How Thick Is Double Glazing Glass? A Complete Breakdown

How thick is double glazing glass? It is one of the most commonly asked questions in the UK glazing industry, and the answer matters far more than most homeowners realise. The thickness of your sealed unit – both the individual glass panes and the cavity between them – directly affects thermal insulation, acoustic performance, security, and whether the unit will physically fit into your window frame. Get the thickness wrong, and you face problems ranging from poor energy performance to a unit that simply cannot be installed.

The standard double glazing thickness in the United Kingdom is 24 mm, written as 4-16-4. That notation means two panes of 4 mm glass separated by a 16 mm cavity – a configuration that has become the default for residential replacement windows over the last two decades. But "standard" does not mean "only option", and many situations call for a different thickness entirely. This guide breaks down every common configuration, explains what the numbers mean, and helps you choose the right double glazing thickness mm for your specific project.

Understanding the Notation – What the Numbers Mean

Every sealed unit configuration in the UK is described using a simple three-number notation separated by hyphens. The first number is the thickness of the outer pane in millimetres, the middle number is the cavity width in millimetres, and the last number is the thickness of the inner pane in millimetres. So a 4-16-4 unit consists of a 4 mm outer pane, a 16 mm cavity, and a 4 mm inner pane, giving a total overall thickness of 24 mm.

This notation system is universal across the UK glazing industry. When you order a 28 mm sealed unit or request a quote from any reputable supplier, you will encounter it on every quotation, technical specification, and product label. Understanding it is the first step to understanding how thick is double glazing glass in any given application.

For triple glazed units, the notation extends to five numbers – for example, 4-12-4-12-4 – representing three panes and two cavities. The principle is identical: add all the numbers together to get the total unit thickness.

Standard UK Double Glazing Configurations

While the 4-16-4 configuration dominates the UK replacement market, a range of other configurations are commonly specified depending on the window frame depth, the required performance, and the budget. Here is a breakdown of the most widely used double glazing thickness options in the United Kingdom:

4-12-4 (20 mm total)

The 4-12-4 configuration produces a 20 mm sealed unit and is the thinnest standard option commonly available. It is typically specified for older window frames – particularly timber frames and early-generation uPVC profiles – where the rebate depth cannot accommodate a thicker unit. The 12 mm cavity is adequate for air-filled units and provides reasonable thermal performance, but it does not achieve the same insulation as wider cavities when filled with argon gas. A 4-12-4 unit with Low-E glass and argon achieves a centre-pane U-value of approximately 1.3 W/m²K.

4-16-4 (24 mm total)

The 4-16-4 is the industry standard in the UK. The 16 mm cavity is the optimal width for argon gas – the point at which convection suppression is most effective, delivering the best balance of thermal performance and cost-effectiveness. A 4-16-4 unit with soft-coat Low-E glass and argon achieves a centre-pane U-value of approximately 1.1 W/m²K, comfortably meeting current Building Regulations Part L requirements. This configuration fits virtually all modern uPVC, aluminium, and engineered timber window frames manufactured in the UK over the past 15 to 20 years.

4-20-4 (28 mm total)

The 4-20-4 configuration produces a 28 mm sealed unit and is specified for frames with deeper rebates, often found in high-specification aluminium systems and some premium uPVC profiles. Surprisingly, the wider 20 mm cavity does not necessarily improve thermal performance when filled with argon – in fact, performance can slightly decrease above 16 mm because convection currents begin to re-establish. However, a 20 mm cavity does offer improved acoustic performance (the larger air gap reduces sound transmission) and is well suited to krypton gas fills, where the denser krypton performs less effectively in narrower cavities. A 28 mm sealed unit with Low-E glass and argon achieves a centre-pane U-value of approximately 1.2 W/m²K.

