What you need
A steel tape — not a cloth one, which stretches. A pencil and paper, or the notes app. Ten minutes for a flat’s worth of openings.
You are producing rough numbers and photographs, not a manufacturing schedule. Anyone who offers to make windows from measurements you took yourself is skipping the step where problems get found.
Measure the opening, not the old frame
The existing frame is being removed. What matters is the structural opening behind it — the masonry or concrete the new frame will be fixed into.
Where the old frame is bedded into plaster, measure to the plaster line and note that you have done so. A surveyor will expect to find 10–25 mm more opening once the frame is out.
Width: three measurements
Measure the width at the top, the middle and the bottom of the opening.
Write down all three. Then note the smallest — that is the one that constrains the new frame, because the frame has to pass through the tightest point.
Height: three measurements
Same again: left, centre and right.
Measure from the sill up to the underside of the lintel. If there is a sloping sill — common in older Mumbai buildings, where the sill was formed to shed rainwater — measure to the highest point of the slope and note that it slopes.
What the differences tell you
This is the part worth doing for your own information.
- Under 5 mm variation — a square opening. Straightforward.
- 5–15 mm — normal for an older building. Easily handled by packing.
- Over 20 mm — the opening has moved, or been made good unevenly. Worth mentioning when you call, because it affects the frame size, the packing and what the reveal will look like afterwards.
If one opening in a flat is dramatically out compared with its neighbours, that sometimes points at a structural issue rather than a window one.
Photograph four things
More useful than any of the numbers:
- The whole window, from inside, with the sash closed
- The sill, from inside, showing how the frame meets it
- The outside, if you can reach it safely — never lean out
- Any damage — rot at the bottom rail, rust at a fixing, a cracked pane, damp on the reveal
The fourth one frequently changes the answer from “replacement” to repair, which is a much smaller bill.
Also note
Which way it currently opens — sliding, side-hinged, top-hung, or fixed.
Which direction the room faces. West and south-west elevations need different glass, and this single fact shapes the specification more than the dimensions do.
What you can hear from the room with the window shut.
The floor number, and whether there is a service lift. In Mumbai buildings this often sets the programme more than the manufacturing does.
Then what
Send the numbers, the photos and those four notes. That is enough for a realistic indication and for us to know what to bring to the survey.
The survey itself is where the real measurements happen: three widths and three heights again, checked for square and plumb, with the actual fixing substrate established. That is covered in window installation, and why it matters is covered in choosing a window contractor.
If you are still deciding between materials first, start with uPVC vs aluminium.