How to Measure

To measure your windows before ordering secondary glazing or getting a FastQuote, you just need a tape measure.

Our measuring guide will help you get the most accurate measurements and best results.

Our Size Guide shows the minimum and maximum sizes suggested for different window applications.

Measuring Guide Explainers

When measuring your window it is important to apply Tolerance Deductions to the sizes taken, this simplifies the installation and improves the final appearance. The tolerance deductions accommodate out-of-square/distortions in the primary window frame (it may be necessary to adapt this tolerance if the window frame is extremely distorted). DO NOT WORRY ABOUT THE GAP between frame and wall, this is fully sealed by the trimming process and will not affect performance. Taking a lesser deduction is OK if you are confident about measuring and your primary window construction. However, additional work and adjustments to the secondary frame may be required, and remember there is a manufacturing drift of +/- 3mm from the confirmed unit build sizes. It’s better to take width and height measurements at several points, checking for variances. The smallest / narrowest measurements should be used. It’s better to have the unit made slightly too small, than too large, gaps are easier to hide and covered by edging trim, and a few mm of inaccuracy will not be noticed and will not affect performance. If the unit is wedged in too tightly, it can distort the frame, reducing efficiency and operation. Also, a gap around the unit allows for seasonal expansion and contraction, in addition, due to the wide variation in window design and property age some onsite adjustments may be unavoidable.

Tolerance: reducing the total measured width and height by 10mm (giving 5mm clearance per side)  and reduce the first and last panels by 5mm. E.g. measuring a window 1195mm wide x 1245mm high would require a manufacture of 1185mm x 1235mm, the measured panels must add up to the build size width or height which ever appropriate, (10mm off the total, 5mm of the first and last panel). Measuring details (internally facing the window) should be submitted by width x height x pane separation sizes. Centre points of horizontal sliders (sliding side to side) are taken from the left side to centre of mullion. Sash/vertical sliding windows (Sliding up/ down) the mid-rail measurement is always taken from the top to centre of mid rail. NB If the window panes are not equal in size (offset) still follow these instructions. Take your time, the measurements are important – we use them as our manufacturing instructions.

Take your measurements for secondary glazing when the primary window is in a finished state. Any painting, plastering, dry-lining, bulk heads or boxing out need to be complete before measuring, make any required adjustments to accommodate any protruding items, e.g. handles, either by unit position or using an XL sub-frame. Need more help? Email with information about the primary window.

Please remember!

1. Measure left to right
2. Measure from inside looking out
3. TOLERANCE: The widest part of the secondary glazing frame must fit inside the narrowest point of the area it is being fitted, better too small than too big! (There is a manufacturing drift of +/-3mm from the make sizes confirmed), gaps are hidden by the trim for a clean neat finish.
4. Do not worry about the tolerance adjusted gap, this will be fully covered and sealed by the perimeter trimming process and does not affect performance.
5. When measuring a split in the height of a unit, measure from the top down
6. Horizontal sliders slider left-right, vertical sliders slider up-down
7. ALWAYS give your measurements in mm!