6-12-6 (24 mm total)

This configuration uses thicker 6 mm glass panes with a narrower 12 mm cavity, giving the same 24 mm overall thickness as the standard 4-16-4 but with different performance characteristics. The thicker glass provides significantly better acoustic performance – thicker panes vibrate less, reducing sound transmission at lower frequencies – making this an excellent choice for properties on busy roads or near railway lines. The 6-12-6 is also stronger and more resistant to wind loading, which is relevant for large or exposed panes. Thermally, however, the narrower 12 mm cavity delivers slightly lower insulation than a 16 mm cavity with the same gas fill.

6-16-6 (28 mm total)

The 6-16-6 combines the acoustic benefits of thicker 6 mm glass with the thermal sweet spot of a 16 mm argon-filled cavity, producing a 28 mm sealed unit that offers excellent all-round performance. This is a popular specification for high-end residential projects where both thermal and acoustic performance are priorities, and where the frame depth can accommodate the thicker unit. It is also commonly specified for commercial glazing applications where wind loading and impact resistance are design considerations.

Asymmetric configurations

Asymmetric configurations use panes of different thicknesses – for example, 6-16-4 (26 mm total) or 6.8-16-4 (26.8 mm total). The primary reason for using asymmetric glazing is acoustic performance. When both panes are the same thickness, they resonate at the same frequency, creating a "coincidence dip" where sound passes through more easily. By using panes of different thicknesses, the coincidence frequencies are separated, and overall sound transmission is reduced across a wider range of frequencies. Acoustic laminate glass – typically 6.4 mm or 6.8 mm thick, comprising two layers of glass bonded with an acoustic PVB interlayer – is often used as the outer pane in an asymmetric unit for maximum noise reduction.

Double Glazing Thickness Comparison Table

The following table summarises the most common double glazing thickness configurations available in the UK, their typical performance ratings, and the situations they are best suited to:

How Cavity Width Affects Thermal Performance

The cavity width is arguably the most critical dimension in determining the thermal performance of a sealed unit. The cavity acts as an insulating barrier, and its effectiveness depends on two factors: the gas it contains and how well convection currents are suppressed within it.

In an air-filled cavity, heat transfers across the gap primarily through convection – warm air rises on the warm side, cools as it contacts the cold outer pane, sinks, and circulates. This convection loop transports heat from inside to outside. Replacing air with argon gas reduces this effect because argon is denser than air and has lower thermal conductivity. Krypton gas is denser still and performs even better, but at significantly higher cost – see our guide to argon gas double glazing and other gas options for a detailed comparison.

The sweet spot for argon gas is a 16 mm cavity. At this width, the gas is thick enough to provide meaningful insulation but narrow enough to suppress convection effectively. Below 12 mm, convection currents are more difficult to suppress, and thermal performance drops noticeably. Above 20 mm, convection currents begin to re-establish even in argon-filled cavities, and performance plateaus or slightly decreases.

For krypton gas, the optimal cavity width is approximately 12 mm. Krypton's higher density means it suppresses convection effectively in narrower gaps, making it the ideal choice for 20 mm overall unit thicknesses (4-12-4) where frame depth is limited. Krypton also performs well at 16 mm, but the cost premium over argon is substantial – typically three to four times the price – and the marginal improvement over argon at 16 mm is modest.

For xenon gas – the rarest and most expensive option – the optimal cavity is even narrower, around 8 to 10 mm, which makes it relevant only for very specialised applications such as ultra-slim heritage glazing.

How Glass Thickness Affects Acoustic Performance

While cavity width dominates thermal performance, glass thickness is the primary factor in acoustic performance. Thicker glass has more mass, which means more energy is required to set it vibrating – and it is the vibration of the glass that transmits sound from outside to inside. A 6 mm pane of glass has 50 per cent more mass per unit area than a 4 mm pane, and it will block noticeably more noise as a result.

However, simply using thicker glass only gets you so far. As discussed above, the most effective acoustic strategy is asymmetric glazing – using panes of different thicknesses so that they resonate at different frequencies. A unit with a 6.8 mm acoustic laminate outer pane and a 4 mm float inner pane outperforms a unit with two identical 6 mm panes, because the different resonant frequencies prevent the coincidence dip that allows sound to pass through.

