Why are duct dimensions specified in inches even in metric countries?

Ductwork and HVAC equipment globally originated in the US and duct sizes became standardised in inch increments. Even in Australia and other metric countries, ductwork is manufactured and specified in inch-based sizes (10x10, 12x16, 24x24 etc.) and fan equipment is rated in CFM. Some Australian specifications now use mm and L/s, but inch and CFM remain dominant in practice for rectangular ductwork.

What is the maximum duct velocity I should use for a quiet system?

For low-noise residential HVAC: 400-600 FPM for return ducts, 500-700 FPM for supply ducts. For commercial applications where some noise is acceptable: up to 900 FPM for main ducts, 600 FPM at terminal devices. Above 800 FPM in occupied spaces, audible air noise becomes noticeable. High-velocity systems (used in limited building heights) operate at 1,500-2,500 FPM but require special attenuators at terminals.

How do I calculate the airflow required for a room?

Minimum residential ventilation under the NCC: 10 L/s per person for habitable rooms. For commercial: see AS 1668.2 for minimum outside air rates by occupancy type. For cooling: the room sensible heat load (in kW) divided by the temperature difference between supply and room air, converted to L/s using the specific heat of air: L/s = kW / (1.2 x deltaT in K). A thermal engineer performs this calculation for commercial buildings.

Airflow / Ventilation Calculator — Free Tradies Calculator

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A mechanical subcontractor needs the airflow in CFM through a 16-inch x 10-inch duct at 500 FPM velocity before specifying the fan unit. You have the duct dimensions on the drawing and the design velocity from the engineer.

Airflow / Ventilation Calculator

HVAC

Duct width (inches)

Standard duct sizes: 6", 8", 10", 12"

Duct height (inches)

Air velocity (ft/min)

Typical: 600–900 ft/min supply · 400–600 return

CFM = Duct area (ft²) × Velocity (ft/min) Duct area = (W × H) ÷ 144 (inches² → ft²) Example: 10" × 8" duct at 700 ft/min → Area = 0.556 ft² → 389 CFM (184 L/s)
Velocity guide: Supply air: 600–900 ft/min · Return air: 400–600 ft/min · Exhaust: 500–800 ft/min

Reference: ASHRAE — HVAC Fundamentals · NCC — Ventilation Requirements

ℹ️ Results are estimates for planning purposes. Verify with current standards and a qualified professional.

1 What this calculator does

Calculates airflow (CFM and L/s) through a rectangular duct from duct dimensions and air velocity. Shows duct cross-section area in both ft² and m². Used for mechanical ventilation system sizing and fan selection.

2 Formula & professional reasoning

Duct area (ft²) = Width (in) x Height (in) / 144
Airflow (CFM) = Duct area (ft²) x Velocity (FPM)
Airflow (L/s) = CFM x 0.4719
Duct area (m²) = ft² x 0.0929

Airflow is the product of the duct cross-section area and the air velocity. The /144 converts square inches to square feet. CFM (cubic feet per minute) is the US/AU industry standard for mechanical ventilation -- most fan data sheets and HVAC references use CFM for duct sizing. L/s (litres per second) is the metric equivalent used in some Australian specifications. The calculator uses inches for duct dimensions because ductwork is specified and manufactured in inch dimensions even in metric-primary countries.

3 Worked examples

⚠️ Illustrative example only — not clinical or professional instruction.

Basic

Standard rectangular duct

Given: Width: 16 inches | Height: 10 inches | Velocity: 500 FPM

Working: Area: (16 x 10) / 144 = 160/144 = 1.111 ft² | CFM: 1.111 x 500 = 556 CFM | L/s: 556 x 0.4719 = 262 L/s

Answer: 556 CFM | 262 L/s | Duct area: 1.11 ft² (0.103 m²)

💡 556 CFM through a 16x10 duct at 500 FPM is typical for a low-velocity HVAC system. Maximum recommended velocity for low-noise residential: 500-700 FPM.

Standard

Check velocity in an existing duct

Given: Known duct: 24 x 12 inches | Required airflow: 1,200 CFM | Solve for velocity

Working: Area: (24 x 12)/144 = 2.0 ft² | Velocity: 1,200 / 2.0 = 600 FPM

Answer: Duct velocity: 600 FPM at 1,200 CFM

💡 600 FPM is within the 400-800 FPM target range for main supply ducts. Acceptable for most commercial applications.

Advanced

Size a duct for a required airflow

Given: Required: 800 CFM | Maximum velocity: 600 FPM | Solve for duct area

Working:

Required area: 800 / 600 = 1.333 ft² | In square inches: 1.333 x 144 = 192 in² | Options: 16x12 = 192 in² (exact) or 14x14 = 196 in² (close)

Answer: Minimum duct area: 1.33 ft² | Use 16x12 or 14x14 duct

💡 Standard duct sizes come in 2-inch increments. Select the first standard size that meets or exceeds the calculated minimum area.

4 Sanity check

Design velocity ranges

Low velocity supply: 400-700 FPM | Main supply ducts: 600-900 FPM | Return ducts: 400-600 FPM | High velocity: 1,500-2,500 FPM

Higher velocities create noise -- keep below 800 FPM for occupied spaces.

Common duct conversions

1 CFM = 0.4719 L/s | 1 L/s = 2.119 CFM | 1 ft² = 0.0929 m²

Minimum ventilation rates (residential)

AU NCC: 10 L/s per person for habitable rooms | 25 L/s for bathrooms and laundries

Round vs rectangular duct

Round duct has lower friction and noise at the same area | Rectangular duct is used where headroom is tight

5 Common errors

Error	Cause	Consequence	Fix
Using metric duct dimensions (mm) instead of inches	Reading duct sizes from metric drawings without converting	Area calculated 1,550x too large -- airflow vastly overstated	Duct dimensions for this calculator are in inches. Convert mm to inches: divide by 25.4. A 400mm x 250mm duct is 15.75 in x 9.84 in.
Confusing supply and return duct velocity requirements	Using the same velocity limit for all duct types	Return duct sized too small -- creates excessive pressure drop and noise	Return ducts operate at lower velocities than supply ducts (400-600 FPM vs 600-900 FPM) to minimise suction noise at grilles. Size return and supply ducts separately.
Not accounting for fitting pressure drop in duct sizing	Sizing only for straight-duct velocity without equivalent lengths for bends and fittings	System underperforms -- fan cannot achieve design airflow due to excess resistance	For each 90-degree bend, add 10-15 equivalent feet of straight duct to the total duct length for pressure drop calculations. Complex duct runs with many bends require a full system pressure drop analysis.
Using FPM velocity that exceeds manufacturer rating for the fan or grilles	Calculating airflow without checking terminal unit ratings	Excessive noise at grilles or fan overloads -- system complaints from occupants	Always check the velocity and pressure drop against the fan curve and the grille manufacturer's maximum rated airflow.