How to Measure Glass Units
Frame-specific measuring technique for uPVC, aluminium and timber — how to access the glass, exactly where to measure, and the 10mm rule that converts an opening size into an order size.
Every Frame Is Measured Differently
The general method — measure at three points, use the smallest figure, order as Width x Height — is the same across every frame. What changes is how you safely get to the glass, and exactly where the measuring point sits. Pick your frame type below, or read straight through.
The 10mm Rule
Order size = tight (rebate) opening size minus 10mm on both width and height — 5mm clearance per edge for packers and thermal movement. This is the trade-standard deduction across uPVC, aluminium and timber frames. Always order to this reduced size, never to the tight opening measurement.
Prefer to work it out yourself? Enter your own three-point readings below.
Enter your three width and three height readings in millimetres — the smallest of each becomes your tight size, and the 10mm rule gives your order size.
Occupied property, or the tenant won’t let you deglaze? See the method for measuring without removing the bead further down — it gives the same firm order size.
uPVC Bead Removal & Measuring
Most uPVC casements and sashes are internally beaded, though some commercial systems bead from the outside face — check which before you start. Beads are snap-in, held by friction clips along their length.
| Step | What To Do |
|---|---|
| Bead removal | Glazing shovel or paddle tool, starting at the centre of the longest bead and working outward to the corners — never start at a corner, the clips are strongest there and the bead is more likely to crack. |
| Measure the aperture | Once the old unit is out, this is the tight (rebate) size. Deduct 10mm total per the rule above for the order size. |
| Packers | Check for bridge packers (spanning the full rebate width, used on wide units) as well as standard location packers — note their positions so the installer can match them. |
| Thickness check | Confirm 24mm or 28mm unit depth against the rebate before ordering — a deeper rebate with a shallower unit needs additional packers, not a different bead. |
Aluminium Bead & Gasket Systems
Aluminium systems use either a beaded rebate (similar principle to uPVC) or a pressure-plate and wedge-gasket system, common on commercial curtain walling and shopfronts — identify which system before ordering, since the measuring point differs.
| Step | What To Do |
|---|---|
| Beaded aluminium | Some systems use security-clipped beads that need a release tool or specific pressure to unclip — do not force a bead that won’t lift, check for a security clip first. |
| Pressure-plate / gasket | Release the plate fixing screws, then remove the wedge gasket to free the unit — no bead to unclip. |
| Measure the rebate, not the sightline | The visible glass line (sightline) sits inside the frame’s decorative face and always reads smaller than the true rebate opening. Measuring to the sightline undersizes the order. |
| Thermal break | Note whether the frame has a thermal break (a non-metallic strip separating the inner and outer aluminium sections) — standard on modern systems, absent on older commercial stock, worth flagging on the order for older frames. |
| Typical unit depth | 28mm is standard for aluminium residential systems; commercial curtain walling can vary — confirm against the rebate. |
Timber Sash & Casement Measuring
Timber sashes are either bead-glazed (a timber bead pinned over the rebate) or putty-glazed (the glass bedded directly into a putty fillet, common on older and heritage sashes with no bead at all) — identify which before you start, since the two need different tools.
| Step | What To Do |
|---|---|
| Bead-glazed | Beads are pinned, not clipped — ease them off gently with a chisel or flat pry bar, working along the grain. Timber beads are reusable if removed carefully; split beads need replacing before re-glazing. |
| Putty-glazed | Remove the putty fillet with a hacking knife or a heat gun to soften old, hardened putty — take care not to damage the rebate. Sashes re-glazed this way are usually converted to a bead system, or re-bedded in fresh putty. |
| Allow for paint and swelling | Timber rebates vary more than uPVC or aluminium — repeated repainting narrows the rebate over time, and timber swells seasonally. Measure at three points and take the smallest, but expect more variation between points on timber than other materials. |
Older sashes are commonly re-glazed with slim configurations, from 12mm up to around 20mm depending on the sash design — see our Heritage / Slim-Fit units for the full range and typical U-values, or our standard DGU page for wider rebates. Always confirm the sash can safely carry the extra weight of a sealed unit before upgrading from single glazing.
Rebate Depth → Product Fit
Once you know your rebate depth from the thickness check above, here is what it typically takes — figures drawn from our own product range, not estimates.
| Rebate Depth | Typical Fit | Notes |
|---|---|---|
| 12–20mm | Heritage / Slim-Fit range | 12mm is the thinnest (3-6-3); 14mm (4-6-4 Low-E Argon) is the most popular heritage sash spec; 20mm (4-12-4 Low-E WE Argon) is the widest slim option. |
| 24mm | Standard DGU, 4-16-4 family | The most common modern DGU rebate — Clear Air up to Low-E Argon depending on performance needed. |
| 28mm | Standard DGU 4-20-4 Low-E WE Argon | Our recommended standard specification. A 28mm rebate can also take a slim krypton TGU (4-8-4-8-4) if triple-glazed performance is wanted at DGU depth. |
| 36–40mm | Triple Glazed, argon or krypton | 4-12-4-12-4 (36mm) up to 6-12-4-12-6 (40mm) depending on pane thickness. |
| 44mm+ | Triple Glazed 4-16-4-16-4 | Our standard TGU specification — the best all-round U-value in the range. |
Can’t Remove the Bead? Measure Across It Instead
Sometimes deglazing isn’t practical — an occupied room, a tenant who’d rather you didn’t, or a quick site visit. No need to deglaze to get your size: measure the full width and height frame to frame, across the beads, and deduct 10mm — the same firm order size as removing the bead and measuring the rebate directly (see above). Both routes reach the same figure.
Frame Type At A Glance
| Frame Type | Bead / Seal System | Access Method | Key Check |
|---|---|---|---|
| uPVC (internal bead) | Snap-in clip bead | Glazing shovel/paddle, centre outward to corners | Bridge packers on wide units |
| uPVC (external bead) | Snap-in clip bead, outer face | Same tool, from the outside face | Confirm which face beads before starting |
| Aluminium (beaded) | Clip bead, sometimes security-clipped | Release tool for security clips | Don’t force a bead that won’t lift |
| Aluminium (pressure-plate) | Pressure plate + wedge gasket | Release plate screws, pull gasket | Measure rebate, not sightline |
| Timber (bead-glazed) | Pinned timber bead, reusable | Chisel or flat pry bar, along the grain | Beads narrow over repeat repainting |
| Timber (putty-glazed) | Putty fillet, no bead | Hacking knife or heat gun | Re-bed in putty or convert to bead |
Shared across every frame type: measure width and height at three points each and take the smallest figure, always order as Width x Height, and check existing unit thickness with a vernier caliper at the glass edge rather than by eye.
Ready to Order?
Quote the frame material, the tight-size (rebate) opening and the measured unit thickness — we’ll return a quote within 2 business hours (Mon–Fri, 8am–5pm).