The PVB interlayer in laminated acoustic glass also plays a significant role. Specialist acoustic PVB – as used in products like Pilkington Optiphon – is formulated to absorb sound energy within the interlayer itself, providing additional noise reduction beyond what the mass of the glass alone delivers. A well-specified acoustic sealed unit can achieve a weighted sound reduction index (Rw) of 39 to 42 dB, compared to 29 to 32 dB for a standard symmetric unit – a perceived halving in loudness.

For properties on particularly noisy roads, near airports, or in dense urban environments, the combination of thick asymmetric glass, an acoustic PVB interlayer, and a wide cavity delivers the best results. Our products page includes acoustic unit options that you can order to your exact dimensions.

Triple Glazing Thickness

Triple glazed units add a third pane of glass and a second cavity, increasing the total unit thickness significantly. Typical triple glazing thickness ranges from 36 mm to 44 mm:

• 4-12-4-12-4 (36 mm): The most common triple glazed configuration. Three panes of 4 mm glass with two 12 mm cavities. With two Low-E coated panes and argon gas in both cavities, this achieves a centre-pane U-value of approximately 0.7 W/m²K – a substantial improvement over even the best double glazed options.
• 4-14-4-14-4 (40 mm): A slightly wider cavity version that offers marginally better thermal and acoustic performance. Increasingly specified in new-build projects targeting high energy performance ratings.
• 4-16-4-16-4 (44 mm): The maximum standard triple glazed configuration. With Low-E coatings and krypton gas fills, this can achieve centre-pane U-values of 0.5 to 0.6 W/m²K. This is the specification typically associated with Passivhaus and Future Homes Standard compliance.

The additional thickness of triple glazing has practical implications. Standard uPVC window frames designed for double glazing typically have a rebate depth of 24 to 28 mm and cannot accommodate a 36 to 44 mm unit. Installing triple glazing almost always requires deeper frame profiles, which means either new frames or specialist retrofit systems. The additional weight of the third pane also increases the load on hinges and hardware, which must be specified accordingly.

For most UK homeowners replacing existing windows, a high-specification double glazed unit (4-16-4 with Low-E glass, argon fill, and warm-edge spacer) provides 90 per cent of the thermal benefit of triple glazing at significantly lower cost and without requiring new frames. Triple glazing makes most economic sense in new-build projects where the deeper frames can be specified from the outset, or in ultra-high-performance retrofit projects where every fraction of a U-value point matters. Our supply and installation service covers both standard and specialist configurations.

How to Measure Your Existing Unit Thickness

Knowing the thickness of your existing sealed unit is essential when ordering a replacement, because the new unit must fit within the available rebate depth of your window frame. There are several methods to measure your existing unit:

Method 1 – Remove a glazing bead

The most accurate method. Carefully prise out one of the glazing beads (the plastic or timber trim that holds the sealed unit in the frame) using a stiff putty knife or glazing shovel. Once the bead is removed, you can measure the full thickness of the unit from the outer face of the outer pane to the inner face of the inner pane using a tape measure or digital calliper. Measure at two points to confirm consistency. This method also lets you measure the rebate depth of the frame, which tells you the maximum unit thickness the frame can accept.

Method 2 – Laser thickness gauge

A laser thickness gauge (sometimes called a glass thickness meter) is a small handheld device that you press against the surface of the glass from inside the room. It uses laser reflections from each glass surface to calculate the thickness of each pane and the cavity width individually, without removing any beads or disturbing the unit. These devices are accurate to within 0.1 mm and are the method preferred by professional surveyors. They typically cost £30 to £80 and are a worthwhile investment if you are ordering multiple replacement units.

