How to Measure for Replacement Glass

Accurate measurements are the foundation of every successful glazing job. An incorrect measurement means wasted glass, wasted time, and an unhappy customer. Whether you're a professional glazier or tackling a DIY replacement, this guide covers everything you need to get it right every time.

Why Accurate Measurement Matters

Glass is a cut-to-size product — once it's cut, it can't be un-cut. Even a 2mm error can render a panel unusable. Measuring glass is deceptively simple, but experienced glaziers know that frames are never perfectly square, measurements can shift between visits, and customers often provide wildly inaccurate dimensions. The only reliable approach is to measure yourself, on site, every time.

Tools You Need

Before you start, make sure you have the right equipment: • Steel tape measure — at least 5m, 8m preferred for large panels • Folding rule for rebate depths and awkward corners • Steel square for checking frame squareness • Pencil and notepad, or a phone for photos • The Glazing Calculator for instant area and weight calculations once you have your dimensions

Step 1 — Measure the Rebate, Not the Glass

The rebate (also called a rabbet) is the groove or channel in the frame that holds the glass. Always measure the full rebate opening — width and height — not the existing glass size. Existing glass may have been incorrectly cut originally, may have shifted over time, or may have had extra clearance left from a previous repair.

Measure from the back of the rebate on one side to the back of the rebate on the other. Use a steel tape, not a fabric measuring tape — fabric tapes stretch and lose accuracy over years of use.

Step 2 — Deduct for Clearance

Glass expands and contracts with temperature changes, and frames are rarely perfectly square. Standard practice is to deduct 3mm from both the width and height measurements. This gives 1.5mm clearance on each side, which accommodates thermal movement and minor frame irregularities without rattling. For larger panels over 2m in any dimension, increase the deduction to 4–5mm. For double glazed units in aluminium frames, check your IGU supplier's specifications — some recommend up to 5mm per side for large panels.

Step 3 — Check for Square

Measure both diagonals of the opening — corner to corner in both directions. If the two diagonal measurements are equal, the frame is square. If they differ by more than 3mm, the frame is out of square. You have two options: order the glass slightly smaller with extra clearance to fit the worst case, or discuss frame rectification with the customer before proceeding. Ordering glass for an out-of-square frame without noting it is one of the most common causes of callbacks.

Step 4 — Measure the Rebate Depth

The rebate depth determines what glass configurations will fit. For single glazing, a rebate depth of 10mm is usually sufficient. For double glazed units, you need at least 16–18mm of depth, with most manufacturers recommending 20mm minimum to allow for glazing tape, sealant, and setting blocks. If the existing rebate is too shallow for the required unit, the frame may need modification — capture this in your quote as a separate item before committing to a price.

Step 5 — Record and Photograph Everything

Write down each measurement immediately — never rely on memory. Record the panel location (e.g. "Kitchen south window — 1520 × 980mm"), glass type, thickness, and any special requirements such as obscure glass, tinting, or safety glass. Photograph the opening, the existing glass (including any manufacturer markings), and the frame condition. This documentation protects you if a dispute arises later.

Measuring for Double Glazed Units

IGUs require a few additional checks beyond basic dimensions. Note whether the existing unit has a visible spacer at the edge — the spacer width indicates the air gap size, which you'll need to match or upgrade. Also check whether the frame has a thermal break, as this affects how the new unit should be bedded and sealed.

Measuring Unusual Shapes

For arched, circular, or other non-rectangular panels, take multiple measurements at different points and photograph the opening carefully. For large arches, a cardboard or thin MDF template is often the most reliable approach — cut the template, test fit it, then send it with the glass order. For circular panels, measure the diameter in at least two directions to confirm the opening is truly circular rather than slightly oval.

Calculating Area and Weight

Once you have your measurements, use the Glazing Calculator to calculate the panel area in m² and the weight in kg. Weight is critical for two reasons: it determines whether you need mechanical lifting equipment on site, and it affects hardware specifications such as hinges and patch fittings. Entering width and height in millimetres takes seconds and gives you instant figures to pass to your supplier.

Frequently Asked Questions

Do I measure inside the frame or the rebate?

Always measure the rebate — the channel that holds the glass — not the visible glass area or the frame's outer dimensions. The rebate measurement gives you the maximum glass size before clearance deductions.

What if the existing glass is smaller than the rebate?

The original glass may have been incorrectly measured, or it may have been installed with extra clearance. Always re-measure the rebate and calculate fresh — never use the existing glass size as your reference.

How much clearance should I allow for a large panel?

For panels up to 1m in any dimension, 3mm total (1.5mm per side) is standard. For panels 1–2m, allow 4mm total. For panels over 2m, allow 5–6mm or follow your supplier's guidelines.

Can I take measurements from photos?

No. Photos are useful for documentation and reference, but dimensions taken from photos are unreliable. Always measure on site with a steel tape.

What's the best way to handle out-of-square frames?

First, discuss with the customer — frames significantly out of square often indicate structural movement that should be addressed before new glass is fitted. If proceeding with the existing frame, increase clearance on the long diagonal and note the condition on your job sheet.