Last Updated on April 15, 2025

How To Measure A Window For Replacement

Whether you’re a homeowner looking to upgrade your property, a landlord maintaining your rental portfolio, or managing a commercial building in the UK, knowing how to measure a window for replacement is essential. 

Accurate measurements ensure a perfect fit, help avoid costly mistakes, and speed up installation. In this expert guide, we’ll take you through the process, offering pro tips to get it right the first time.

What Tools Do You Need to Measure a Window for Replacement?

Before you begin, gather these tools:

• Steel measuring tape (not a fabric one)
  
• Pencil and notepad (or phone/tablet)
  
• Spirit level (optional but useful)
  
• Step ladder (if needed for higher windows)

How Do You Measure the Width of a Window for Replacement?

To measure the width of your window accurately, you’ll want to take three separate measurements from inside your home — at the top, middle, and bottom of the window opening. 

Always measure from the internal edge of the frame, or if the frame is being replaced entirely, from brick-to-brick (brickwork opening).

For each section:

1. Place your steel measuring tape against the left-hand edge and pull it across to the right-hand edge.
   
2. Note down each width.
   
3. Use the smallest measurement to avoid overestimating — it ensures the new window will fit without requiring trimming.
   

Tip: In older properties, window openings often shift or become misshapen over time. Measuring in multiple places helps account for these irregularities.

This method works for most standard casement, tilt-and-turn, and fixed windows, giving installers an accurate size to work from.

How Do You Measure the Height of a Window for Replacement?

Like the width, height should be measured in three places. The left side, the centre, and right side from inside the home.

• Start at the top edge of the internal head (or lintel) and measure down to the bottom of the window sill or the internal cill board.
  
• If you’re removing the existing frame, measure from brick-to-brick vertically.
  
• Record all three height readings and again use the smallest one as your final height.
  

If there’s an internal window board or trim that won’t be retained, don’t include it in your measurement.

Measuring from plastered reveals? Be aware that plaster can conceal valuable millimetres. You will need to measure to the structural opening if possible.

Do You Need to Measure the Window Depth?

While depth isn’t always necessary for a quote, it becomes important in certain scenarios:

• Triple-glazed or flush-fit windows often require precise depth fitting to maintain energy efficiency and aesthetics.
  
• Sash windows, bay windows, and heritage replacements may require custom frame depths to match existing architecture or comply with conservation requirements.
  

To measure window depth:

1. Open the window and measure from the exterior face of the outer frame to the interior face.
   
2. Include the full depth of the frame — not just the glazing unit.
   

Knowing the depth helps ensure the new frame won’t protrude or sit too deep into the reveal, which could affect insulation, sealing, and appearance.

How Do You Measure Different Types of Windows in the UK?

Not all windows are created equal and neither are the ways we measure them. Window types vary significantly in design and structure across UK homes, from post-war estates to Victorian terraces.

Let’s break down how to approach the most common styles:

How to Measure uPVC Windows for Replacement

When measuring uPVC windows, especially if you plan to fully replace the frame, measure the brickwork opening, not the frame itself.

Steps:

1. From the outside, measure brick-to-brick across the width and height.
   
2. Take three measurements for each direction and use the smallest.
   
3. Deduct 10mm on each side to allow for expansion, fitting foam, and sealing.
   

Why deduct 10mm? This fitting tolerance ensures the new frame can be installed properly and sealed without forcing it into place, which could lead to warping or air leaks.

This is especially important in properties with rendered or pebble-dashed exteriors where brick-to-brick isn’t visible.In such cases, interior measurements may be used as estimates, but a professional survey is advised.

How to Measure Sash Windows for Replacement

Sash windows are common in period properties and require a more delicate approach:

1. Measure the visible sash opening — not the full box frame.
   
2. Take width and height measurements in three places.
   
3. Note down the sash weight and opening mechanisms if visible (e.g. cords, spiral balances).
   

For timber sash replacements or upgrades to double-glazed sashes, exact size and depth of the weight box may be critical.

Since regulations for heritage buildings (such as conservation areas or listed properties) may apply, it’s highly recommended to involve a window specialist early in the process.

How to Measure Bay or Bow Windows

Bay and bow windows can curve outward or form angular projections making them more complex to measure.

To measure for quotes:

1. Break the window into sections and measure each windowpane individually.
   
2. Record each unit’s width, height, and angle (if possible).
   
3. For square bay windows, measure the return walls and angle of projection.
   

Measuring bow windows? Note the curvature and total width across the arc. Approximate angles may be enough for a quote, but a survey is essential for manufacturing.

Due to the structural considerations and design complexity, a professional site survey is usually required before final measurements are confirmed.

What Are Common Mistakes When Measuring Windows?

Even small errors can cost time and money. Avoid these frequent measuring mishaps:

Measuring only the glass instead of the full frame or aperture. This leads to underestimating the replacement size.

Using a cloth or flexible tape, which can sag and result in inaccurate dimensions.

Failing to consider uneven openings or out-of-plumb walls, especially in older homes.

Assuming all windows in a property are identical — they rarely are, even if they look the same.

Always double-check your notes and measurements, and measure twice (at least) before requesting quotes or placing orders.

What Should You Do With Your Window Measurements?

Once you’ve taken accurate measurements, you’re ready to:

• Request quotes from local or national window installers.
  
• Use them to assess cost estimates and compare window styles.
  
• Identify potential issues that might affect installation, like oversized apertures, damaged lintels, or existing frame materials.

Pro tip: Keep a copy of your measurements, photos of each window, and any relevant notes — these can be extremely useful for installers during the quoting process.

Ready to turn your measurements into savings? At Eco Happy, we make it simple. Submit your details today and get a free, no-obligation window replacement quote from trusted installers in your area. Whether you’re a homeowner or landlord, we’ll help you find the right fit. 

Should You Measure and Install Windows Yourself or Hire a Professional?

Measuring your own windows is a great way to start the process and gather information for quotes. But when it comes to final orders and installation, even small inaccuracies can have big consequences.

Here’s when to DIY and when to delegate:

Good for DIY:

• Gathering measurements for quotes
  
• Simple casement window replacements
  
• Basic shape and fitting scenarios
  

Best left to professionals:

• Bay, bow, or box sash windows
  
• Listed buildings or conservation areas
  
• Full-frame replacements requiring structural work
  
• A professional installer will also ensure your new windows comply with UK Building Regulations, provide FENSA or CERTASS certification, and offer workmanship guarantees.

What You Should Know About Window Replacement Pricing in the UK?

Once you’ve taken accurate measurements, you’re probably wondering about costs. The price of new windows varies based on size, style, glazing type, and installation factors.

Whether you’re budgeting for one casement window or a full property upgrade, our new windows cost guide gives you a clear picture of what to expect. No guesswork, just solid numbers and expert insights.

Ready to Get a Free Window Replacement Quote?

You’ve done the groundwork — now let the experts do the heavy lifting. Our trusted UK window specialists will verify your sizes, recommend the right window styles for your property, and provide a transparent, no-obligation quote tailored to your needs.

Request a free window replacement quote.