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Double Glazing Unit Calculator — Thickness, Weight & Rebate Depth
Calculate sealed unit thickness, weight and rebate depth required for double glazed units. Essential for specifying frames and assessing handling requirements.
Total DGUs to produce or order
Width of the sealed unit (not the frame)
Height of the sealed unit
Typically 5-8mm for back-bedding compound
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How We Calculate This
This calculator determines the overall DGU thickness, unit weight and the glazing platform width (front-to-back rebate the unit sits in) from the pane thicknesses, cavity width and unit dimensions.
The formulas
Total thickness = Pane 1 + Cavity + Pane 2
Glass weight = Area × total glass thickness (mm) × 2.5 kg/m² (the cavity adds no weight; glass density 2,500 kg/m³)
Laminated panes add the PVB interlayer (density ≈ 1,070 kg/m³) on top of the glass lites, so a 6.4mm laminated pane is the two 3mm glass lites (15.0 kg/m²) plus the interlayer ≈ 15.4 kg/m² — not 16 kg/m².
Glazing platform width (guide) = Total thickness + glazing-tape depth + 2mm tolerance
The platform-width figure is a rule-of-thumb guide, not a code minimum. NHBC Standards 2025 §6.7.5 set a minimum rebate depth of 15mm, minimum edge clearance of 5mm (3mm for well-drained, ventilated frames, glass ≤2m) and minimum edge cover of 12mm. Always confirm against the frame manufacturer’s profile.
Common DGU make-ups
- 4/16/4 (24mm) — standard argon-filled unit
- 4/12/4 (20mm) — common for air-filled units
- 6/16/6 (28mm) — toughened or thicker-float unit
- 6.8/16/4 or 6.4/16/4 — acoustic units (asymmetric, laminated outer pane)
- 4/6/4 (14mm) or 4/8/4 (16mm) — slim/heritage units
Frequently Asked Questions
The most common DGU make-up in the UK is 4/16/4 (4mm glass, 16mm cavity, 4mm glass) giving a total thickness of 24mm. This is optimum for argon-filled units. Heritage or slim units may use 4/6/4 (14mm total) or 4/8/4 (16mm total) for restricted rebate depths.
A standard 4/16/4 DGU weighs approximately 20 kg per square metre (glass only, not including the frame), so a typical 1.2m x 1.2m (1.44 m²) unit is about 29 kg. The cavity is ignored — weight comes only from the glass: total glass thickness (mm) x 2.5 kg/m². Laminated panes weigh slightly more than the same nominal float thickness because of the PVB interlayer (for example 6.4mm laminated is about 15.4 kg/m², not the 16 kg/m² that 6.4mm of solid glass would give). Glass density is 2,500 kg/m³ and PVB is about 1,070 kg/m³.
There is no single fixed figure — it depends on the frame profile. NHBC Standards 2025 (clause 6.7.5) set a minimum rebate depth of 15mm for double-glazed units in timber, a minimum edge clearance of 5mm around the unit (reducible to 3mm in well-drained, ventilated frames for glass up to 2m), and a minimum edge cover of 12mm to keep the spacer below the sight line and shield the edge seal from sunlight. As a rough guide the front-to-back glazing platform must accommodate the DGU thickness plus the glazing tape or back-bedding depth (typically 5-8mm) plus a small tolerance, so a 24mm DGU needs roughly 32mm of platform. Always confirm against the specific frame manufacturer specification, which varies between profiles.
Existing timber frames may have rebates as shallow as 15mm, which is too shallow for standard DGUs. Slim heritage units (14-16mm) may fit with modifications. The rebate must be deep enough and the frame strong enough to support the additional weight. A FENSA-registered installer should assess suitability.
Maximum DGU size depends on glass thickness and handling weight. Typical practical limits are approximately 3000mm x 2000mm for 4mm glass and larger for thicker glass. Very large units may require toughened glass for wind load resistance. Most UK glass suppliers limit individual pane weight to around 100-120 kg for standard delivery.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.