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Glass thickness for windows: how the right figure is worked out

How a fabricator works out pane thickness from size, wind pressure and glass type, the glass ranges AluK systems accept, and the questions to ask before you sign off a quotation.
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THE SHORT ANSWER

Thickness is calculated for each pane, not picked from a list

There is no single correct glass thickness for windows. The figure comes from the pane’s size, the wind pressure at your site, how the glass is supported and whether it is annealed, toughened or laminated. A fabricator checks the pane for strength and for deflection, then confirms the frame can carry it.
Our verdict: ask for the check, not a number. A thickness quoted before anyone has measured the opening is a guess. An Indian design-education guide puts single panes for homes at 4 to 6 mm, and AluK’s SC40 slider accepts single glass of 4, 5, 6 or 8 mm, but which one suits a given pane depends on the inputs below. This guide adds the method to the glass types compared in our window glass options guide.

What decides the thickness of a window pane

Four inputs go into the check. Change any one of them and the answer can change.
  • Pane area and shape. Glass behaves as a plate bending under wind. For a given thickness, the wind load a pane can take falls as its area grows, and guides say design charts also adjust for the pane's proportions.
  • Design wind pressure. In India, wind loads come from IS 875 (Part 3). The AIS windload calculator, built on that data, asks for the city, the building's location and its size, so identical windows on different sites can need different glass.
  • Support. How the glass is held is an input too. An Indian guide notes that framed units use thinner glass than frameless doors because the frame carries part of the structure.
  • Glass type. In the load-resistance method used with ASTM E1300, annealed glass has a type factor of about 1.0, heat-strengthened about 2.0 and fully toughened about 4.0.

Strength and deflection: the two checks a pane must pass

An Indian facade-engineering lesson, which calls itself a revision aid rather than a design reference, sets out the logic. The strength check asks whether the glass will break. It looks at tension on the back face of the pane under load, designed to a breakage probability of 8 in every 1,000 panes. The stiffness check asks whether the glass will bend too far, and the lesson gives a deflection limit of span/60 to span/90 of the short edge.
Thickness affects the two checks differently. Stiffness rises with the cube of thickness, so going from 6 mm to 8 mm makes a pane about 2.4 times stiffer. Strength rises roughly with the square of thickness. The pass condition is simple to state: the glass’s non-factored load, read from the maker’s charts, multiplied by the type factor, must be at least the design wind pressure.
In a double-glazed unit, the wind load is shared between the panes in proportion to the cube of their thickness. Two equal panes take about half each. In a 6 mm and 10 mm pair, the thinner pane takes about 18% and the thicker about 82%. Unequal panes also help with noise, as our soundproofing guide explains, so the structural and acoustic choices interact. One more caution from the same lesson: do not mix European methods (EN 16612 and EN 16613) with ASTM E1300 in a single calculation.

ILLUSTRATIVE EXAMPLE

An illustrative example: one 1.5 m by 2.0 m pane

Illustrative only. This example comes from the same teaching lesson. Its capacity figures were read from design charts that the lesson does not reproduce, so it shows how the method works and must not be used to size your window.
Take one pane of 1.5 m by 2.0 m and a design wind pressure of 3.0 kPa. The lesson compares the three options listed below. The same 6 mm thickness fails as annealed glass and passes once heat-strengthened, while 8 mm adds spare capacity. A real check uses your site’s pressure from IS 875 (Part 3) and the glass maker’s data; the AIS windload calculator returns a pass or fail with the minimum configuration from those inputs.
  • 6 mm annealed: capacity about 1.6 kPa, so it fails
  • 6 mm heat-strengthened: about 3.2 kPa with a type factor of about 2.0, so it passes
  • 8 mm heat-strengthened: about 5.2 kPa, which passes with margin

Glass thickness ranges on AluK systems

Once the check gives a build-up, it has to fit the frame. These are the glass ranges AluK publishes for systems we work with. A range tells you what the frame can hold. It is not a recommendation, and AluK’s pages do not give a thickness for each sash size, so we confirm the glass for each opening at specification.
SystemTypeGlass range listedSash size and weight listed
SC40SlidingSingle glass: 4, 5, 6 or 8 mm1000 x 2000 (no unit stated), up to 120 kg
SC45SlidingUp to 24 mm1200 x 2800 (no unit stated), up to 120 kg
SC95NISlidingUp to 28 mm (6 to 24 mm on the 85 mm interlock)1500 x 3500; weight figures conflict on the page
S115NISliding28 to 36 mm1500 x 3500, up to 300 kg per shutter
Infineo S146LMSlidingUp to 32 mm3000 x 3500 mm, 350 or 450 kg
SC140TTLift and slide10 to 40 mm2500 x 3500 mm, 300 kg (400 kg with additional trolley)
Versatile casementCasement5 to 40 mmSashes up to 130 kg
C87K-CS-NICasementUp to 32 mmStandard sash 800 x 1800 mm
67IWCasement, tilt-turnUp to 60 mm1400 x 1400 mm casement, 1500 x 1500 tilt-turn; 100 / 130 kg
40NCasement4 to 26 mm700 x 1500 mm, 32 kg
BSF70NIFolding13.52 to 40 mm3000 mm high x 1200 mm wide per sash, 130 kg

Sources: AluK India product pages (in.aluk.com) and the AluK Home India casement page. Where a page gives a sash size without a unit, we show none. Ranges are frame capacity, not advice for any one pane.

