Bathroom Extractor Fan Sizes Explained: 100mm, 125mm & 150mm Guide

Choosing the correct bathroom extractor fan size ensures adequate ventilation performance, compliance with UK Building Regulations Part F, and long-term reliability. Extractor fans are available in 100mm (4"), 125mm (5"), and 150mm (6") sizes, each suited to different room volumes, duct configurations, and extraction requirements. Whether you're a professional installer specifying for new builds, a landlord upgrading rental properties, or a homeowner selecting the right fan for a bathroom renovation, this guide explains how to calculate the correct fan size and which models deliver optimal performance for your specific application.

Standard Extractor Fan Sizes: 100mm, 125mm, 150mm

100mm (4 inch) extractor fans are the most common size for residential bathrooms, delivering 15-25 litres per second (l/s) extraction rate. This meets Part F Building Regulations requirements for bathrooms with toilets (15 l/s minimum) and provides adequate moisture extraction for rooms up to 12-15m³. Most standard residential bathrooms use 100mm fans from brands like Xpelair, Envirovent, Vistalux, and Blauberg.

125mm (5 inch) extractor fans deliver 25-40 l/s extraction, suited to larger bathrooms (15-25m³), commercial washrooms, or installations with longer duct runs (3-6 metres) where additional pressure and airflow are required. 125mm fans are also specified for utility rooms and kitchens requiring higher extraction rates than standard bathrooms.

150mm (6 inch) extractor fans provide 40-60+ l/s extraction for large commercial washrooms, multi-bathroom installations, or kitchens requiring powerful extraction adjacent to cooking areas. 150mm fans are less common in residential applications but essential for challenging installations with very long duct runs or high moisture loads.

How to Calculate the Correct Fan Size

Calculate room volume by multiplying length × width × height in metres. For example, a bathroom measuring 2.5m × 2m × 2.4m = 12m³. Part F requires air changes per hour based on room volume and usage. For bathrooms, the fan must extract the room volume at least 6-8 times per hour during operation.

For a 12m³ bathroom, minimum extraction rate = 12m³ × 6 air changes ÷ 60 minutes = 1.2m³/min = 20 l/s. A standard 100mm fan delivering 15-20 l/s meets this requirement. For larger bathrooms or those with showers generating high moisture loads, specify fans delivering 20-25 l/s or upgrade to 125mm models.

Always account for duct length and bends when calculating fan size. Each metre of ducting and each 90-degree bend reduces effective airflow. Consult manufacturer duct length tables to verify the fan will achieve required extraction rates for your specific installation.

100mm Extractor Fans: When to Use

100mm fans are ideal for standard residential bathrooms up to 15m³, installations with short duct runs (under 3 metres) and minimal bends, separate WCs and cloakrooms, en-suites and small bathrooms in flats and apartments, and budget-conscious projects where cost control is important. Most residential applications suit 100mm fans, making them the default choice for straightforward bathroom ventilation.

Popular 100mm models include Envirovent Silent range, Xpelair VX100 and Simply Silent, Blauberg Calm 100, and Vistalux VISF100. These fans deliver 15-20 l/s extraction with options for timer or humidity control.

125mm Extractor Fans: When to Use

125mm fans are essential for larger bathrooms (15-25m³) with high moisture loads, installations with duct runs between 3-6 metres, ducting with multiple 90-degree bends requiring higher pressure, utility rooms and laundry areas, commercial washrooms and public facilities, and multi-bathroom installations where one fan serves multiple rooms. 125mm fans provide the additional airflow and pressure needed for challenging installations where 100mm fans would underperform.

Brands like Vent-Axia, Envirovent, and Blauberg offer 125mm models with advanced features including EC motors, continuous extract (dMEV), and silent operation for premium applications.

150mm Extractor Fans: When to Use

150mm fans are specified for very large bathrooms or commercial washrooms (over 25m³), kitchens requiring powerful extraction adjacent to cooking areas (Part F requires 30 l/s for kitchens), installations with very long duct runs (over 6 metres) or complex routing, multi-room extraction systems serving multiple bathrooms or wet rooms, and industrial or commercial applications with high moisture or odour loads. 150mm fans are less common in residential settings but essential for specific high-demand applications.

