How To Measure Aluminium Windows: The Best 2026 Guide

How to measure aluminium windows: Getting the measurement wrong is the single most common reason a window order goes wrong — and unlike most mistakes, it’s usually avoidable. This guide walks through exactly how to measure an opening for an aluminium window before you order, what tools you need, the mistakes we see most often, and when it’s worth asking Coetz™ to confirm your measurements before you commit.

How to measure aluminium windows, step by step

Getting the size right is the most important part of ordering. This guide on how to measure aluminium windows walks you through measuring the structural opening accurately, allowing for the frame and tolerances. Follow these how to measure aluminium windows steps and you will order a window that fits your opening perfectly.

Because every Coetz™ window is manufactured to order rather than kept as generic stock, an incorrect measurement isn’t a quick fix — it means waiting for a new unit to be manufactured. Ten minutes spent measuring carefully up front is by far the cheapest insurance against that delay.

Tools required
Measuring width
Measuring height
Brick opening vs reveal
Common mistakes
Measurement checklist
When to request a custom quotation
Why Choose Coetz™
Related products
Related articles
Frequently asked questions

Tools required: how to measure aluminium windows

You don’t need anything specialised to measure a window opening accurately, but a couple of basics make a real difference:

A steel tape measure (at least 5m) — a fabric or retractable plastic tape can sag and give an inaccurate reading over wider openings.
A pencil and paper, or your phone’s notes app, to record each measurement as you go rather than trying to remember them.
A spirit level or level app, to check whether the opening is actually square — older openings often aren’t.
A second person, if possible, for openings wider than about 1.5m — it’s far easier to get an accurate reading with someone holding the other end of the tape.

A ladder or step stool is also worth having on hand for taller openings, so you can measure the top of the opening properly rather than estimating it from the ground — a surprisingly common shortcut that leads to inaccurate height readings.

Measuring width: how to measure aluminium windows

Measure the width of the opening at three points: near the top, in the middle, and near the bottom. Brick and block openings are rarely perfectly consistent from top to bottom, especially in older buildings, so a single measurement can easily be wrong by 10-20mm. Record all three measurements and use the smallest of the three as your working width — it’s much easier to pack out a slightly undersized frame than to force an oversized one into a tighter opening.

Write each of the three readings down as you take them, labelled clearly (top/middle/bottom), rather than trying to remember which was which afterwards — it’s a small habit that avoids a surprising number of avoidable mistakes.

Measuring height: how to measure aluminium windows

Apply the same approach vertically: measure the height at the left side, the centre, and the right side of the opening, and again use the smallest measurement. If you’re replacing an existing frame, measure the raw brick or block opening once the old frame is removed where possible, rather than measuring the old frame itself — old frames can be out of square in ways that don’t reflect the true opening size.

Brick opening vs reveal: how to measure aluminium windows

Two terms come up constantly in window measuring, and confusing them is one of the most common causes of an incorrect order:

Brick (or block) opening — the raw, unfinished gap left in the wall structure itself, before any plaster, render or finishing.
Reveal — the finished opening after plastering, rendering or any other wall finish has been applied, which is always slightly smaller than the brick opening.

Standard-size aluminium windows are manufactured to fit a specific brick opening size, with an allowance built in for the frame to sit correctly once plastered in. If you measure the finished reveal instead of the actual brick opening, you can end up ordering a window that’s too small for the wall it’s going into. Always confirm which measurement you’re taking, and say so clearly if you’re sending measurements to Coetz™ for a quote.

If you’re not sure which one you’re looking at, a simple rule of thumb helps: if you can see raw brick, block or unfinished concrete around the edge of the opening, you’re measuring the brick opening. If the edges are plastered, painted or otherwise finished, you’re measuring the reveal — and you’ll need to account for the difference before ordering.

Common mistakes: how to measure aluminium windows

Measuring only once, at one point. A single width or height reading can miss the fact that an opening narrows or widens by 10-20mm from top to bottom.
Measuring the old frame instead of the opening. An old, out-of-square frame doesn’t tell you the true size of the brick opening behind it.
Confusing brick opening with reveal. As above — this is the single most common cause of a wrongly-sized order.
Rounding measurements. “About 1.2m” isn’t precise enough — record the actual number in millimetres.
Assuming the opening is square. Especially in older homes, checking with a level takes a minute and can save a costly reorder.
Not accounting for existing trim, fixtures or plaster overhang. Anything protruding into the opening needs to be measured around or removed before you take a final reading.

Any one of these mistakes on its own is usually recoverable with a bit of extra site work. Combined, they compound quickly — which is exactly why a methodical, repeatable process matters more than doing it quickly.

Measurement checklist: how to measure aluminium windows

Work through this checklist for each opening before you order:

Width measured at top, middle and bottom (smallest recorded)
Height measured at left, centre and right (smallest recorded)
Confirmed whether the measurement is the brick opening or the finished reveal
Checked the opening for square with a level
Measurements recorded in millimetres, not estimated
Noted which direction the window needs to open (if replacing an existing window)

Keep this list next to you while you measure, or send us your numbers against each point and we’ll flag anything that looks off before you order.