5 mm vs 6 mm vs 8 mm: how the common thicknesses are used

Flat glass is made in standard metric thicknesses, which guides list as 4, 5, 6, 8, 10, 12, 15 and 19 mm. For homes, an Indian design-education guide puts single panes at 4 to 6 mm, names 8, 10 and 12 mm for big or structural panes, and gives 10 to 12 mm for frameless doors.
So 5 mm, 6 mm and 8 mm are not a quality ladder. A 5 mm toughened pane can be right for a modest window if the frame and the wind check allow it; a large pane may need more, or a different glass type. The check decides. Thicker glass also does not, by itself, keep heat out. Guides say energy performance depends on the unit, its coatings and its gas fill, which our guide to double glazed windows in India covers.
Glass in a balcony railing is a different structural case from a framed window, and this guide does not cover it. See our glass railings page and confirm that build with the fabricator or a structural engineer.

TOUGHENED AND LAMINATED

If the check calls for toughened or laminated glass

Toughened glass is made by heating glass to about 620 to 650 °C and then quenching it. Afterwards it cannot be cut, drilled, ground or notched, so the final size must come from site measurements before the order goes in. Standard sizes help with planning, as our guide to standard window sizes in India shows, but toughened glass is made to the measured opening.
For overhead glass, large facade panes and panes that are hard to replace, one Indian guide recommends heat-soak testing at about 290 °C for a few hours to flush out nickel sulphide flaws. Safety glass for buildings must also conform to IS 2553 (Part 1):2018 and bear the BIS Standard Mark under the Safety Glass (Quality Control) Order, 2020; a toughened pane should carry a permanent etched mark in a corner. Our laminated glass guide covers that rule and how laminated thickness is written.

What to ask your fabricator about glass thickness

Which wind pressure did you design for?

Ask for the figure and where it came from. If the answer is that the same thickness is always used, the pane has not been checked. A glass maker’s calculator or an engineer’s calculation should sit behind the number, based on IS 875 (Part 3).

Which method and limits?

Ask which method was used, what deflection limit was applied and whether the type factor assumes annealed, heat-strengthened or toughened glass. One method should run through the whole calculation, because mixing European and American methods in one check is a known error.

Is the glass inside the frame's rating?

Ask for the system’s published glass range and sash weight limit beside the proposed build-up. A system aluminium window comes with these limits published, which makes the comparison straightforward. Heavier glass needs hardware rated for it, and the quote should name the build-up, for example 6+12+6, pane by pane.

How will the glass be marked?

Ask whether each toughened or laminated pane will carry the BIS Standard Mark, and ask for the fabricator’s test certificate on a large order. A missing mark on toughened glass is a warning sign worth raising before installation, not after.

FAQ

Common questions about glass thickness

Plain answers, including what nobody can say without checking the pane.
There is no single standard. An Indian design-education guide puts single panes for homes at 4 to 6 mm, and AluK’s SC40 sliding system accepts single glass of 4, 5, 6 or 8 mm. Double-glazed units are thicker overall because they include a gap: 6+12+6 is 24 mm. The right figure for any pane comes from its size and the wind load on it.
It can be, for a modest pane in a frame rated for it, particularly if the glass is toughened, which is about four times stronger than annealed glass. We found no published Indian limit saying 5 mm is safe up to a given size, so the answer comes from a wind load check on the actual pane, not from the thickness alone.
It is stiffer and stronger. Going from 6 mm to 8 mm makes a pane about 2.4 times stiffer, and strength rises roughly with the square of thickness. Whether that is better depends on need: 8 mm adds weight and cost, and on a pane that already passes at 6 mm it buys margin rather than safety you were missing.
Not by itself. Guides say energy performance depends on the unit, its coatings and its gas fill rather than glass thickness. In a hot climate the useful figure is the solar heat gain coefficient, and one Indian guide shows that coatings, not extra glass, bring it down. Treat thickness as a structural choice and heat control as a coating choice.
Each pane is typically 3 to 10 mm, with a centre gap that insulates best at about 16 to 19 mm. A unit written as 6+12+6 has two 6 mm panes and a 12 mm gap, 24 mm in all. Check the total against the frame: AluK’s SC45 slider takes glass up to 24 mm, while its Versatile casement takes 5 to 40 mm.
The fabricator or glass processor, from a check of each pane against the design wind pressure, with a structural engineer involved on large or exposed glazing. The window system then sets the limits the result must fit. We confirm thickness for each opening at specification, after measuring, and write the build-up for every pane on the quote.

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Share sizes, photos and the site location through the quote form or at the Belagavi showroom, and we will list the thickness and type proposed for every pane.
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