Insulated glass unit formula: how to read 4-16-4 and more

An insulated glass unit formula is a shorthand that lists the thickness of each pane and the width of each spacer between them, in millimetres. For example, 4-16-4 means two 4 mm panes with a 16 mm spacer, a single-chamber unit with a total thickness of 24 mm. Below we explain how to read the formula, what the letters mean and how to write it correctly when you place an order.

What does an insulated glass unit formula mean?

The formula always follows the same pattern: glass, spacer, glass. Numbers in the 1st, 3rd and 5th positions are pane thicknesses; numbers in the 2nd and 4th positions are spacer widths, which equal the depth of the air chamber between two panes.

By convention the formula is typically written from the outside in: the first number is the outer pane facing the street, the last is the inner pane facing the room. When all panes are identical the order does not matter. Once a tinted, reflective or tempered pane is involved, however, the sequence is what tells the factory where that pane goes.

The spacer is an aluminium profile filled with desiccant; the unit edge is sealed first with butyl and then with polysulphide. You can read how each step is done on our insulated unit assembly service page.

4-16-4 vs 4-10-4-10-4: single or double chamber?

The number of chambers equals the number of spacers in the formula. Two panes and one spacer make a single-chamber unit (often called double glazing); three panes and two spacers make a double-chamber unit (triple glazing).

Formula	Panes	Chambers	Total thickness
4-12-4	2	1	20 mm
4-16-4	2	1	24 mm
5-14-4	2	1	23 mm
6-16-6	2	1	28 mm
4-10-4-10-4	3	2	32 mm
4-12-4-12-4	3	2	36 mm

A single-chamber unit such as 4-16-4 is lighter and simpler, and is usually sufficient for balconies, unheated spaces and budget projects.

A double-chamber unit such as 4-10-4-10-4 is the more common choice for homes in Baku’s climate: it noticeably reduces winter heat loss, summer cooling load and street noise. For a fuller comparison, see our guide on how to choose an insulated glass unit.

How do you calculate total unit thickness?

Simply add every number in the formula. For 4-10-4-10-4 that is 4 + 10 + 4 + 10 + 4 = 32 mm. The figure matters for three reasons:

1. The glazing rebate. Every window profile accepts units within a specific thickness range, and the glazing bead is chosen to match. Check the profile manufacturer’s table before ordering: a 32 mm unit will not fit a sash designed for 24 mm.
2. Weight. Each millimetre of glass weighs roughly 2.5 kg per square metre, while the spacer adds very little. A 4-16-4 unit (8 mm of glass in total) is therefore about 20 kg/m², and a 4-10-4-10-4 unit (12 mm of glass) about 30 kg/m². Hinges and hardware need to be rated accordingly.
3. Handling and installation. Thicker, heavier units need more care in transport and on site, so plan ahead for large items.

Note that only the glass determines weight: a 4-20-4 unit is thicker than a 4-12-4, yet weighs almost the same.

Which spacer widths and glass types does Magna use?

We assemble units with 6, 8, 9, 10, 12, 14, 16, 18 and 20 mm spacers. Panes are 4, 5 or 6 mm thick and can be any type we produce: clear float glass, frosted, tinted, reflective or tempered.

As a general guide:

• 6–9 mm — slim profiles and interior partitions;
• 10–12 mm — each chamber of a double-chamber unit;
• 14–16 mm — the usual optimum for a single-chamber unit;
• 18–20 mm — special projects and profiles with deep rebates.

One more tip: mixing pane thicknesses (for example 6-12-4) typically improves sound insulation, because identical panes resonate at the same frequency. A thicker outer pane also helps with wind load on large lights. For more on this, read how to choose glass thickness.

What do the letters mean: 4T, 4M1, Low-E and others

A letter next to a number describes the type of pane. There is no single standard, but these are the markings you are most likely to see:

• T (or “temp”) — tempered glass, e.g. 4T-16-4. Tempered glass cannot be cut afterwards, so all sizes must be final before tempering.
• M1 — a float glass quality grade under the GOST standard used across the CIS; in our region it is often written simply to mean ordinary clear glass, as in 4M1-16-4M1.
• Low-E, i, K — low-emissivity, energy-saving coated glass, with the coating usually facing into the chamber. This is not currently part of our listed range; if your project needs it, ask us.
• Ar — an argon-filled chamber, as in 4-16Ar-4. Please also ask us about gas filling before you order.
• Colour — usually written in words, such as “4 bronze” or “4 reflective blue”.

Because codes differ between suppliers, the safest approach is to describe every pane in words as well.

How to write the IGU formula in an order

For an accurate quote, give the following for each item:

1. Item number, width × height (mm) and quantity.
2. The formula, written outside to inside: e.g. 4-16-4 or 4-10-4-10-4.
3. The type and colour of each pane: “outer 4 mm bronze, inner 4 mm clear”.
4. Which pane is tempered: outer, inner or both.
5. A drawing if you need decorative bars (muntin units).
6. Any special requirements, such as Low-E or gas filling, as a separate note.

A typical line might read: Item 3 — 1200 × 1450 mm — 6 pcs — 4T bronze / 16 / 4 clear.

The mistakes we see most often:

• confusing spacer width with overall thickness (a “24 mm unit” and a “16 mm spacer” are different things);
• writing the formula inside to outside, so the tinted pane ends up facing the room;
• forgetting to specify a tempered pane, which is a safety issue for floor-length windows and doors;
• choosing an overall thickness that does not fit the profile rebate.

Ordering insulated glass units from Magna

Send your sizes and formula through the quote form — plain text, an Excel sheet or a photo of a drawing is enough. Our software calculates pane sizes, spacer length, sealant and desiccant use for every item and optimises the cutting plan, and the quote is ready within one business day.

Not sure which formula to choose? Tell us the profile system and what the room is used for, and we will help you pick the right insulated glass unit.