While there’s no single standard window size in the UK, manufacturers do produce windows in commonly used dimensions based on window type and property style. Learning about these standard sizes helps with budgeting, reduces lead times, and ensures compliance with building regulations. This guide breaks down the typical measurements you’ll encounter when shopping for windows in 2025.
Key Takeaways
- UK window dimensions vary by type, with casement windows typically 500-1000mm wide and sash windows 450-1200mm wide.
- Window installations must comply with building regulations for safety, ventilation, and energy efficiency (U-value of 1.2 W/m²K).
- Standard window sizes offer cost savings of 15-30% and shorter lead times compared to non-standard options.
Standard UK Window Dimensions at a Glance
This comprehensive table shows the typical size ranges you’ll encounter when shopping for windows in the UK:
| Window Type | Width Range (mm) | Height Range (mm) | Common Specific Sizes (W×H) |
|---|---|---|---|
| Casement | 500–1000 | 500–1450 | 600×900, 1200×1200 |
| Sash | 450–1200 | 600–1800 | 900×1200, 1200×1800 |
| Bay | 2400–4800 | 1200–2400 | 3000×1200, 3600×2100 |
| Tilt & Turn | 600–1000 | 600–1200 | 600×900, 900×1200 |
Material variations can affect these dimensions slightly. For instance, aluminium frames tend to be slimmer than uPVC ones, potentially offering larger glass areas within the same overall dimensions.
Casement Windows
Casement windows are the most popular choice in British homes. They open outward on side hinges, similar to a door, providing excellent ventilation and unobstructed views.
Standard casement windows typically range from 500mm to 1000mm in width and 500mm to 1450mm in height. The most commonly installed sizes include 600mm × 900mm, 900mm × 1200mm, and 1200mm × 1200mm.
These windows work well in virtually any room and can be configured as side-hung or top-hung (awning windows). Our uPVC windows are available in all standard casement sizes to suit your needs.
Sash Windows
Sash windows, with their vertical sliding panels, are a quintessential feature of period properties. They bring classic elegance to Georgian, Victorian, and Edwardian homes.
Standard sash windows typically measure between 450mm and 1200mm in width and 600mm to 1800mm in height. Common specific dimensions include 900mm × 1200mm and 1200mm × 1800mm.
These traditional windows are designed with proportions that complement historic architecture. Modern versions maintain the classic appearance while offering improved insulation and security features.
Bay and Bow Windows
Bay windows project outward from the main walls of a building, creating a bay in the room and allowing more light to enter from multiple angles.
Standard bay windows generally range from 2400mm to 4800mm in width and 1200mm to 2400mm in height. A typical three-section bay window might measure around 3000mm × 1200mm.
The exact dimensions often depend on the number of panels and the angle of projection. Bay windows make excellent features in living rooms and bedrooms, adding character and extra space to your home.
Tilt and Turn Windows
Tilt and turn windows offer versatile opening options. They can tilt inward at the top for ventilation or open fully inward like a door for cleaning or emergency exits.
Standard sizes mirror many casement window dimensions, typically ranging from 600mm to 1000mm in width and 600mm to 1200mm in height. Common configurations include 600mm × 900mm and 900mm × 1200mm.
These windows are particularly popular in modern homes and apartment buildings. Our aluminium windows are available in tilt and turn designs that combine contemporary styling with practical functionality.
UK Building Regulations for Windows
Window installations must comply with several UK building regulations to ensure safety, energy efficiency, and adequate ventilation.
Safety and Emergency Egress
According to Approved Document B, windows serving as emergency exits in bedrooms must have an unobstructed openable area of at least 0.33m². No dimension should be less than 450mm, and the bottom of the openable area must be no higher than 1100mm from the floor.
This requirement is crucial in bedrooms and other habitable rooms where windows might serve as emergency escape routes.
Ventilation Requirements
Approved Document F stipulates that windows must provide adequate ventilation. For most rooms, the openable area should be at least 1/20th of the floor area.
Trickle vents are often required in replacement windows to maintain background ventilation. These small, controllable vents allow fresh air to enter even when windows are closed.
Thermal Performance
Under the current Building Regulations Part L, new and replacement windows must meet minimum energy efficiency standards. The maximum permitted U-value (a measure of heat loss) for windows is now 1.2 W/m²K, representing a significant improvement over previous standards.
Windows installed by FENSA-registered companies automatically receive certification of compliance with these thermal performance requirements.
Standard Sizes vs Custom Windows
The choice between standard and custom window sizes involves several considerations that affect cost, installation time, and aesthetic outcomes.
Benefits of Standard Sizes
Standard window sizes offer numerous advantages:
- Lower costs (typically 15-30% less expensive than custom options)
- Shorter lead times (usually 2-3 weeks versus 4-6 weeks for custom)
- Faster installation as fitters are familiar with common dimensions
- Easier replacement of glass or hardware in the future
According to Homebuilding & Renovating, most UK builders opt for standard sizes due to their cost efficiencies and faster installation processes.
When Custom Windows Make Sense
Custom windows become necessary in certain situations:
- Period properties with non-standard openings
- Listed buildings requiring exact historical matches
- Unique architectural designs with special requirements
- Replacement windows in older properties with unusual dimensions
While custom windows cost more and take longer to produce, they ensure perfect fits and can maintain the character of distinctive properties.
How to Measure Windows Correctly
Accurate measurements are essential, whether you’re choosing standard or custom windows. Here’s a simplified approach:
For width measurements, measure the distance between the internal reveals at the top, middle, and bottom of the opening. Use the smallest measurement.
For height, measure from the sill to the underside of the lintel at the left, middle, and right sides. Again, use the smallest measurement.
For replacement windows, it’s often best to measure the existing frames rather than the openings, depending on your installation method.
Making Your Window Choice
When planning your window project, consider these factors:
For most modern homes, standard window sizes will meet your needs perfectly while offering better value and faster installation. For period properties or unique designs, custom windows might be worth the extra cost and time.
Next Step
To discuss your window requirements or arrange a free, no-obligation survey and quote for your property, contact our team today. We’ll help you navigate the options and find the perfect windows for your home.
