Standard Window Sizes in the UK Explained

If you are planning new windows, it helps to know what counts as a standard size and why your own openings might not match any of them. The truth about standard window sizes in the UK is that they are more of a starting point than a rule, because most homes end up needing something made to measure. Here is what the common dimensions are and how to handle an opening that does not fit the chart.

We measure and fit windows across York and North Yorkshire, so this is the practical version, not the sales version.

Is there really a "standard" UK window size?

Not in the strict sense. Unlike some countries, the UK does not have one fixed set of window sizes that every home uses. What we do have are commonly used dimensions that builders and manufacturers reach for, especially in new-builds and estate homes built to repeat plans. Older properties, which describes a huge share of homes around York, were often built with openings to suit the house rather than a catalogue, so their windows vary enormously.

So when people talk about standard window sizes, they usually mean the popular sizes rather than a legal standard.

Common UK window sizes

As a rough guide, these are dimensions you will often see quoted for stock and popular windows. Widths and heights are approximate and given in millimetres, which is how the trade works.

• Small windows (bathrooms, landings): around 500mm to 600mm wide by 500mm to 900mm high.
• Medium windows (bedrooms): around 900mm to 1,200mm wide by 1,050mm to 1,200mm high.
• Large windows (living rooms): around 1,200mm to 1,800mm wide by 1,200mm to 1,500mm high.
• Bay windows: made up of several sections, so the overall span varies widely.

Treat these as ballpark figures. They are handy for picturing sizes, but they are not what you order from. Every window we fit is measured on site.

Why bespoke sizing is so often needed

Here is the important bit. Even where a "standard" size exists, your actual opening usually needs a made-to-measure window, and there are good reasons for that.

• Older homes were built by hand. Period terraces, cottages and Victorian semis rarely have identical openings, even within the same house. Two bedroom windows that look the same can differ by centimetres.
• Settlement and alterations. Over decades, houses move, and previous owners change things. Openings go slightly out of square.
• Replacement is a tight fit. A new window has to sit within the existing brick or stone opening with the right allowance for fitting and sealing. Guessing from a chart risks a poor fit and draughts.

This is why any reputable installer measures every opening individually rather than working from standard sizes. It costs nothing and it is the only way to get a window that fits and seals properly.

What this means for your project

The practical upshot is simple: use standard sizes to plan and budget, but expect your windows to be made to your exact openings. Made to measure sounds expensive, but for most home windows it is the normal way things are done and it does not carry a big premium over stock. What you are paying for is a window that fits your house, not someone else's.

Whether you are looking at uPVC windows for value and low upkeep, or aluminium windows for slim frames and larger openings, the size will be tailored to your property either way. The same goes for period-style casement windows, where matching the original proportions matters as much as the raw dimensions.

A note on Building Regulations

Window size is not just about looks. There are rules covering things like ventilation, the size of openings needed for escape in habitable rooms, and glazing safety in certain positions. These requirements change over time and depend on the room and the property, so check the current Building Regulations and, where relevant, confirm with your local authority before finalising sizes. A good installer will flag anything that needs attention.

Rough costs

Prices depend on size, material, glazing and the number of windows, so any figure is a ballpark. As a rough 2026 guide, replacement windows often fall somewhere between about 400 and 1,200 pounds per window fitted, with large or aluminium units at the higher end. Bespoke sizing is usually included in that, not a big extra. For accurate numbers on your openings, request a quote.

FAQs

What is the most common window size in a UK house?

There is no single answer, but medium windows of roughly 900mm to 1,200mm wide and around 1,050mm to 1,200mm high are very common in bedrooms. Living-room windows are usually larger and bathroom windows smaller.

Do I need a made-to-measure window or a standard one?

For most homes, made to measure. Even where a standard size exists, real openings vary, especially in older properties, so measuring on site gives a window that fits and seals properly.

Are window sizes measured in inches or millimetres?

The trade works in millimetres. You might still hear old imperial sizes mentioned, but any modern order will be in millimetres, measured to your exact opening.

Does a bigger window cost a lot more?

Larger windows use more material and glass, so they do cost more, but the jump is usually gradual rather than dramatic. Material and glazing choices often affect the price as much as size.

Want to talk sizes and styles for your home? Download our latest brochure to see the range, or request a free quote and we will measure up.