Window Mullion Count Calculator | Panel Width Layout
Estimate the number of equal window panels needed for a target panel width, then calculate the resulting panel width and internal mullion count. This is a layout tool, not a structural mullion-sizing calculation.
Window subdivision inputs
Mullion layout result
Mullion count method
Internal mullions = panels − 1
Bay width = (overall width − total internal mullion width) ÷ panels
Why panel count is a layout decision
A target panel width can produce different practical layouts depending on whether the user wants to stay under a maximum width or simply get as close as possible to a preferred module.
Why mullion width matters
Each internal mullion occupies horizontal space. Ignoring that width can make the calculated glass bays wider than the actual frame permits.
Worked example
For a 4.0 m window with a target panel width of 1.0 m and 60 mm mullions, four panels require three internal mullions:
Actual bay width = 3.82 ÷ 4 = 0.955 m
Frequently Asked Questions
Does this size a structural mullion?
No.
Can the panel width be limited to a maximum?
Yes. The default mode selects enough panels to stay at or below the target.
Can I use a different mullion width?
Yes.
Does this include transoms?
No. It handles horizontal subdivision only.