Window Glass Thickness: How to Choose Between 4, 5 and 6 mm

The right window glass thickness depends mainly on the size of the pane, the wind load it has to resist and where the window is installed. The short answer: 4 mm is typically enough for a standard residential window, 5–6 mm is the usual choice for large panes, upper floors and exposed façades, and floor-to-ceiling glazing or doors call for tempered glass. Below we walk through the same reasoning we use with customers at the Magna factory every day.

Which glass thicknesses are used in windows?

Most windows are glazed with clear float glass. We keep 3.5, 4, 5, 6, 8 and 10 mm in stock, and each thickness has its typical role:

Thickness	Approx. weight per m²	Typical use
3.5–4 mm	9–10 kg	Standard residential windows, panes in insulated units
5 mm	~12.5 kg	Larger panes, outer pane of an insulated unit
6 mm	~15 kg	Upper floors, exposed façades, glazed screens
8–10 mm	20–25 kg	Shopfronts, doors, partitions — usually tempered

A useful rule of thumb: every millimetre of glass adds roughly 2.5 kg per square metre. That figure matters for the frame, hardware and installation, especially on opening sashes where hinges and handles carry the full load.

How does pane size affect the thickness you need?

The larger the pane, the more it deflects under wind pressure and the higher the risk of breakage. The guideline we use in practice: if a pane is wider than 1.2 m or larger than 1.5 m², specify at least the outer pane in 5–6 mm. For small and medium sashes, 4 mm is typically fine.

Proportions matter as well. A narrow but very tall glazing element may look small in area, yet the long unsupported span can still call for thicker glass. So base the decision on the shape of the pane, not just its square metres.

Single glazing, for example on unheated balconies or in older frames, is a slightly different case. In an insulated unit the wind load is shared between the panes, whereas a single pane carries all of it alone. For single glazing of the same size it therefore typically makes sense to go one step thicker.

Floor level and wind load: why higher means thicker

Wind speed, and therefore the pressure on the glass, increases with height. Baku is well known for its strong khazri and gilavar winds, so seafront buildings, open sites and the upper floors of tall buildings usually get thicker glass than a sheltered ground-floor window of the same size.

A simple working approach:

1. Lower floors, sheltered courtyard side — 4 mm for standard sizes.
2. Mid-level floors or large sashes — 5 mm outer pane.
3. Upper floors, exposed façades, sea-facing elevations — 6 mm outer pane, tempered where needed.

For tall buildings and curtain-wall systems, glass thickness should be set by the designer on the basis of a project-specific wind load calculation. Our guidance is intended for initial selection and comparison.

Choosing thickness in an insulated glass unit

Most modern windows use an insulated glass unit rather than a single pane. We assemble units from 4, 5 and 6 mm glass; the most common builds are 4-16-4 (single chamber) and 4-10-4-10-4 (double chamber).

Points worth knowing:

• Insulation comes from chambers and spacers, not thickness. Moving from 4 to 6 mm glass does not make a unit noticeably warmer, but it does make it stronger and heavier.
• Asymmetric units perform better acoustically. Panes of different thickness, such as 6-16-4, resonate at different frequencies and together damp street noise more effectively. For windows facing a busy road, this is a simple and effective upgrade.
• The outer pane takes most of the load. On large units we usually increase the outer pane first.
• Watch the weight. A 4-10-4-10-4 unit weighs roughly 30 kg per m²; moving all three panes to 6 mm adds considerably to that and affects the choice of hardware.

For chamber and spacer choices in detail, see How to choose an insulated glass unit.

When does a window need tempered glass?

Extra thickness adds strength, but it does not change how glass breaks: ordinary float glass fails into large, sharp shards. Tempered glass, the thermally toughened safety glass covered by EN 12150, is 4–5 times stronger than annealed glass and breaks into small, blunt granules. In terms of safety, this is often a more important decision than thickness.

Tempering is typically recommended for:

• floor-to-ceiling windows and sashes with low sills;
• balcony doors and glazed entrance doors;
• children’s rooms, schools, nurseries and other public buildings;
• large, exposed panes on upper floors;
• tinted or reflective panes in strong sun, which absorb more heat and are more prone to thermal stress cracking.

We temper 4–10 mm glass in our own tempering furnace and can assemble tempered panes into insulated units. Remember that tempered glass cannot be cut, drilled or edge-worked afterwards, so every dimension must be exact in the order. More background: What is tempered glass and where is it essential.

Pre-order checklist

Before you settle on a thickness, answer these questions:

1. What are the width and height of each item in millimetres? Are any panes wider than 1.2 m or larger than 1.5 m²?
2. Which floor is the window on, and is the façade exposed to wind or the sea?
3. Single glazing or an insulated unit? If a unit, which build?
4. Are there floor-to-ceiling windows, doors or children’s rooms that need tempering?
5. Would an asymmetric unit help on noisy elevations?
6. Can the frame and hardware carry the weight of the chosen glass?

Let us help you choose the right thickness

Window glass thickness is not a case of “the thicker, the better”: unnecessary thickness adds weight and cost, while too little creates risk. The reliable way is to look at the actual sizes and conditions.

Send your dimensions as text, an Excel sheet or a photo of a drawing through the quote form. Our software builds the cutting plan, calculates waste and material usage, and we send you a quote within one business day. If you are unsure which thickness fits, mention it in your request or get in touch and we will compare the options with you.