Square Bay Window Measuring Guide

Top Tip
Firstly, you should note that roller blinds are generally not recommend for square bay windows, unless you have particularly wide corner mullions. This is because the width of the fabric is narrower than the overall width of the blind which means there will be areas of glass that are not fully covered by the fabric.
This is particularly noticeable with blackout roller blinds.

What you will need: A metal tape measure, a pencil & a piece or paper


Step 1

Using a metal tape measure, measure the width of all three windows directly into the corners. Make sure to check the width at the top, middle and bottom of each window and record the smallest measurement.

Measure square box bay window widths


Step 2

Calculating the Width to Order Your Blinds

Calculating the width to order your square bay blinds

If you have any handles or vents on the windows, measure in millimetres how far out from the window frame they protrude. You’ll need to add this measurement to the ‘depth’ of your blind in the next step.

Window Handle Projection

The ‘depth’ of your chosen blind type is shown in the table below. This depth is simply the distance your blind will protrude from where it is fitted. You’ll need to know this depth to make the correct allowance when measuring.

Blind Type   Depth
Roller Blinds (Face-Fix)   70mm
Roller Blinds (Top-Fix)   50mm
25mm Real & Faux Wood Blinds   55mm
35mm Real & Faux Wood Blinds   55mm
50mm Real & Faux Wood Blinds   65mm
25mm Aluminium Venetian Blinds   30mm
Roman Blinds   50mm
Vertical Blinds   75mm

Add these two measurements together and note down the result as Dimension ‘A’. If there are no handles or any other obstructions on the windows, just use the blind ‘depth’ as Dimension ‘A’.

Depth Calculation

Make the Final Deductions for the Width of Each Blind

Centre Blind: Roller Blinds Only - Deduct 4mm from the Width & Order as ‘Exact Size’

Centre Blind: All Other Blinds - Deduct 10mm from the Width & Order as ‘Exact Size’

Left Blind: Deduct Dimension ‘A’ from the Width & Order as ‘Exact Size’

Right Blind: Deduct Dimension ‘A’ from the Width & Order as ‘Exact Size’


Step 3

For the drop of your blinds, simply measure from the top of where you want the blinds to fit, down to where you would like the bottom of the blinds to rest (this is usually the window-sill, if you have one). Take this measurement in at least three places across the bay window and use the smallest measurement.

For Vertical Blinds only, reduce this measurement by 12mm to ensure the fabric louvres don’t drag or sit on the window-sill.

Note, all of the blinds for your bay window should be ordered as ‘Exact Size’.

Measure the Drop of the Square Bay Window Blinds