Whether you are fitting a kitchen extractor fan, upgrading your bathroom ventilation, or designing a full HVAC system, getting your duct sizes right is one of the most important decisions you will make. Choose a duct that's too small, and you will struggle with poor airflow, condensation, and noisy systems. Go too large, and you waste money on materials while reducing efficiency.

This guide covers everything you need to know about flexible ducting sizes — from standard dimensions and ducting size calculations to sizing ducting for air conditioning and the step-by-step variations you should be aware of. By the end, you will know exactly which size flexible duct your project needs.

What Is Flexible Ducting and Why Does Size Matter?

Flexible ducting (also called flex duct or flexible duct) is a lightweight, bendable tube used to move air from one point to another in ventilation, extraction, and HVAC systems. Unlike standard & rigid metal ductwork, aluminium, combination, & semi-rigid flexible ducting can navigate tight corners, route through confined spaces, and connect to grilles and fans with ease.

The size of the duct — primarily its diameter — directly determines how much air it can carry. Getting the right duct size matters because:

• An undersized duct creates excessive static pressure, reducing airflow and overworking fans.
• An oversized duct reduces air velocity, causing poor extraction performance and potential condensation inside the duct.
• Correct duct sizing calculation ensures energy efficiency, low noise levels, and long system lifespan.  

Key Insight: Choosing the right type of flexible ducting as a buyer is one thing. But the right duct size is another. It's not just about fitting the duct physically into a space — it is about matching the duct's airflow capacity to the ventilation or air conditioning system it serves. 

Flexible Duct Sizing: How to Choose the Right Size

• Step 1 - Check your fan or unit's airflow rating. Look at the box or instruction sheet that came with your extractor fan or ventilation unit. It will show an airflow number (usually CFM (Cubic Feet per Minute) or m³/h). This is your starting point.
• Step 2 - Match that number to a duct size. Use a simple lookup table (like the one in this guide) to find the recommended duct diameter for your airflow rating. No calculation needed — just match the number to the table.
• Step 3 - Consider your run length and bends. Think about how far the duct needs to travel and how many corners it has to turn. Short and straight? Stick with the table size. Long run or several bends? Go one size up.
• Step 4 - When in doubt, size up. A slightly larger duct never hurts performance. A slightly smaller one always does.

Standard Flexible Duct Sizes: What Are the Common Dimensions?

Flexible ducting is available in a range of standard diameters. These are measured by the duct's internal diameter (ID), which determines airflow capacity. Below are the most commonly used flexible duct sizes in residential and light commercial applications:

Duct Diameter	Typical CFM Range	Common Applications
75mm (3")	30–60 CFM	Small bathroom fans, spot ventilation
100mm (4")	60–100 CFM	Bathroom extractor fans, tumble dryers
125mm (5")	80–150 CFM	Kitchen hood connections, utility rooms
150mm (6")	150–250 CFM	Kitchen extractors -house-wide ventilation
200mm (8")	200–400 CFM	Larger HVAC runs, commercial kitchens
250mm (10")	400–700 CFM	Main supply/return trunks, commercial
300mm (12")	700–1200 CFM	Large HVAC systems, industrial ventilation

 Can't find your fan's rating? Here are your alternatives:

•  Search the model number online — the manufacturer's product page almost always lists the airflow rating.
• Use a room-type guide — many ducting suppliers publish simple room-by-room recommendations (bathroom, kitchen, utility) with a suggested duct size.
• Ask at the trade counter — a merchant or ducting supplier can recommend the right size based on your room type and fan brand.
• Call the fan manufacturer — they will tell you the recommended duct size for their product directly.

Flexible Ducting Sizes Comparison: Which Should You Choose?

The table below provides a head-to-head comparison of the most common flexible ducting sizes to help you make the right choice for your specific application:

Duct Size	Best For	Noise Level	Key Consideration
82mm (3")	Small bathrooms	Very low	Limited to short runs only
100mm (4")	Bathrooms, WCs	Low	Most common residential size
125mm (5")	Kitchens, utility rooms	Low–medium	Good for moderate kitchen runs
152mm (6")	Kitchen hoods, MVHR	Medium	Versatile; suits most kitchens
200mm (8")	HVAC branch ducts	Medium	Ideal for longer runs
250mm (10")	Main trunks, commercial	Medium–high	Typically requires support straps
305mm (12")	Large HVAC, industrial	High if under-specified	Professional installation advised

Flexible Ducting vs Rigid Ducting: Does Size Work the Same Way?

When comparing flexible and rigid ducting, the fundamental principles of duct sizing are the same — both use diameter, airflow, and velocity as the primary variables. However, there are some practical differences to be aware of:

Factor	Flexible Ducting	Rigid Ducting
Friction Loss	Higher due to corrugated inner wall	Lower — smooth inner surface
Sizing Approach	Size up by ~10% to compensate for friction	Standard calculations apply directly
Bends & Turns	Can navigate bends, but avoid sharp kinks	Requires fittings for each turn
Installation	Easier, faster, and no specialist tools	More complex, as it requires cutting and joining
Noise	Can vibrate at high velocities	Generally quieter at equivalent velocities
Durability	Shorter lifespan; can sag or kink	More durable and maintains shape
Cost	Lower initial cost	Higher initial cost

Key takeaway: When using flexible ducting in place of rigid ducting, consider sizing up by one diameter step if the run is long, has multiple bends, or is part of an HVAC system where pressure loss is critical.

