Complete Guide to Ductwork Sizing

Why duct sizing matters

Ductwork sizing is one of the most fundamental tasks in ventilation design. Get it wrong and the consequences cascade through the entire system: undersized ducts push air velocities too high, generating noise, increasing pressure drop, and forcing fans to work harder — all of which raises energy consumption and specific fan power (SFP). Oversized ducts waste ceiling void space, increase material and installation costs, and can cause low-velocity turbulence issues at fittings.

The aim is to find the right balance between noise, energy, space, and cost for each section of ductwork. This guide walks through the standard methods used in UK building services, the velocity limits to design to, the DW144 standard duct sizes, and a worked example you can follow.

The three sizing methods

UK building services practice recognises three main approaches to duct sizing, each suited to different system types and complexity levels:

• Velocity method — the most common approach for general HVAC. You select a target velocity based on the application and size the duct to achieve it. Simple, quick, and effective for single-branch or straightforward systems.
• Equal friction method — sizes all duct sections to maintain a constant friction rate (Pa/m). This produces more balanced pressure drops across branches and is preferred for larger, multi-branch systems.
• Static regain method — sizes each section so that the static pressure regain from velocity reduction offsets friction loss. Used in high-velocity systems and very long duct runs where maintaining static pressure at each outlet is critical.

In practice, most UK building services engineers use the velocity method for initial sizing and refine with equal friction calculations for complex systems. Static regain is relatively uncommon outside specialist high-velocity installations.

Velocity method (most common)

The velocity method is the standard starting point for duct sizing. The process is straightforward:

1. Determine the required volume flow rate (Q) for the duct section, in l/s or m³/s.
2. Select a maximum air velocity (v) based on the application (see the velocity table below).
3. Calculate the required cross-sectional area.
4. Calculate the equivalent circular diameter.
5. Round to the nearest DW144 standard size.
6. Check the actual velocity at the selected standard size.

The key formulas:

Where: A = cross-sectional area (m²), Q = volume flow rate (m³/s), v = air velocity (m/s).

Where: d = circular duct diameter (m), A = required cross-sectional area (m²).

Once you have the theoretical diameter, round to the nearest DW144 standard size and recalculate the actual velocity to confirm it stays within limits. If the next size down pushes velocity above the maximum, go up to the next standard size.

Equal friction method

The equal friction method sizes every duct section in the system to the same friction rate, measured in Pa/m. This approach naturally produces more balanced pressure drops across parallel branches, reducing the need for extensive damper adjustment at commissioning.

The typical design friction rate for low-velocity systems is 1.0 Pa/m. For high-velocity systems, rates of 2–4 Pa/m may be used, though noise implications must be carefully considered. Source: CIBSE Guide B, Section 4 [VERIFY: exact section reference for recommended friction rates].

In practice, the equal friction method requires duct sizing charts (available in CIBSE Guide C) or software. You enter the flow rate and the target friction rate, and the chart gives you the required diameter. The method works well for systems with many branches of similar length, such as office floor distribution networks.

Static regain method

The static regain method is used mainly in high-velocity systems and long supply duct runs where maintaining adequate static pressure at each outlet is critical. The principle is to size each duct section so that the velocity reduction at a branch take-off produces a static pressure regain that approximately offsets the friction loss in the preceding section.

This method maintains roughly constant static pressure along the length of the duct, which improves air distribution uniformity at terminal devices. However, it results in progressively larger ducts as you move away from the fan, which can create space challenges.

Static regain sizing is less common in general UK building services work. It is most relevant for large supply ducts in industrial, commercial kitchen extract, or high-velocity air conditioning systems. For most office, education, and residential projects, the velocity or equal friction method is sufficient.

Recommended duct velocities

The table below shows guidance velocities for different duct applications. These values balance noise, pressure drop, and practical system performance.

Source: CIBSE Guide B, Table 4.20 [VERIFY: exact table number in current edition].

