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Measuring sash windows help


Really really hope someone could help me! We are renovating an old Victorian property and need to replace windows. We have a mix of aluminium windows and a couple of old timber sashes. I have found a company who manufacture pretty realistic looking UPVC sashes, they have a lead time and budget to fit us... My problem is they don’t measure and supply only!!! I have been back and forth trying to work out how to measure and order the right window sizes and I’m quickly running out of time!

Anyone could please give me some tips? Is the external brick to brick plus 60mm width and 30mm height the best way to go or measuring internal openings and adding say 20mm width and height is the way to go?

Thank you!!


Thanks for all the answers. What I’m understanding is there is properly several ways to do this 🙃!

I have tried the external brick to brick and adding on 60mm width and 30mm height, but what I’m finding is that when I compare with my internal measurements up to the lintel then on some of the windows there wouldn’t be enough height! For example external measurement is 1630h and internal is 1650h... if I deduct 20mm internally to ensure that I get get the window in its 1630...exactly as the external measurement. So if I’m to add another 30m height the window will not fit from inside!

5 Answers from MyBuilder Window & Door Fitters

Best Answer

You have 2 options here, the first one (which I would recommend) is to measure brick to brick, make sure the aperture is square and deduct 10mm in height and width. This will allow the new frame to fit and be fixed into the solid external wall, I find this better as the extra weight with new sash and double glazing will be well supported. The down side is it limits amount of light into the room slightly due to width of new frame if small window.
The second option would be to measure brick to brick add on 40mm height and width (so not too much of new frame is hidden) then line inside with 4x2 timbers once old box and weights are removed. Fix the timbers through the front face into external skin either side of the window, then fix the new frame to the timbers.
Both options offer necessary support, usually depends on how you want the overall look and finish inside.
Hope this helps,
Best of luck 👍


Answered 17th Jul 2019


This is an absolute mine field you are trying to navigate, sliding sash windows are very hard to measure if you inexperienced, I can imagine your property is period & each aperture will be different, by measuring yourself you are taking what could be a very expensive gamble, my advise to you, is to only use a company who offer a surveying service, then if anything comes wrong it would be to down to the company to correct at their expense & not yours, so paying a little more, is always worth it in the long run.


Answered 18th Jul 2019

Hi There,

20mm to the height. 40mm to the width. But can go up too 80mm. depending on internal measurement.
Internal measurements is supposed to be bigger. Best bet is to add 40mm to the width and 20mm to the height. And order some add ons just in case you need to build the frame out.
Kind regards,

Darius D.


Answered 18th Jul 2019

Measure from out side ingo to ingo add 10mm each side then from cill to top ingo from out side add 10mm cheers


Answered 17th Jul 2019

The best way is brick to brick add 2 30mm add ons to the sides and 30mm on the head alone


Answered 18th Jul 2019

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