What size duct do I need?
Enter the airflow in litres per second and select the duct type — the calculator finds the minimum cross-section area and the corresponding round duct diameter using the velocity method.
Next standard size: 150 mm round duct
Required area: 13056 mm² · Exact diameter: 128.9 mm
Rectangular equivalents
| Width | Height needed |
|---|---|
| 200 mm | 66 mm |
| 300 mm | 44 mm |
| 400 mm | 33 mm |
Height = required area ÷ width. Round up to the nearest stock size.
Velocity method only — this gives the minimum duct cross-section for the design velocity. A full Manual D calculation also accounts for friction loss, static pressure, and duct run length. Always have duct sizes reviewed by your HVAC contractor.
Recommended duct design velocities
Planning targets for residential HVAC systems. Higher velocity = smaller duct but more noise and friction loss. Lower velocity = quieter, lower pressure drop, but larger duct required.
| Duct type | FPM (imperial) | m/s (metric) |
|---|---|---|
| Supply main trunk | 700–1,000 | 3.6–5.1 |
| Supply branch duct | 500–800 | 2.5–4.1 |
| Return air — main | 600–900 | 3.1–4.6 |
| Return air — branch | 400–600 | 2.0–3.1 |
| Flexible duct (max) | ≤600 | ≤3.1 |
| Register / diffuser face | 300–500 | 1.5–2.5 |
These are design guidance figures from ACCA Manual D and ASHRAE residential duct design guidelines — not code minimums. Local codes and the specific system design may require different values; always have ductwork designed and installed by a licensed HVAC contractor.
Standard round duct sizes
| Diameter (in) | Diameter (mm) | Area (in²) | At 700 FPM → CFM |
|---|---|---|---|
| 4" | 102 | 12.6 | 61 |
| 5" | 127 | 19.6 | 95 |
| 6" | 152 | 28.3 | 137 |
| 7" | 178 | 38.5 | 187 |
| 8" | 203 | 50.3 | 244 |
| 9" | 229 | 63.6 | 309 |
| 10" | 254 | 78.5 | 382 |
| 12" | 305 | 113.1 | 550 |
| 14" | 356 | 153.9 | 748 |
| 16" | 406 | 201.1 | 977 |
Area = π × (diameter/2)². CFM at 700 FPM = area (ft²) × 700. Values rounded to the nearest whole number. Want the full explanation? Read the duct sizing guide →
Common questions
- How do I calculate duct size?
- Use the velocity method: divide the required airflow (CFM) by the design velocity (FPM) to get the minimum cross-section area in square feet. Multiply by 144 to convert to square inches. For a round duct, the diameter in inches equals 2 × √(area ÷ π). For example, a branch supply duct carrying 150 CFM at 700 FPM needs an area of 30.9 in² and a diameter of about 6.3 inches — round up to a 7-inch round duct.
- What CFM do I need per room?
- A common rule of thumb is 1 CFM per square foot of floor area for standard 8-foot ceilings, or about 0.5–1 CFM per square foot for energy-efficient homes. A 150 sq ft bedroom typically gets 100–150 CFM; a 200 sq ft living room gets 150–200 CFM. The precise number comes from a Manual J load calculation, which accounts for insulation, windows, orientation, and climate — a rule of thumb is a starting point only.
- What is the best velocity for HVAC ducts?
- Typical design velocities are 700–900 FPM (3.6–4.6 m/s) for main trunk ducts, 500–700 FPM (2.5–3.6 m/s) for branch supply ducts, and 400–600 FPM (2.0–3.1 m/s) for return air and flexible ducts. Higher velocities increase static pressure loss and duct noise; lower velocities require larger duct sizes. These are planning guidelines — a full Manual D duct design also checks friction loss per 100 feet of duct run.
- What size duct do I need for a bedroom?
- A typical bedroom of 150–200 sq ft needs around 100–150 CFM. At a branch supply velocity of 700 FPM, 100 CFM requires a 5.1-inch round duct (use a 6-inch) and 150 CFM requires a 6.3-inch round duct (use a 7-inch). Metric equivalent: 47 L/s at 3.6 m/s needs a ≈129 mm round duct (use 150 mm).
- What is the difference between round and rectangular duct?
- Round duct is more efficient — a given diameter has less surface area per unit of area, so it has lower friction loss and is easier to seal. Rectangular duct fits more easily into tight ceiling cavities and floor joists. For the same airflow and velocity, a rectangular duct has the same cross-section area as the equivalent round duct — the calculator shows both. When in doubt, round duct performs better; use rectangular where geometry demands it.
Reference & education only. Not professional, engineering, or code-compliance advice. Estimates are based on published model codes; local amendments and your Authority Having Jurisdiction (AHJ) govern. Always verify against the current adopted code and a licensed professional before doing work.
Last reviewed 2026-07.