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IGU Thickness & Spacer Calculator

Give it any two of the three, glass thickness, spacer/cavity width, or overall unit thickness, and it solves for the third, with ASTM C1036 actual-thickness tolerances built in.

IGU configuration
Glazing type
What do you want to solve for? Pick one
Enter every pane and cavity below, the tool adds them up and gives you the overall IGU thickness.
ASTM C1036 nominal vs. actual glass thickness
Source: ASTM C1036 Table 2 (Dimensional Tolerances for Rectangular Shapes, Type I, Transparent Flat Glass). Applies to clear and tinted annealed float glass; heat-treated glass (ASTM C1048) uses the same nominal designations.
Common spacer / cavity widths
3/16–5/16 in
(4.76–7.94 mm)		Narrow cavities, common on small/decorative units, lower thermal performance.
7/16–5/8 in
(11.11–15.88 mm)		Typical "sweet spot" for argon-filled cavities, best conductive + convective balance. 1/2 in (12.7mm) is the NFRC 100 simulation default.
11/16–3/4 in
(17.46–19.05 mm)		Common for high-performance double or triple units; diminishing returns above ~5/8in for a single gas fill.
>7/8 in
(>22 mm)		Convective gas movement starts to offset insulating gains, rarely worth it for a single cavity.
There is no ASTM dimensional standard governing spacer/cavity width, available sizes are set by individual spacer manufacturers. North American systems are typically sold in fractional-inch increments (3/16in to 1in); European/metric systems in round mm increments (6mm to 24mm). NFRC 100 uses a 12.7mm (1/2in) default airspace for simulation. Confirm available sizes with your spacer system supplier.
Overall IGU thickness
,
mm
How this is calculated:
Overall thickness = the sum of every pane's glass thickness plus every sealed cavity's width. For nominal glass, the tool uses the standard nominal value (e.g. "6mm") for the headline number, and separately shows the real-world range allowed by ASTM C1036, actual 6mm float glass can legally measure 5.56–6.20mm, so a real unit's true thickness will land somewhere inside the shown range rather than the exact nominal sum.

Custom/actual entries are treated as exact measured values with no added tolerance, use this when you've measured the real glass with a caliper rather than ordering by nominal callout.

Do not use this tool as the final word on frame/glazing-pocket fit. Confirm spacer availability with your IG fabricator and glazing pocket depth with your frame/window system supplier before finalizing a spec.
Also try
Glass Weight Calculator→Glass Selection Wizard→