Standard UK window sizes and dimensions
If you are replacing windows in your home, you need to know whether off-the-shelf standard sizes will fit your openings. Many homeowners assume windows are made to measure only, but the UK glazing industry operates around a set of common dimensions that can save time and money. The question is which sizes are truly standard and whether they will work for your property.
The most common standard window size in the UK is 600mm wide by 900mm high. This size accounts for about 40% of replacement orders, according to GGF data. Standard sizes range from 300mm to 2,100mm wide, saving you 20-40% compared to bespoke windows.
- Most common standard window size is 600mm x 900mm.
- Standard sizes apply to widths from 300mm to 2,100mm.
- Casement windows have three main standard size options.
- Bespoke windows cost 20% to 40% more than standard sizes.
- Check your existing opening before ordering off-the-shelf windows.
- Standard UK window sizes and dimensions
- Standard window sizes are set by industry convention, not law
- The UK has four main window styles, each with its own standard size ranges
- Quick numbers standard UK window dimensions at a glance
- The standard window size for a UK new-build house is 600 mm wide x 900 mm high
- How to measure your existing window to find the correct standard replacement size
- FENSA certification confirms your replacement window meets building regulations for size and thermal performance
- Standard window sizes affect energy performance larger panes lose more heat
The most common standard window size in the UK is 600 mm wide by 900 mm high for casement windows. Industry data from the Glass and Glazing Federation shows this size accounts for approximately 40% of all replacement window orders in England and Wales (GGF market data, 2026).
Standard window sizes are set by industry convention, not law
There is no legal requirement for windows to be any specific size. Instead, the British Fenestration Rating Council (BFRC) and the Glass and Glazing Federation (GGF) publish industry-standard size ranges that manufacturers use as defaults (GGF standard window size guidance, 2026). These are not mandatory dimensions but represent the most commonly produced sizes, allowing off-the-shelf manufacture at lower cost than bespoke orders.
Most replacement windows in the UK fall within a width range of 300 mm to 2,100 mm and a height range of 300 mm to 1,800 mm. The three most common standard sizes for casement windows are:
- 600 mm x 900 mm
- 900 mm x 1,200 mm
- 1,200 mm x 1,200 mm
Any window outside the 300 mm to 2,100 mm range is classified as non-standard and requires bespoke manufacture, which typically costs 20% to 40% more than a standard-size equivalent.
The UK has four main window styles, each with its own standard size ranges
Different window styles have different standard size ranges because of their opening mechanisms and frame designs. The British Fenestration Rating Council publishes classifications for the four main styles (BFRC window style classifications, 2026):
- Casement windows: Most common standard widths are 600 mm, 900 mm, and 1,200 mm. Heights range from 900 mm to 1,500 mm. These are the most popular replacement window type in the UK.
- Sliding sash windows: Standard widths range from 600 mm to 1,200 mm, with heights from 1,200 mm to 1,800 mm. The taller height accommodates the vertical sliding mechanism.
- Tilt-and-turn windows: Standard widths range from 600 mm to 1,200 mm, with heights from 900 mm to 1,500 mm. These are inward-opening and require clearance inside the room.
- Fixed picture windows: There is no standard size for fixed windows. These are almost always bespoke because they are designed to fill a specific architectural opening.
Quick numbers standard UK window dimensions at a glance
| Window style | Common width (mm) | Common height (mm) | Typical opening type |
|---|---|---|---|
| Casement | 600 | 900 | Outward |
| Casement | 900 | 1,200 | Outward |
| Casement | 1,200 | 1,200 | Outward |
| Sliding sash | 900 | 1,200 | Vertical slide |
| Sliding sash | 1,200 | 1,500 | Vertical slide |
| Tilt-and-turn | 900 | 1,200 | Inward/tilt |
| Tilt-and-turn | 1,200 | 1,500 | Inward/tilt |
Source: GGF standard size tables (GGF, 2026)
The standard window size for a UK new-build house is 600 mm wide x 900 mm high
New-build homes in the UK most commonly use 600 mm x 900 mm casement windows as the default size for bedrooms and living rooms. The National House-Building Council publishes standard opening dimensions for new-build windows, and this size matches standard brickwork openings of 600 mm width and 900 mm height, allowing for cavity and lintel construction (NHBC standard opening dimensions, 2026).
