To measure, you should just need a tape measure and a spirit level. We recommend following our measuring guide to help you get the most accurate measurements and the best results for your secondary glazing.
Tolerance deductions
Tolerance Deductions improve the installation process and final appearance. They accommodate any out of square/distortions in the primary window frame ( it may be necessary to increase this tolerance if the frame is extremely distorted). Taking a lesser deduction is ok if you are confident about your measuring and your primary window construction, however additional work/adjustments to the secondary frame may be required. Tolerance: Reducing the total measured width & height by 10mm & centre points/first panel points by 5mm. Example: measuring a window width of 1195mm x height 1245mm would require an enquiry/manufacture of 1185mm x 1235mm. (NB deduction is 10mm not 10cm).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.

These downloads may be helpful

About Tolerance Deduction

About Splayed/angled reveals

How to measure for splayed reveals

A Measure Width, wall to wall, then deduct 10mm
B Measure height, sill to top, then deduct 10mm
C Meausre from wall to centre of first mullion, then deduct 5mm
D Measure centre first mullion to centre second mullion
E As D continued, until the penultimate pane.

Horizontal Slider Measuring Downloads