Nominal |
|---|---|---|---|
| 200 CFM | 32.0 sq in | 6.38 in | 7 in |
| 300 CFM | 48.0 sq in | 7.82 in | 8 in |
| 400 CFM | 64.0 sq in | 9.03 in | 10 in |
| 500 CFM | 80.0 sq in | 10.09 in | 11 in |
Limits
- Area sizing only; no pressure loss math.
- Round ducts; rectangular needs conversion.
- Targets are screening guides, not a Manual D.
- No grille, filter, or coil loss included.
Common mistakes
- Sizing every run at trunk velocity.
- Rounding diameter down to save metal.
- Mixing CFM with m/s in one division.
- Forgetting fittings in the pressure budget.
- Starving bedrooms with tiny branches.
Try it yourself
Quick checks. Answers sit tucked under each question.
Branch: 150 CFM at 700 fpm?
Area 30.9 sq in. Diameter 6.27 in (use 7 in nominal).
Large trunk: 600 CFM at 1200 fpm?
Area 72.0 sq in. Diameter 9.57 in (use 10 in nominal). Velocity 1,200 fpm.
400 CFM at 1000 fpm? Work it by hand.
Area = 400/1000 = 0.4 ft2 = 57.6 sq in. Diameter = sqrt(4 x 57.6/pi) = 8.56 in, use 9 in.
Questions
How is duct size found?
Area equals airflow divided by velocity, then diameter follows from circle geometry. This page does both and names the next nominal size up.
What velocity should I use?
Near 800 to 1100 fpm for trunks and 500 to 800 fpm for branches in homes. Faster runs hiss; slower runs waste sheet metal.
How is nominal size picked?
Round up to the next whole inch here (7.82 in becomes 8 in). Nominal sizes match stocked pipe and fittings.
Does it work in metric?
Yes. Pick metric and enter L/s with m/s: 141.6 L/s at 4.57 m/s gives 0.0310 m2 and 198.6 mm, so 200 mm.
Does it give friction loss?
No. Length, bends, fittings, and roughness set pressure loss, which needs Manual D tables or software after this first size.
What about rectangular ducts?
Rectangular ducts need equal area with aspect limits plus their own loss tables. Size the area here, then convert shape in Manual D.