Extractor Fan Sizes Explained: 4 Inch vs 5 Inch vs 6 Inch for Bathrooms and Kitchens

Overview

If you are asking what size extractor fan do I need, start with one key idea: fan size and fan performance are related, but they are not identical. A 4 inch extractor fan usually connects to 100mm ducting, a 5 inch extractor fan to 125mm ducting, and a 6 inch extractor fan to 150mm ducting. In general, a larger duct size allows more airflow and can help reduce resistance, but the right choice still depends on the fan design and the installation conditions.

For most UK homes, the usual pattern looks like this:

• 4 inch extractor fan: common for standard bathrooms, en-suites and WCs.
• 5 inch extractor fan: a useful middle ground where a bathroom needs more extraction than a basic 4 inch model, or where a utility room or larger shower room produces heavier moisture.
• 6 inch extractor fan: more common in kitchens, larger wet rooms, and situations where you need stronger airflow through longer or more restrictive duct runs.

That said, size alone does not solve ventilation problems. A well-chosen 4 inch fan installed with a short, straight duct run can outperform a badly installed 6 inch unit fighting through bends, crushed flexible ducting and a restrictive external grille. If your concern goes beyond one room and into broader indoor air quality, it can also be worth comparing extractor fans with whole-home options in our guide to the best ventilation system for a house in the UK.

As a rule of thumb, think about fan selection in this order:

1. Room type: bathroom, shower room, kitchen, utility, cloakroom.
2. Moisture or odour load: occasional use, daily showers, frequent cooking, poor existing ventilation.
3. Duct route: through-wall, short ceiling duct, long run to a roof terminal, multiple bends.
4. Replacement constraints: existing cut-out size, existing duct diameter, wiring, access.
5. Usability: noise, overrun timer, humidity sensor, boost control, ease of cleaning.

Once you look at those five points together, the "best" extractor fan size becomes much clearer.

Checklist by scenario

Use this section as a practical shortlist. It will not replace a room-by-room assessment, but it will help you narrow down the right direction before you buy.

1. Small bathroom, en-suite or downstairs WC

Usually consider: a 4 inch extractor fan.

A 4 inch fan is often the default for compact bathrooms and cloakrooms because it suits typical domestic ducting, fits many existing openings, and is widely available in basic, timer and humidity-sensor versions. It is often a sensible choice when:

• the room is small,
• the duct run is short and fairly straight,
• you are replacing an older 100mm fan,
• the main aim is everyday moisture control rather than heavy-duty extraction.

Choose carefully if:

• the room has a powerful shower used multiple times a day,
• there is no opening window,
• condensation lingers on mirrors, walls or windows long after bathing,
• the duct run includes several bends or a long route through the loft.

In those cases, a stronger 4 inch centrifugal or mixed-flow model may be more suitable than a basic axial fan. If mould is already appearing, it is also worth reading our guide to mould and ventilation fixes that last, because the same moisture patterns often affect nearby rooms.

2. Family bathroom with bath and separate shower

Usually consider: a strong 4 inch model or a 5 inch extractor fan.

This is where many homeowners get stuck. A standard bathroom may cope perfectly well with 100mm extraction, but a busier family bathroom creates more humidity and often needs better performance, especially in winter. A 5 inch extractor fan can make sense when you want more airflow than a typical basic bathroom fan provides but do not need to step up to a larger kitchen-style setup.

A 5 inch fan is worth considering if:

• the bathroom is medium to large,
• two or more people shower in close succession,
• the room is internal or poorly ventilated,
• you are planning a new installation and are not tied to an existing 100mm opening,
• you want a quieter system achieving the target airflow with less strain.

Checklist before choosing 5 inch:

• Confirm there is space for 125mm ducting.
• Check the wall sleeve, ceiling void or boxed-in route can accommodate the larger diameter.
• Make sure the external grille or roof terminal matches the duct size.
• Verify whether the fan is axial, centrifugal or mixed-flow, since that affects real performance.

A 5 inch extractor fan is less universal than a 4 inch model, but it can be a very practical answer where a bathroom repeatedly feels damp after use.

3. Standard kitchen extraction point

Usually consider: a 6 inch extractor fan, or follow the appliance manufacturer guidance if paired with a cooker hood system.

Kitchens generate grease, heat, steam and odours, so they generally call for more extraction than bathrooms. A 6 inch extractor fan is a common fit for kitchen wall fans and utility areas with heavier moisture loads. The larger duct size can help move more air and cope better with demanding use.

6 inch is often the better option when:

• you cook frequently,
• the room is open-plan or semi-open to another living space,
• the extraction route is longer,
• there is a need to clear odours more quickly,
• you are replacing an existing 150mm kitchen fan.

For kitchens, remember that duct design matters especially strongly. Grease build-up, backdraught shutters, long horizontal runs and poorly chosen external terminals can all reduce actual performance.

4. Utility room, laundry area or drying room

Usually consider: a 4 inch or 5 inch fan, depending on how much moisture the room handles.

If the room mainly houses a washing machine and has occasional damp air, a decent 4 inch fan may be enough. If it doubles as a drying room, contains a tumble dryer vent arrangement, or regularly fills with humid air from indoor clothes drying, moving up to a 5 inch extractor fan may be more sensible.

Ask yourself:

• Do clothes regularly dry indoors here?
• Does the room feel humid for long periods?
• Is there visible condensation on colder surfaces?
• Does the fan need to work through a long ceiling or loft run?

If humidity remains high throughout the home rather than in one room, a local extractor fan might only be part of the answer. You may also want to compare broader ventilation approaches such as PIV or MVHR.

5. Replacement fan where the hole size already exists

Usually consider: matching the existing duct size first, then checking whether the fan type can be improved.