Common Flexible Ducting Sizing Mistakes to Avoid

Even experienced installers make duct sizing errors. Here are the most common flexible ducting mistakes to avoid:

• Matching duct size to fan outlet size only: The fan outlet may not match the ideal duct size for the run length. Always calculate based on airflow and run length.
• Ignoring bends in the duct sizing calculation: each bend adds additional length and resistance. A heavily bent 150 mm duct may perform worse than a straight 125 mm run.
• Leaving too much slack in the flex duct: loose, wavy flexible ducting significantly increases airflow resistance. Keep the flex duct as straight and taut as reasonably possible.
• Using uninsulated flex duct for AC systems can lead to condensation, energy loss, and potential mould — always use insulated flexible ducting for cooling applications.
• Not accounting for return air duct size: many DIY installers focus solely on supply ducts and undersize the return duct, causing a system imbalance.
• Choosing duct size by appearance rather than calculation: a duct that 'looks right' for a space is not necessarily the correct size. Always calculate your duct sizing prior to purchasing.

 FAQs

1. What standard flexible duct sizes are available?

Standard flexible duct sizes include 75mm (3"), 102 mm (4"), 125mm (5"), 150mm (6"), 203mm (8"), 250mm (10"), and 300mm (12"). The most common sizes for residential use are 100 mm and 150 mm, covering most bathroom and kitchen ventilation applications.

2. What duct size do I need for a bathroom extractor fan?

Most standard bathroom extractor fans work perfectly with a 102mm (4") flexible duct. For larger bathrooms or higher-output fans, a 125mm (5") duct may be more appropriate. Always match the duct size to the fan's rated airflow, not just its outlet size.

3. Can I use a smaller duct than the fan outlet size?

It is strongly advisable not to reduce the duct size to less than the fan's outlet diameter. Doing so increases static pressure and reduces the fan's effective airflow, often below the minimum required for adequate ventilation. If anything, sizing up slightly (especially on longer runs) improves performance.

4. What is the difference between duct size and duct sizing?

Duct size refers to the physical dimensions of a duct (e.g. 150mm diameter). Duct sizing is the process of determining the correct duct size for a given application by calculating airflow requirements, duct length, and system pressure. Duct sizing is the methodology; duct size is the result.

5. How do I size flexible ducting for an air conditioning system?

First, find the AC unit's rated airflow (CFM). Then calculate the airflow needed for each room based on its cooling load. Size each branch duct using the standard diameter formula, using a target air velocity of 400–600 FPM for bedrooms and 600–900 FPM for other areas. Always use insulated flexible ducting for AC applications.

6. Does a flexible duct need to be sized differently from a rigid duct?

Yes, slightly. Because flexible ducting has a corrugated inner surface, it creates more friction (and therefore more pressure drop) than rigid ducting of the same diameter. As a rule of thumb, when replacing rigid with flexible duct, consider sizing up one step (e.g., from 150mm to 200 mm) to compensate, particularly on longer runs.

7. What happens if my flexible duct is too small?

An undersized flexible duct will cause excessive static pressure, reduced airflow, overheating of fans, increased noise, and higher energy consumption. In ventilation applications, it can also result in inadequate air changes, potentially causing condensation, dampness, and poor indoor air quality.

8. How long can a flexible duct run be?

For 100mm residential flexible ducting, a maximum run length of 3 metres (approximately 10 feet) is generally recommended for optimal performance. Longer runs increase resistance and reduce airflow. If a longer run is unavoidable, size up the duct diameter and minimise bends wherever possible.

9. What is the most common flexible ducting size for kitchen extraction?

The most common flexible ducting size for kitchen extraction is 150mm (6"), which suits most domestic cooker hood fans rated between 150 and 250 CFM. For high-output professional-style kitchen extractors, 200mm (8") flexible ducting may be required.

Conclusion

Choosing the correct flexible ducting size is not guesswork — it is a straightforward process of matching airflow requirements to duct diameter, accounting for run length and bends, and verifying air velocity falls within acceptable limits.

Whether you are completing a bathroom installation with a 100 mm flex duct, sizing a kitchen extraction system with 150mm or 200mm ducting, or working through the calculations for a full HVAC or air conditioning system, the principles are the same:

• Calculate your required airflow first.
• Apply the duct sizing calculation to determine the correct diameter.
• Account for equivalent length (bends and run length).
• Check air velocity is within recommended limits.
• Round up to the nearest standard flexible duct size.

For complex systems or commercial installations, use a professional air duct size calculator or consult an HVAC engineer. For most residential applications, the guidance and formulas in this article will give you everything you need to specify flexible ducting confidently and correctly.

Need expert advice on flexible ducting sizes for your project? Contact our team for personalised guidance at sales@flexibleducting.com or call us on 0117 330 2277.