When to request a custom quotation: how to measure aluminium windows

If your measurements don’t match one of Coetz™’s standard sizes — common in renovations, older homes, or unusually shaped openings — a custom quote is the right next step rather than trying to force a standard size to fit. It’s also worth requesting a custom quote (rather than ordering standard) whenever you’re not fully confident in your own measurements, the opening isn’t square, or you’re measuring a large or unusually positioned window where a mistake would be costly to correct. Send us your measurements and we’ll confirm the right specification before you commit to an order.

This is especially worth doing for larger openings, openings in older properties where walls have settled unevenly over time, or any project where more than one window needs to line up visually — a small measuring error becomes far more noticeable across a row of matching windows than on a single opening.

Why Choose Coetz™

Coetz Glazing supplies aluminium windows and doors directly to homeowners, builders, contractors and property developers — supply-only, made to order, with no installation middleman marking up the price.

Made to order — every unit is manufactured specifically for your order, standard size or custom.
Collection or delivery — collect from our Laser Park, Roodepoort premises, or arrange delivery. See the Delivery page for current options.
Measurement help — not confident in your numbers? Send them to us before ordering and we’ll confirm them with you.
Trade and bulk orders — builders, contractors and developers can apply for a trade account.

New to aluminium windows generally? Start with our Ultimate Guide to Aluminium Windows in South Africa. Need help measuring? Contact the Coetz™ team on WhatsApp.

Once you have accurate measurements, browse the live ranges:

Frequently asked questions

Should I measure the brick opening or the finished reveal?

Standard-size aluminium windows are manufactured to fit the brick (or block) opening, not the finished reveal after plastering. Confusing the two is one of the most common causes of an incorrectly sized order.

How many times should I measure a window opening?

Measure width at the top, middle and bottom, and height at the left, centre and right — six measurements per opening — and use the smallest width and smallest height as your working dimensions.

What if my measurements don’t match a standard size?

Request a custom quote. Coetz™ manufactures to custom dimensions for openings that fall outside the standard range, rather than forcing a standard size to fit.

Can Coetz™ help me confirm my measurements before I order?

Yes. Send us your measurements before ordering and we’ll flag anything that looks inconsistent or unusual before you commit.

Do I need to remove the old window before measuring for a replacement?

Where possible, yes — measuring the raw brick opening once the old frame is removed gives a more accurate result than measuring the old frame itself, which may be out of square.

Ready to order? Browse Coetz™ aluminium windows by size, or send us your measurements for a custom quote.

About Coetz™

Coetz™ manufactures premium aluminium windows and doors for homeowners, builders, architects and contractors across South Africa. Our products are manufactured to order and supplied directly to customers nationwide using genuine aluminium profiles and safety glass.

How to measure aluminium windows when the opening is not square

Measure the width at the top, middle and bottom, and the height at the left, centre and right, then work from the smallest figure in each direction. Knowing how to measure aluminium windows in an out-of-square opening is what keeps a unit from binding once it is in.

How to measure aluminium windows for a replacement rather than a new build

For a replacement, measure the structural brick opening once the old frame is out, not the old frame itself. Frames are often packed and shimmed, so measuring the frame repeats somebody else’s error. This is the single most common mistake in how to measure aluminium windows for a retrofit.

How to measure aluminium windows if you cannot reach the full height

Measure in two stages against a fixed reference such as the sill, and add the figures rather than estimating. If you are unsure, send us the photographs and the partial dimensions and we will work it through with you.

Do I need a tolerance when working out how to measure aluminium windows?

Yes. Allow a clearance gap around the frame for packing and sealing rather than ordering to the exact opening. Our team confirms the allowance with you before anything is manufactured.

How to measure aluminium windows: a final check before you order

Before the order goes in, repeat every dimension once more and write both figures down. Knowing how to measure aluminium windows accurately is only half of it — recording the numbers clearly, with width stated first, is what stops a transposed measurement reaching the factory. If anything looks marginal, send the figures through and we will check them against the standard sizes before manufacture.

How to measure aluminium windows: a practical checklist

Clients ask us how to measure aluminium windows on almost every enquiry, and the honest answer depends on the opening, the traffic through it and what the building already has in place. The checklist below is what our estimators actually work through.

The structural opening. Measure the brickwork opening rather than an existing frame, and check the reveal is square and plumb.
Configuration. Confirm the opening arrangement, which panes open, and which way they open before anything is manufactured.
Colour. Five finishes are stocked — black, white, bronze, charcoal and natural anodised — and some carry a premium.
Glass. Coetz™ glazes as standard with 6.38 mm clear laminated safety glass.
Supply only. Aluminium windows and doors are supplied, not installed, so allow for your own installer.

If you are still asking how to measure aluminium windows for one specific opening, send the sizes and a photo of the existing entrance. That is normally enough for us to answer it, and to quote, without a site visit.

How to Measure Aluminium Windows Before Ordering