Duct sizing calculator
Airflow, velocity and available height: you get the required area, round duct diameter and stock size, and the width of an equivalent rectangular duct.
Calculated diameter
230.33 mm Ø
- Stock round duct250 mm Ø
- Calculated diameter230.33 mm Ø
- Required cross-section416.67 cm²
- Rectangular width for that height8.201 in
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How it's calculated (visible formula)
area = airflow ÷ velocity; Ø = √(4 × area ÷ π); rectangular width = area ÷ height
- Net geometric measurement, no waste.
- Adjustable waste % for cuts and breakage.
- Purchase rounded up to sellable units (boxes, bags, buckets, boards).
How to calculate it, step by step
- Work out airflow for the run.
- Pick velocity for the duct type.
- Area = airflow ÷ velocity.
- Get round diameter and stock size.
- If it won't fit, work out a rectangle for the available height.
Worked example: 353 CFM (600 m³/h) at 790 fpm (4 m/s), 8 in available height
- Area: 0.0417 m² (65 in²)
- Ø ≈ 9.1 in → 10 in stock (250 mm)
- Rectangular ≈ 8.2 × 8 in
→ A 10 in round duct or about 10 × 8 in rectangular.
Common mistakes
- High velocity to bedrooms — noisy.
- Using equal-area rectangles for very flat ducts (more friction).
- Ignoring elbows and grilles in total pressure.
Pro tips
- Step down trunk size as branches leave.
- Insulate ducts through unconditioned space.
- Use long-radius elbows to cut loss and noise.
- Seal joints with mastic or foil tape.
From calculation to quote
Add labour, overhead and your margin, and generate the professional PDF.
FAQs
- What duct velocity should I use?
- 500–700 fpm for branches, 700–900 fpm for residential trunks.
- Round or rectangular?
- Round has less friction; rectangular fits low ceilings.
- What is equivalent diameter?
- The round size with the same friction — a bit smaller than equal-area.
- Flex or rigid duct?
- Rigid for trunks; flex only for short runs to diffusers.
- Why does my duct whistle?
- Too much velocity or undersized grilles.
- What duct size for 200 CFM?
- About 8 in round at typical branch velocity.
Sources and standards
Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.