Duct Size Calculator — Duct Cross-Section from Airflow
Enter the airflow and the mean velocity your design basis allows: get the free area the duct needs, the round diameter and — at the aspect ratio you choose — the sides of a rectangle with the same area.
What this tool estimates
The free cross-sectional area a duct needs to carry your airflow at the mean velocity you choose, as a round internal diameter and, if you give an aspect ratio, the sides of a rectangle of the same area.
What it cannot decide
- What velocity to design for. Noise, pressure drop, ceiling space and fan energy set it; the tool has none built in.
- Friction loss, static pressure or fan duty — those need the whole duct system: lengths, fittings, dampers, filters and terminals.
- Kitchen-exhaust, smoke-extraction or other fire-mode duct requirements, which follow their own code and authority conditions.
- Insulation, lining or sheet gauge. Sizes here are internal free dimensions, not rounded to fabricated sizes.
Result
Same area is not same friction: a rectangle of equal area has a longer perimeter, so more pressure loss per metre at the same velocity. No pressure-loss result is given here — that needs the duct system.
| Step | Value |
|---|
| Velocity | Area | Round Ø | Rectangle |
|---|
Method version 6 October 2026. A screening estimate, not a design: see the limits below.
How this calculator works — and where it stops
What it calculates
The free cross-sectional area a duct needs to carry an airflow at the mean velocity you choose, as the internal diameter of a round duct and — when you give an aspect ratio — the sides of a rectangle with the same area, with the inside perimeter of each.
Inputs
- Airflow in m³/s, m³/h, CFM or L/s.
- Design mean velocity in m/s or ft/min — your choice from the design basis, more than 0. The tool has no built-in velocity.
- Optional: the rectangle's aspect ratio, long side ÷ short side (1 or more).
Method
- Airflow is converted to m³/s (1 m³/s = 3,600 m³/h; 1 CFM = 1.6990107955 m³/h; 1 L/s = 3.6 m³/h) and velocity to m/s (1 ft/min = 0.00508 m/s).
- Free area A (m²) = airflow (m³/s) ÷ velocity (m/s).
- Round duct internal diameter D = √(4A ÷ π).
- Rectangle of the same area at aspect ratio r: short side = √(A ÷ r); long side = r × short side.
- Inside perimeter — π × D for the round duct, 2 × (long + short) for the rectangle — is the sheet surface per metre of straight duct, before seams, joints and overlaps.
- Sensitivity: area, diameter and rectangle are recomputed with the velocity 10% and 20% either side, at the same airflow.
Assumptions
- The velocity is the mean (average) velocity across the free area at the design airflow.
- Dimensions are internal and free. Internal insulation, acoustic lining or anything else inside the duct needs its thickness added.
- The rectangle has the same area as the round duct, so the same mean velocity — not the same friction loss.
Limitations
- No pressure-loss, static-pressure or fan-duty result: those need the whole duct system — lengths, fittings, dampers, filters and terminals.
- It does not choose the velocity. Allowable noise, pressure drop, ceiling space and fan energy set it, and they differ between mains, branches and outlets.
- A rectangle of equal area has a longer perimeter, so more friction per metre than the round duct; a rectangle with the same friction is larger, and sizing it is a duct-design step not done here.
- Kitchen-exhaust, smoke-extraction and other fire-mode ducts follow their own code and authority requirements for construction and velocity.
- Sizes are not rounded to standard or fabricated sizes: round up, then check the velocity again.
Worked example
Produced by running this calculator with the inputs below.
Inputs
- Airflow 1 m³/s (3,600 m³/h; 2,118.88 CFM)
- Mean velocity 5 m/s (984.25 ft/min) — the plan's test input, not a recommended velocity
- Aspect ratio 2 : 1 for the rectangle
Working
- Free area = 1 ÷ 5 = 0.2 m².
- Round duct diameter = √(4 × 0.2 ÷ π) = 0.50463 m = 504.63 mm.
- Rectangle at 2 : 1: short side = √(0.2 ÷ 2) = 316.23 mm; long side = 2 × 316.23 = 632.46 mm.
- Inside perimeter: round 1.585 m, rectangle 1.897 m — 20% more sheet per metre for the same area.
Result. 0.2 m² of free area: a 504.63 mm round duct, or 632.46 × 316.23 mm at 2 : 1 — before rounding to a fabricated size.
Sensitivity — what moves the answer
- Diameter moves with the square root of velocity: at 4 m/s (−20%) the area is 0.25 m² and the round duct 564.19 mm; at 6 m/s (+20%), 0.1667 m² and 460.66 mm.
- Doubling the airflow at the same velocity doubles the area and multiplies the diameter by √2, about 1.41.
- The aspect ratio leaves the area unchanged but stretches the perimeter: at 2 : 1 the rectangle carries 20% more sheet per metre than the round duct of the same area.
How engineers use the result
To check that a proposed duct or opening is in the right range for its airflow before the layout is detailed, and to compare round and rectangular options for the ceiling void and the sheet they use — a quick cross-check on duct sizes in a drawing or BOQ.
When a professional design must replace it
Before ductwork is fabricated or priced: an HVAC engineer should size the network by a recognised duct-design method against the fan's available pressure, check noise at the outlets, confirm clearances in the ceiling void, and apply any fire-damper, smoke-extraction or kitchen-exhaust requirements.
Sources
- NIST Special Publication 811, Appendix B.9 — 1 ft³/min = 4.719 474 × 10⁻⁴ m³/s; 1 ft/min = 5.08 × 10⁻³ m/s; 1 ft² = 9.290 304 × 10⁻² m² — unit conversions only; checked 6 Oct 2026
- National Building Code of India 2016 (BIS), Part 8 Building Services, Section 3 — Air Conditioning, Heating and Mechanical Ventilation — its planning provisions include supply and return air ducts; no figure from it is built into this tool
- BIS, Guide for Using National Building Code of India 2016 — summarises Part 8, Section 3 and Part 4 (air conditioning, ventilation and smoke control)
- Continuity of flow: airflow = area × mean velocity — definition; no code clause is applied by this tool
Questions people ask before using it
What do I need before using the duct size calculator?
The design airflow for the duct section — from the heat-load or ventilation design, not a fan's maximum — in m³/s, m³/h, CFM or L/s; the mean velocity your design basis allows for that section, which is usually lower near occupied spaces and outlets than in a plant-room main; and, for a rectangular duct, the aspect ratio the ceiling void allows.
Which site conditions change the duct size?
The depth of the ceiling void and what crosses it (beams, sprinkler mains, cable trays), the noise limit in the rooms the duct passes, the pressure the fan can deliver against the rest of the system, internal insulation or acoustic lining that eats into the free area, and any fire-mode or kitchen-exhaust duty, which brings its own construction and velocity requirements.
What should I send an HVAC engineer for a duct design review?
The layout with the duct routes and ceiling-void depths, airflows per room or outlet, the AHU or fan schedule with its external static pressure, noise criteria for the spaces served, the BOQ or drawings with proposed sizes and sheet gauges, and any fire-damper, smoke-extraction or kitchen-exhaust requirements on the project.
Optional: ask an engineer to look at it
The result above is yours with no form. If you want an engineer to check it against your drawings, schedules or bills, send a request. Nothing is sent until you press the button.
Request saved
It reached our team with exactly what the preview showed. An engineer will use the number you gave if they need your drawings or data.
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The work itself
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The terms behind it
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- Full MEPF Package for JMH Prime, Jeewan Mala Hospital — Karol Bagh, New Delhi — Jeewan Mala Hospital
- Electrical & Mechanical for Tata International — Tata International
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