Bay window sizes: how to ensure a perfect fit

Simon Edward • 4 August 2025

Getting a bay window installed or replaced in your property? Read our guide to make sure you get the perfect fit.



Getting a bay window installed or replaced in your property? Read our guide to make sure you get the perfect fit.

Up and down the UK, bay windows add a touch of vintage class to homes, hotels and other buildings.


These traditional-looking windows can enhance both the inside and the outside of a property. They increase the amount of natural light coming into the front of the house. They create space, in some cases providing extra seating or storage.


Perhaps above all, they add vintage character and charm. But just because they have a premodern look doesn't mean they're built to premodern standards.


In fact, today's bay windows are made from modern materials like uPVC and toughened glass. This makes them robust and energy-efficient. On top of this, many are fitted with multi-point locking systems for extra security.


If you're looking to install a bay window or replace an existing one, you'll need to make sure it's a perfect fit. If you don't, you'll have to undo your work at a high cost. Here's how to make sure you get it right first time.

Standard sizes for bay windows

There are standard sizes for bay windows. These range in width from a minimum of three feet, six inches (101.6 cm) to 10 feet, six inches (320 cm) and in height from a minimum of three feet (91.4 cm) to a maximum of six feet and six inches (198.1 cm).


These standard sizes can serve as a rule of thumb when you're in discussions with an installer or supplier. In some cases, however, you may want to get a custom design. This could be the case whether you're installing a bay window as an extension or replacing an existing window.


Custom designs

While bay windows can come in standard sizes, you also have the option to get a customised design.


This customisation covers all aspects of the design and installation. You can choose how big the window should be, the materials it's made from and the angle at which it's installed. This last factor will affect the amount of natural light entering your room.


In terms of materials, you can choose between uPVC, aluminium and timber for the frame and different types of glass for the window. For instance, you may want frosted glass for extra privacy or triple glazing for extra energy efficiency.


The bay window can be configured in different shapes. Splay bays, circular bays and single bend bays are just some of the options. They can also be customised with window sills, seating areas and integrated storage spaces.


Picture of a bay window with a seating area.

Customisation also covers the colour of the window frames. Here at Calder Windows, our frames come in all the colours (and shades) of the rainbow. Truly, the only limit is your imagination.

How to measure a bay window for blinds and curtains

Measuring blinds and curtains for bay windows isn't always an easy job, thanks to the bay window's unusual angles.


You'll need three sets of measurements for blinds and curtains:

  • The width of each window panel
  • The total length of the curtain pole or track
  • The "drop" (vertical length) of the curtains or blinds


When measuring for blinds, you should also take handles, vents and other obstructions into account.


Once you've got these measurements, you can either take them into the store with you or use them to order curtains or blinds online.

Do you need planning permission to get a bay window installed?

If you want to install a new bay window at the front of your house, you'll need to apply for planning permission. However, if you're replacing an existing bay window or installing one to the side or rear of your property, you won't need planning permission.


The exception to this is if you live in a conservation area or listed building. In these cases, more stringent rules apply and you might need planning permission – even if you're replacing an existing bay window.


In this situation, you should contact the Conservation Officer at your local council to find out if consent is required. You may also need to apply for
Listed Building Consent.

How Calder Windows can help

Here at Calder Windows, we've been installing, repairing and replacing bay windows for more than 30 years.


We stock bay windows with frames made from uPVC and windows of toughened glass. These combine traditional aesthetics with modern functionality.


Each one has a multi-point locking system for your peace of mind. They're built to last, are low on maintenance and available in a range of attractive colours and finishes.


What's more, they're energy efficient. By keeping the heat in during the colder months and out when it's warm, you'll spend less on energy bills and cut your carbon footprint. This is all thanks to the high-quality glass we use.


And as if that wasn't enough, all our uPVC bay windows come with a 10-year warranty – so if you run into any problems, we've got you covered.


Frequently asked questions


  • What is a bay window?

    A bay window is a type of window that projects outward from the wall of a building. It creates a bay or alcove within the room. Most bay windows consist of three or more windows arranged in a curve.


  • How much are bay windows?

    The price of bay windows varies depending on the materials used and the size of the project. Your best bet is to get in touch with a bay window installer like us and get a quote.


    Are you in the market for an experienced, friendly team to repair, replace or install a bay window? Get in touch with Calder Windows today for a quick, competitive quote – or explore our range online.


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