Standard Window Sizes in the UK
Most replacement windows in the UK are made to measure, so there's no single standard size. Stock uPVC windows come in widths of 610, 905, 1,190 and 1,770 mm and heights of 610, 820, 1,040 and 1,190 mm. Older timber windows follow a grid of widths around 630, 1,200 and 1,770 mm, with heights rising in 150 mm steps, two brick courses at a time. Below, the common sizes are drawn to scale next to an adult.
Drawn to the same scale, with the sill at 800 mm (the dashed line): the lowest an opening window can start without extra guarding. The figure is 1.75 m tall.
Stock uPVC window sizes
Supply-only sellers keep a range of set sizes you can buy without fitting. These are the sizes both stock ranges we checked list, width x height. Both include the cill in the height of their double-glazed windows, so allow for it when you measure:
- 610 x 610 mm
- 610 x 820 mm
- 610 x 1,040 mm
- 905 x 1,040 mm
- 1,190 x 1,190 mm
- 1,770 x 1,040 mm
- 1,770 x 1,190 mm
Stock windows are cheaper, but only if your opening matches. Most installers make every window to measure instead, because openings in British homes vary so much. See our window cost guide for both.
Traditional timber window sizes
For decades, timber casement windows were made to a set grid, sized to fit brickwork. Heights rise in 150 mm steps because a brick course is 75 mm (a 65 mm brick plus 10 mm of mortar). Both makers whose tables we found have since closed their window production, so treat these as the sizes you'll find in older homes rather than a range on sale today.
| Range | Sizes (mm) |
|---|---|
| Standard | 630, 1,200, 1,770 and 2,340 |
| Narrow | 488, 915, 1,344 and 1,770 |
| Heights | 450, 600, 750, 900, 1,050, 1,200, 1,350 and 1,500 |
Nominal sizes from Howarth's 2008 tables; JELD-WEN's 2017 tables give the same grid as actual frame sizes, about 5 mm smaller (625, 1,195 and 1,765 mm wide, for example).
Sash window sizes
Sliding sash windows are now mostly made to measure, especially to match an older house. The traditional ranges were about 410, 635, 860 and 1,085 mm wide, in heights from 1,050 to 1,650 mm in 150 mm steps. See our sash window cost guide.
Escape window sizes
In a house whose upper floors are no more than 4.5 m up, each upstairs room you live or sleep in (not the kitchen) needs either a window you could climb out of in a fire or direct access to a protected stairway. Approved Document B asks for an opening of at least 0.33 m², at least 450 mm high and wide, with the bottom of the opening no more than 1,100 mm above the floor. The window has to stay open on its own.
If you replace a window that was big enough to escape through, the new one should give at least the same opening, or the minimum above.
Is my window big enough to escape through?
Open the window fully and measure the clear gap you could climb through, not the frame.
Building regulations for new windows
- Sign-off: replacement windows have had to meet building regulations since 1 April 2002. A FENSA or CERTASS registered installer certifies the work; otherwise building control has to inspect it, which you pay for.
- Energy: in England, a new window needs a U-value of 1.4 W/m²K or better, or a Window Energy Rating of band B.
- Trickle vents: if the old windows had vents, the new ones need vents at least as big. If not, aim for 8,000 mm² in living rooms, bedrooms and kitchens and 4,000 mm² in bathrooms.
- Guarding: opening windows that start lower than 800 mm above the floor need guarding up to 800 mm.
New editions of the ventilation and energy guidance take effect on 24 March 2027, with the same window figures.
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Frequently asked questions
What is the standard window size in the UK?+
There isn't one. Most replacement windows are made to measure for each opening. Stock uPVC windows come in a few set sizes, with widths of 610, 905, 1,190 and 1,770 mm and heights of 610, 820, 1,040 and 1,190 mm. Older timber windows follow a grid of widths around 630, 1200 and 1770 mm, with heights in 150 mm steps.
What is the minimum size for an escape window?+
The opening you climb through needs to be at least 0.33 m², and at least 450 mm high and 450 mm wide, with its bottom no more than 1,100 mm above the floor (Approved Document B). A 450 mm wide opening needs to be about 734 mm tall. The window must also stay open on its own.
How high should a window sill be?+
If an opening window starts lower than 800 mm above the floor, it needs guarding to at least 800 mm (Approved Document K). Roof windows in loft conversions are treated separately. For an escape window, the bottom of the opening should be no more than 1,100 mm up.
Do new windows need trickle vents?+
If the old windows had trickle vents, the new ones need vents at least as big. If they didn't, Approved Document F asks for about 8,000 mm² of vent in living rooms, bedrooms and kitchens, and 4,000 mm² in bathrooms, or 4,000 mm² in each habitable room if the home has continuous mechanical extract.
Are uPVC windows made to measure?+
Most are. Installers measure each opening and have the windows made to fit, because British window openings vary so much. Some supply-only sellers stock a range of set sizes for DIY fitting, which are cheaper but only work if your opening matches.
What size is a standard sash window?+
Sash windows are usually made to measure now. The older standard ranges were about 410, 635, 860 and 1,085 mm wide, in heights from 1,050 to 1,650 mm in 150 mm steps.
Do I need building regulations approval for new windows?+
Replacement windows have had to meet building regulations since 1 April 2002. A FENSA or CERTASS registered installer can sign the work off for you; otherwise you need building control to inspect it. In England, new windows need a U-value of 1.4 W/m²K or better, or a Window Energy Rating of band B.
Sources
- GOV.UK: Approved Document B, Volume 1 (escape windows, para 2.10)
- GOV.UK: Approved Document K (guarding at windows)
- GOV.UK: Approved Document F, Volume 1 (ventilation)
- GOV.UK: Approved Document L, Volume 1 (conservation of fuel and power)
- GOV.UK: competent person schemes
- Planning Portal: building regulations for doors and windows
- FENSA: information for homeowners
- JELD-WEN: external joinery technical specification guide (2017)
- Just Value Doors: stock uPVC windows
- Sizes and rules checked on 30 October 2026.
Written by Chris Kervin, Co-Founder, Less.co.uk
Last updated: