How to Measure Windows Correctly

DEA's are required to measure all window areas within a property as part of their assessment. These measurements must then be entered into the relevant software to complete the assessment before an EPC can be lodged.

In addition to recording the height and width of each window, Assessors must also note details such as the window type (e.g., double glazing) and its orientation, along with any other necessary information.

A window is an opening in an external wall or roof of a building, fitted with glass or similar material, usually in a frame, that admits light.

A door may be treated as a window if it is considered to be highly glazed.

Examples of highly glazed doors are patio doors, fully glazed doors or French doors. If in doubt, measure it and treat as a window if glazing area is 60% or more.

Convention 2.15 states: 

"For arched windows enter the actual width and enter the height as the average between shoulder height and the height to the top point".

"For other unusually shaped windows, calculate the area by the most appropriate means. Enter the area as the width and enter the height as 1".

"For circular windows, calculate the area by using the radius (which is half the diameter)".

"For oval windows, multiply the longest radius by the shortest radius and multiply that by 3.14".

"For semi-circular or half oval windows, apply the approach above as if the other half were
present and halve the result".

Taking window measurements can be done using a standard tape measure or a laser measuring device.

Alternatively, LIDAR scanning tools — available through apps such as MagicPlan — can automatically measure window dimensions when scanning a room. While this technology is excellent, we always recommend manually checking these measurements for accuracy, to ensure they are 100% correct.

We also advise using a compass to record the orientation of the property’s front door during the assessment. This can be done with a traditional compass or a calibrated compass app on your mobile phone. To determine the orientation, stand at the front door facing outwards and note the direction it faces on your floorplan.

Most windows you encounter will be rectangular, although some may be square, circular, oval, or an alternative shape. We’ll begin by focusing on standard shapes and explore how to measure other window types later in this article.

Start by measuring the width and height of the window to the nearest two decimal places — for example, Width: 1.82 m, Height: 0.89 m.

Position your chosen measuring device between the relevant points, either inside or outside the property.

Be sure to include the window frame in your measurement, not just the glass, as the frame contributes to the overall heat loss of the area.

Once you have recorded these measurements, enter them into the software, which will automatically calculate the window area for you.

You’ll often come across windows of various shapes during your assessment of a property. These will require similar measurements and additional details to be recorded; however, the process for inputting this information into the software can differ slightly.

The software typically requires the height and width of a window to calculate its area. However, if you were to enter these dimensions for a circular window, the software would calculate the area as if it were a square, resulting in an inaccurate measurement.

Therefore, once you have the area calculated for the different window shapes below you'll need to find the square root so you can use this for your height and width.

Thankfully, you can also rely on the following convention when measuring unusually shaped window:

Convention 2.15 states: "For other unusually shaped windows, calculate the area by the most appropriate means. Enter the area as the width and enter the height as 1".

Circular window? No need to worry! 

The guidelines are clear—convention 2.15 states: 

For circular windows, calculate the area using the radius (half of the diameter). First calculate
the square of the radius (multiply it by itself) then multiply that by 3.14.

Measurement Required: Radius (R) of the circle (half the diameter).

Formula: (RxR) x 3.14

You’ll need to take 2 measurements for an oval shaped window to work out the area.

Convention 2.15 states: For oval windows, multiply the longest radius by the shortest radius and multiply that by 3.14. 

Measurements Required:

1. The longest dimension (the major axis)

2. The widest dimension (the minor axis)

Formula: Area=3.12×a×b

Arched windows can be tricky—so how should you handle them?

Convention 2.15 advises: "For arched windows, enter the actual width and use the height as the average between the shoulder height and the height to the top point." 

To work out the area of a rhombus (stretched diamond) you’ll need to take 2 measurements, one of each of the diagonals.

Measurements Required:

1. Diagonal 1

2. Diagonal 2

Formula: Area=1/2​×d1​×d2​

To calculate the area of a triangle, multiply the base by the height, and then divide by two. 

Measurements Required:

1.  Base of the triangle
2.  Height of the triangle

Formula: Area = (1/2) * base* height.

It may be that you come across windows that are a mixture of shapes. For example, you may find a rectangular base to the window with a triangular shape at the top - all part of the same window.

In this instance, you should split the window up into different shapes such as a rectangle and a triangle and obtain the area of them each, before adding them together.

You would then need to work out how to get the height and the width of this area in a way to enter it into the software.

As mentioned, you’ll also need to record the orientation of each window. This refers to the direction it faces when looking from inside the property outwards.

For example, if a window is next to the front door and the front door faces North, that window will also be North-facing (assuming it faces the same direction).

The video above provides an excellent overview of the additional window information required.

You should consider the full range of compass points: North, North-East, East, South-East, South, South-West, West, and North-West. These details can be noted next to each window on your floorplan or entered directly into the software as you progress.

In addition, record the glazing type (e.g., single or double glazing), whether you can identify the age of the windows from date stamps or other evidence, whether permanent shutters are present, the frame material (e.g., metal, PVC, or timber), and whether the window is draught-proofed.

While it is not mandatory to photograph every window, gathering as much photographic evidence as possible will help you complete your assessment accurately and provide robust supporting evidence should your work be audited.

So there you have it, here’s how you can measure unusual shaped windows, to ensure that you can enter in the height and width into your software to produce the correct area in m2.

If you come across a shape that’s not covered above, then reach out to us, and we’ll get it added into the article.