Duct Size Calculator Widget

Embed a duct sizing calculator for HVAC installers, students and DIY ventilation projects. Users enter the airflow and a design velocity and get the free area, the round diameter, and a rectangular duct of equal friction for a side they choose, with a check against residential maximum velocities.

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<iframe src="https://a2z.tools/embed/w/duct-size-calculator" title="Duct Size Calculator by A2Z Tools" width="100%" height="820" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>

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How it works

Sizing by velocity starts from continuity: the free area a duct needs is the airflow divided by the speed you allow, so 400 CFM at 700 fpm needs 0.571 ft² (82.3 in²), a round duct of 10.2 in. Metric inputs work the same with m³/h converted to m³/s and velocity in m/s. Rectangular ducts are not simply the same area: corners add friction, so the widget finds the rectangle with the same pressure loss per length at the same airflow, using the Huebscher equivalent diameter De = 1.30 (ab)^0.625 / (a + b)^0.25 printed in the ASHRAE Fundamentals duct-design chapter. You fix one side - often the depth that fits a joist space - and the other side is solved numerically until De equals the round diameter. Ducts flatter than 4:1 trigger a warning. For homes, the velocity is compared with the maximums the DOE Building America Solution Center lists from ACCA Manual D: 700 fpm for supply trunks, 600 fpm for rigid supply branches and 500 fpm for rigid returns.

Calculation method

  • Area A = Q / V (ft² = CFM / fpm; m² = (m³/h / 3600) / (m/s))
  • Round diameter D = sqrt(4A / pi), shown in inches or millimetres
  • Huebscher equivalent diameter De = 1.30 x (a x b)^0.625 / (a + b)^0.25; b solved so that De = D for the chosen side a
  • Same-velocity rectangle: b = A / a (equal area, higher friction)
  • Residential maximum velocities (ACCA Manual D Table A1-1, via DOE): supply trunk 700 fpm, rigid supply branch 600, flexible supply branch 700, rigid return 500, flexible return 600; 1 fpm = 0.00508 m/s

Worked examples

Residential supply trunk

Inputs: 400 CFM at 700 fpm, fixed side 8 in

Result: Round 10.2 in; area 82.3 in²; equal-friction rectangle 8 x 11 in; within the 700 fpm trunk maximum

Huebscher De of 8 x 11 in equals the 10.2 in round duct.

Larger main

Inputs: 1,000 CFM at 800 fpm, fixed side 10 in

Result: Round 15.1 in; equal-friction rectangle 10 x 19.7 in; same-velocity rectangle 10 x 18 in

800 fpm is above the 700 fpm residential trunk maximum, so the widget warns.

Metric extract duct

Inputs: 1,800 m³/h at 4 m/s, fixed side 250 mm

Result: Round 399 mm; area 0.125 m²; equal-friction rectangle 250 x 554 mm

A 400 mm round duct is the practical choice.

Limitations

  • Velocity sizing only - friction loss, fittings, duct length and fan static pressure are out of scope.
  • The velocity check uses residential limits; commercial and industrial systems follow other criteria.
  • Next-size-up suggestions round to whole inches or 10 mm and do not follow any particular manufacturer's size series.

Where publishers use it

  • HVAC contractor websites explaining why a too-small duct is noisy
  • Mechanical and building-services course pages on the velocity method
  • Bathroom, kitchen hood and grow-tent fan guides choosing a duct diameter
  • Basement-finishing articles fitting a rectangular trunk between joists
  • Dust-collection and workshop ventilation pages

Questions

What size duct do I need for 400 CFM?

At 700 fpm the duct needs 0.571 ft² of free area, which is a 10.2 in round duct, so a 10 in duct runs slightly fast and a 12 in duct slower. An 8 in deep rectangular duct needs to be about 11 in wide for the same friction.

Why is the rectangular duct bigger than the round one?

A rectangle with the same area has more wall surface and corners, so it has more friction. The Huebscher formula gives the rectangle with the same friction: for 1,000 CFM at 800 fpm the round duct is 15.1 in, and a 10 in deep duct must be 19.7 in wide, not the 18 in that equal area would suggest.

What velocity should I design for?

For homes, the DOE Building America Solution Center lists ACCA Manual D maximums of 700 fpm for supply trunks, 600 fpm for rigid supply branches, 700 fpm for flexible branches, 500 fpm for rigid returns and 600 fpm for flexible returns. Commercial systems use other limits set by noise criteria, so choose 'Commercial / other' to skip the check.

Does it calculate friction loss or static pressure?

No. It sizes by velocity only. Pressure drop depends on duct length, fittings, material and the fan curve; use ACCA Manual D or the ASHRAE equal-friction method with a friction chart for that.

Can I use metric units?

Yes. 700 m³/h at 3.5 m/s needs 0.0556 m² - a 266 mm round duct, so round up to the next diameter your supplier stocks. A 200 mm deep rectangle must be 299 mm wide for equal friction.

What aspect ratio is acceptable?

The widget warns above 4:1. Very flat ducts need more sheet metal for the same airflow and lose more pressure; keep rectangles as close to square as the space allows.

Sources

  1. Compact Air Distribution - US DOE Building America Solution Center (PNNL) . Maximum design velocities from ACCA Manual D Table A1-1: supply trunks 700 fpm, rigid supply branches 600, flexible supply branches 700, rigid returns 500, flexible returns 600. Checked 2026-10-01.
  2. A Computer Model of Smoke Movement by Air Conditioning Systems (SMACS), NBSIR 87-3657 (J. H. Klote, 1987) - National Bureau of Standards (now NIST) . Points to Huebscher, 'Friction Equivalents for Round, Square and Rectangular Ducts', ASHVE Transactions 54 (1948), as the relation for rectangular ducts. The formula itself is printed in the ASHRAE Handbook - Fundamentals duct-design chapter, which is not freely available online.

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A2Z Tools Duct Size Calculator
https://a2z.tools/embed/duct-size-calculator

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