Duct Size Must Match Fan Size

Always use ducting matching the fan outlet diameter. A 100mm fan requires 100mm ducting, 125mm fan requires 125mm ducting, and so on. Using undersized ducting (e.g., 100mm duct on a 125mm fan) creates restriction that reduces airflow and prevents the fan achieving its rated extraction rate. Using oversized ducting (e.g., 150mm duct on a 100mm fan) reduces air velocity and may cause condensation accumulation in the duct.

For installations requiring duct size transitions, use gradual reducers rather than abrupt changes to minimize airflow disruption. Verify the final duct configuration achieves Part F minimum extraction rates using manufacturer performance tables.

Impact of Duct Length on Fan Size Selection

Duct length significantly affects fan performance. A 100mm axial fan may deliver 20 l/s with 1 metre of straight ducting but only 12-15 l/s with 4 metres of ducting and two bends. For installations with duct runs over 3 metres, consider upgrading to 125mm fans or specifying centrifugal/inline fans that maintain pressure and airflow through longer ducting.

Always consult manufacturer duct length tables before finalizing fan size. These tables show extraction rates at various duct lengths and bend configurations, ensuring you select a fan that achieves Part F compliance for your specific installation.

Upgrading from 100mm to 125mm: Is It Worth It?

Upgrading from 100mm to 125mm fans costs an additional £20-50 per fan but delivers significant performance benefits for challenging installations. The larger impeller and motor generate higher airflow and pressure, maintaining extraction rates through longer or more complex ducting. For bathrooms with chronic condensation issues, large shower enclosures, or installations requiring duct runs over 3 metres, the upgrade is justified by improved performance and reduced moisture problems.

For standard residential bathrooms with short, straight ducting, 100mm fans are adequate and cost-effective. Assess your specific installation requirements before deciding whether to upgrade.

Frequently Asked Questions

What size extractor fan do I need for my bathroom?

Most residential bathrooms up to 15m³ require 100mm (4 inch) fans delivering 15-20 l/s. Larger bathrooms (15-25m³) or those with long duct runs benefit from 125mm (5 inch) fans delivering 25-40 l/s. Calculate room volume and consult manufacturer specifications to verify the fan meets Part F requirements.

Can I use a 100mm fan with 125mm ducting?

Not recommended. Oversized ducting reduces air velocity and may cause condensation accumulation. Always match duct diameter to fan outlet size for optimal performance.

What is the difference between 100mm and 125mm extractor fans?

125mm fans deliver higher airflow (25-40 l/s vs 15-20 l/s) and generate more pressure, making them suitable for larger rooms, longer duct runs, and challenging installations. 100mm fans are adequate for standard residential bathrooms with short ducting.

How do I measure my existing extractor fan size?

Measure the diameter of the duct outlet on the fan housing (not the grille). Common sizes are 100mm (4 inches), 125mm (5 inches), and 150mm (6 inches). Verify the measurement before purchasing a replacement fan.

Do larger extractor fans use more electricity?

Not necessarily. Modern EC motor fans from Vent-Axia Lo-Carbon, Envirovent Quro LC dMEV, and Blauberg Calm consume only 2-15 watts regardless of size, making running costs negligible (under £5 per year even with continuous operation).

Can I replace a 100mm fan with a 125mm fan?

Yes, but you'll need to enlarge the wall opening and use 125mm ducting. Verify the larger fan fits the available space and that the electrical supply is adequate. Consult a qualified electrician for installation.

Shop the complete range of bathroom extractor fans in 100mm, 125mm, and 150mm sizes at ACAS Electrical, with models from Envirovent, Vent-Axia, Blauberg, Xpelair, and Vistalux at competitive trade prices with fast UK delivery. For technical advice on fan sizing, contact our team for specification support.