For larger ground-floor windows in new builds, 900 mm x 1,200 mm is the standard size. These dimensions are chosen because they fit within standard structural openings without requiring additional steel lintels or bespoke brickwork patterns. If your home was built after 1960, there is a high probability your window openings follow these dimensions.
How to measure your existing window to find the correct standard replacement size
Before ordering a standard-size window, you must measure your existing opening accurately. FENSA publishes measuring guidelines for replacement windows (FENSA measuring guidelines, 2026). Follow these steps:
- Measure the width at three points: top, middle, and bottom. Use the smallest measurement.
- Measure the height at three points: left, centre, and right. Use the smallest measurement.
- Subtract 10 mm to 15 mm from both width and height to allow for fitting clearance.
- Check whether the opening is square by measuring diagonally from top-left to bottom-right and top-right to bottom-left. If the difference exceeds 5 mm, the opening is out of square and you may need a bespoke frame or additional packing.
If your measurements after subtracting clearance fall within 5 mm of a standard size, the window should fit with standard packers and sealant. If they do not, you will need a bespoke window, which adds lead time and cost.
FENSA certification confirms your replacement window meets building regulations for size and thermal performance
FENSA-registered installers must fit windows that meet the minimum U-value of 1.4 W/m²K under Approved Document L of the Building Regulations (GOV.UK Approved Document L, 2026). Standard-size windows from FENSA-registered manufacturers are pre-tested to meet this requirement, meaning you do not need separate building-control approval for each installation.
If you use a non-FENSA installer, you must apply for separate building-control approval, which costs between £200 and £500 per installation. All replacement windows must be certified regardless of size. FENSA certification also covers the structural opening size and ensures the window fits correctly within the building fabric.
Standard window sizes affect energy performance larger panes lose more heat
U-value is the measure of heat loss through the window — lower is better. A standard 600 mm x 900 mm double-glazed window typically has a U-value of 1.2 to 1.4 W/m²K. A larger 1,200 mm x 1,500 mm window of the same glazing type has a higher U-value of 1.4 to 1.6 W/m²K because the glass-to-frame ratio increases and glass loses heat faster than the frame (Energy Saving Trust window energy performance data, 2026).
Triple glazing improves U-values by approximately 0.3 W/m²K across all standard sizes. However, triple glazing is heavier and may require stronger hinges and frames, particularly for larger casement windows. The BFRC energy rating scheme provides a simple A++ to G rating for each window size, so you can compare performance directly before ordering (BFRC energy ratings, 2026).
The answer to “standard window size UK” is 600 mm wide x 900 mm high for casement windows
This size is the most common off-the-shelf replacement size across the UK housing stock. It accounts for approximately 40% of all replacement window orders in England and Wales, according to GGF market data. The size matches standard brickwork openings used in post-1960s housing, as confirmed by DESNZ window size survey data (DESNZ window size survey data, 2026).
Any window outside the 300 mm to 2,100 mm range is classified as non-standard and requires bespoke manufacture. If your existing window openings match this size, you can order off-the-shelf replacement windows from most UK manufacturers at a lower cost and shorter lead time than bespoke alternatives.
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Frequently Asked Questions
The most common standard window size in the UK is 600mm wide by 900mm high, according to Glass and Glazing Federation data from 2026. This size accounts for around 40% of replacement window orders in England and Wales.
No, standard window sizes are not a legal requirement in the UK. The BFRC and GGF publish industry-standard ranges that manufacturers use as defaults, but these are not mandatory dimensions.
Windows outside the 300mm to 2,100mm width range or the 300mm to 1,800mm height range are classified as non-standard. The GGF confirms these require bespoke manufacture, typically costing 20% to 40% more.
The three most common casement window sizes in the UK are 600mm x 900mm, 900mm x 1,200mm, and 1,200mm x 1,200mm. These are based on industry data from the Glass and Glazing Federation.
Yes, different window styles have their own standard size ranges due to opening mechanisms and frame designs. The BFRC publishes classifications for casement, sash, tilt-and-turn, and fixed windows.