This is one of the most common real-world situations. If you already have a 4 inch opening, replacing it with another 4 inch unit is often the simplest route. Enlarging the opening to fit a 5 inch or 6 inch extractor fan may be possible, but it can add cost and disruption, especially on tiled walls, finished ceilings or external walls with awkward access.

Before changing size, check:

• the duct diameter already installed,
• whether the old fan is underperforming because of poor design rather than small size,
• whether the external grille is blocked or restrictive,
• whether the ducting is damaged, sagging or crushed,
• whether the current fan runs long enough after use.

Sometimes the best upgrade is not a bigger size but a better-performing fan in the same size, with a timer or humidity sensor and better pressure handling.

6. Long duct runs through lofts or ceilings

Usually consider: higher-pressure fan designs, often with 5 inch or 6 inch ducting where practical.

A long or complicated duct run changes everything. Every bend, every metre of duct and every awkward termination adds resistance. In these cases, many disappointing installations come from choosing the fan based only on diameter rather than pressure performance.

Checklist for this scenario:

• Measure the approximate duct length.
• Count the number of bends.
• Check whether the duct is flexible or rigid.
• Check whether the route passes through cold loft space that may need insulation.
• Choose a fan designed to maintain airflow against resistance.

If your fan is ceiling-mounted and vents through the loft, the installation details matter as much as the fan itself.

What to double-check

Before you settle on a 4 inch, 5 inch or 6 inch fan, work through these checks. This is the part most likely to save you from buying twice.

Duct diameter is not just a label

The fan size should match the ducting and terminals. Reducing a 6 inch fan down to smaller ducting can undermine the reason for choosing it in the first place. Likewise, fitting a smaller fan onto oversized ductwork is possible in some situations but may not give the performance you expect.

Axial vs centrifugal vs mixed-flow

Two fans with the same diameter can behave very differently. Axial fans are often suitable for short, direct runs. Centrifugal and mixed-flow designs are often better where pressure resistance is higher. If your duct route is anything other than simple, this distinction matters.

Noise in real use

A fan that is technically powerful but unpleasantly loud may be switched off, isolated, or avoided. That defeats the point. Check the likely sound level, but also think practically about where the noise will be heard: directly in the bathroom, in a nearby bedroom, or transmitted through rigid ducting.

Control method

Basic on-off switching works, but many homes benefit from a timer overrun or humidity sensor. Overrun helps clear residual moisture after a shower. Humidity sensors can help when occupants forget to use the fan. The best control method depends on how the room is used.

External grille and backdraught shutter

A poor-quality external terminal can restrict airflow or allow rattling in windy weather. If you are replacing a fan but not improving the duct end, performance gains may be limited.

Maintenance access

Can the grille be cleaned easily? Can the unit be reached without damaging finishes? Kitchen fans and bathroom fans both lose effectiveness when dust and grime build up.

Whether you have a room problem or a house problem

If condensation appears on windows across several rooms, or if the home feels stale generally, changing one extractor fan size may not be enough. In that case, our guide on how to stop condensation on windows in winter can help you assess whether the issue is local or whole-house. You may also need to compare extractor fans against wider systems such as PIV or MVHR, especially in airtight homes. For those options, see our related articles on PIV system costs and MVHR installation costs in the UK.

Common mistakes

The wrong extractor fan size is often only part of the problem. These are the mistakes that cause the most disappointment in bathroom and kitchen ventilation projects.

Choosing by hole size alone

Matching the existing opening is convenient, but if the room use has changed or the old fan never worked well, convenience may not equal suitability.

Assuming bigger is always better

A 6 inch extractor fan is not automatically the best answer for every damp bathroom. Larger fans may need larger ducting, more space and a better overall layout. If badly fitted, they can still underperform.

Ignoring duct resistance

This is perhaps the most common issue. Long flexible ducting with several bends can make even a decent fan struggle. A smaller but better-specified fan can outperform a larger, poorly matched one.

Forgetting replacement compatibility

Check spigot depth, grille size, fixing points, wiring arrangement and whether the fan is suitable for the mounting position. A nominally correct size can still be awkward to fit.

Not thinking about user behaviour

If the fan only works when the light is on, but people shower with low lighting or leave quickly, moisture may remain. Controls matter almost as much as airflow.

Treating mould as a fan-only issue

Mould can be linked to under-heating, thermal bridging, blocked vents and broader ventilation imbalance. An extractor fan is important, but it is not the only variable.

When to revisit

The best time to review extractor fan sizes is not only when a unit fails. Revisit your decision whenever the room conditions change, because the right setup for one stage of a home may not be right later.

Review your fan choice if:

• you renovate a bathroom or kitchen,
• you change the room layout,
• you add a more powerful shower,
• you notice longer-lasting condensation or odours,
• you plan to improve insulation and airtightness,
• you replace windows and reduce natural background leakage,
• you start drying clothes indoors more often in colder months,
• the existing fan becomes noisy, dirty or unreliable.

A simple action plan is:

1. Measure the existing duct size and opening.
2. Note the room type and how often it produces moisture or odours.
3. Check whether the duct route is short and direct or long and restrictive.
4. Decide whether you need a straight replacement or a performance upgrade.
5. Compare 4 inch, 5 inch and 6 inch options based on the installation, not just the room label.
6. If the issue affects the whole home, review whether a wider ventilation strategy is needed.

For most homes, the reusable takeaway is straightforward: choose a 4 inch extractor fan for many standard bathroom applications, look at a 5 inch extractor fan where moisture load or duct resistance is higher, and reserve a 6 inch extractor fan for kitchens, larger wet areas and more demanding extraction duties. Then confirm the fan type, duct route and controls before buying. That small amount of extra checking is usually what turns an average installation into one that actually clears steam, reduces humidity in the house, and stays practical to live with.