DW144 standard duct sizes

DW144 is the HVCA/BESA specification for sheet metal ductwork in the UK. It defines standard circular duct diameters, construction classes, and fabrication requirements. Using DW144 standard sizes reduces fabrication cost, lead time, and the risk of non-standard components arriving on site.

Standard circular diameters (mm)

80, 100, 125, 150, 160, 200, 250, 315, 355, 400, 450, 500, 560, 630, 710, 800, 900, 1000, 1120, 1250

Rectangular duct sizes

For rectangular ductwork, standard width increments are typically 50 mm up to 300 mm, then 100 mm increments above 300 mm [VERIFY: exact DW144 rectangular increment rules]. When comparing rectangular to circular ducts, the equivalent circular diameter is calculated using:

Where: d_e = equivalent circular diameter (mm), a and b = rectangular side dimensions (mm). Source: CIBSE Guide C [VERIFY: exact reference in Guide C].

The equivalent diameter gives the circular duct that would have the same friction loss per unit length as the rectangular duct at the same flow rate. It is not the same as equal area — rectangular ducts have a higher friction loss per unit area than circular ducts due to corner effects.

Circular vs rectangular

Choosing between circular and rectangular ductwork depends on several practical factors:

Circular ductwork

• Lower pressure drop per unit length for the same flow rate (no corner turbulence).
• Easier to seal — fewer joints, better airtightness.
• Less material per unit of cross-sectional area (a circle is the most efficient shape).
• Preferred where ceiling void depth allows it.

Rectangular ductwork

• Fits in shallow ceiling voids and bulkheads where height is limited.
• More flexible routing around structural elements.
• Commonly used for main risers and large distribution ducts in commercial buildings.
• Higher aspect ratios (e.g. 4:1) are less efficient — try to keep the aspect ratio below 3:1 where possible.

Flat oval

Flat oval ductwork is a compromise: it offers lower profile than circular duct while maintaining better aerodynamic performance than high-aspect-ratio rectangular duct. It is less commonly specified in the UK but can be useful in retrofit projects with tight ceiling voids.

Worked example

Size a circular duct for 500 l/s supply air with a maximum velocity of 5 m/s.

Step 1: Convert flow rate to m³/s

Step 2: Calculate required area

Step 3: Calculate required diameter

Step 4: Select DW144 standard size

The nearest standard sizes are 355 mm (just under) and 400 mm (just over). Check both.

Step 5: Check velocity at 400 mm

This is comfortably within the 5 m/s limit.

Step 6: Check velocity at 355 mm

This marginally exceeds 5 m/s. In practice you have two choices: accept the marginal exceedance (5.05 m/s is only 1% over) if the noise environment allows it, or use the 400 mm duct for a comfortable margin.

You can run this calculation instantly using the Duct Sizer tool, which shows both DW144 options and their resulting velocities.

Common mistakes

• Not accounting for fittings and transitions. The velocity method sizes straight duct sections. Fittings (bends, tees, reducers) add pressure loss that must be calculated separately as velocity pressure loss coefficients or equivalent lengths. These can significantly increase total system resistance.
• Using manufacturer’s nominal sizes instead of DW144 standards. Some manufacturers offer non-standard sizes. Specifying DW144 ensures interchangeable components and avoids site problems when different trades or suppliers are involved.
• Forgetting to check noise implications at higher velocities. A duct may be correctly sized for flow, but if the velocity generates noise above the NR limit for the space, the design fails. Noise regeneration at fittings is particularly problematic.
• Sizing supply and extract independently. In a balanced system, changes to the supply duct sizing affect the total system pressure, which in turn affects the fan duty and extract side. Size the complete system, not individual branches in isolation.
• Not allowing for future capacity. It is good practice to include a small margin (typically 10–15%) for future load increases, especially in speculative office developments. However, massive oversizing wastes space and money and can create low-velocity problems at partial load.