Method 3 – Visual estimation

If you cannot access the edge of the unit or do not have a laser gauge, you can make a reasonable estimate by looking at the spacer bar visible inside the cavity at the edge of the glass. The spacer bar sits between the two panes and is usually visible as a silver or black strip around the perimeter. If the spacer bar appears to be approximately 16 mm wide, the cavity is 16 mm, and the total unit thickness is approximately 24 mm (with 4 mm panes) or 28 mm (with 6 mm panes). This method is less precise but often sufficient for initial quoting purposes.

For detailed step-by-step instructions with diagrams, see our complete measuring guide.

Building Regulations and Minimum Thickness Requirements

Building Regulations Part L (Conservation of Fuel and Power) in England and Wales does not specify a minimum unit thickness directly. Instead, it sets a maximum whole-window U-value of 1.4 W/m²K for replacement windows. Any sealed unit configuration that achieves this target – when combined with an appropriate frame – is compliant, regardless of its physical thickness.

In practice, this means a 4-16-4 unit with Low-E glass and argon gas (centre-pane U-value ~1.1 W/m²K) comfortably meets the requirement, while a 4-12-4 unit with the same specification (centre-pane U-value ~1.3 W/m²K) will also comply in most frames. A basic 4-16-4 unit with clear float glass and air fill (centre-pane U-value ~2.8 W/m²K) would not meet current regulations for a replacement window.

Scotland has separate requirements under Section 6 of the Building Standards, which are broadly similar but with slightly different threshold values. Northern Ireland follows its own regulations under Part F of the Building Regulations (NI). In all cases, the principle is the same: the U-value of the complete window system determines compliance, not the thickness of the glass alone.

Building Regulations compliance is certified through competent person schemes. When a qualified installer fits new or replacement windows, they self-certify that the installation meets Building Regulations, and the details are registered with the local authority. If you are ordering supply-only units and fitting them yourself, you should ensure the specification meets the current requirements and notify your local authority's building control department if required.

What Thickness Should You Choose?

The right double glazing thickness for your project depends on several factors. Here is a decision framework to help you choose:

• Replacing like for like in standard frames: 4-16-4 (24 mm). This is the correct choice for the vast majority of UK replacement projects. It delivers excellent thermal performance with Low-E glass and argon, fits all standard frame profiles, and offers the best value for money.
• Older frames with shallow rebates: 4-12-4 (20 mm). When your frame cannot accept a 24 mm unit, the 20 mm option is the best available compromise. Specify Low-E glass and argon to maximise thermal performance within the reduced cavity.
• Noise is a primary concern: 6.8-16-4 acoustic (26.8 mm) or 6-16-6 (28 mm). Acoustic laminate glass with an asymmetric configuration delivers the best sound reduction. Check the frame rebate depth can accommodate the thicker unit.
• Deep-rebate premium frames: 4-20-4 (28 mm) or 6-16-6 (28 mm). Premium aluminium and some uPVC systems have deeper rebates that allow a 28 mm unit. Use this depth for either acoustic benefit (thicker glass) or for krypton gas fills (which perform well in wider cavities).
• New build or high-performance retrofit: Triple glazing in the 36 to 44 mm range. Specify from the outset with compatible frame profiles. Typically combined with two Low-E coatings and argon or krypton gas for U-values of 0.5 to 0.7 W/m²K.
• Heritage or conservation properties: Slim-profile units (4-6-4 = 14 mm or 4-8-4 = 16 mm) are available for heritage applications where preserving the original frame proportions is essential. These thinner units sacrifice significant thermal performance but allow double glazing where standard-thickness units would be visually inappropriate. Krypton or xenon gas compensates partially for the narrow cavity.

In all cases, the starting point should be measuring the available rebate depth in your frame. This determines the maximum unit thickness you can physically install. From there, choose the configuration that delivers the best performance within that constraint. For pricing across all configurations, see our cost guide.

We supply sealed units in all standard and bespoke thicknesses across London, Manchester, Birmingham, Liverpool, and areas throughout the UK. Free delivery on orders over £150.

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