Free tool
Mulled window size calculator: centerline sizes and frame sizes.
Enter the overall size of a mulled unit, how many openings it has and the width of the mull post. You get each opening's sold size, measured centerline to centerline, and the frame size to build, with a check that frames plus posts come back to the overall size.
Inches. Type decimals or fractions: 36.5, 36 1/2 or 36-1/2.
How the calculator works
- Sold size is measured to the middle of each mull post, so the openings add up exactly to the overall size. An equal split is shown to the nearest 1/32", and the last opening takes whatever keeps the total exact.
- Frame size is the sold size less half a post on each side the opening shares with a neighbor, rounded down to the nearest 1/16" so the assembly never comes out over size. End openings lose half a post once; middle openings lose it twice.
- United inches (width + height) are given on the sold size, which is how per-UI charges are usually billed.
- Across the mulls (the height of a side-by-side unit, the width of a stacked one), every frame is built at the full size.
The full reasoning, including which size your price book should read and which your cut list should read, is in our guide: how to size and price mulled windows.
Note: the mull post width is the one number to get from your own product. Use the actual width of the mullion your plant builds with. Some manufacturers also add a fit allowance; this calculator applies none.
Mulled window sizing — common questions
What is the difference between centerline size and frame size?
The centerline (sold) size of an opening runs to the middle of the mull post on each side it shares, so the openings add up to the overall size. The frame size is the window actually built: the centerline size less half a post for every shared side.
Which size should a mulled window be priced on?
Most window price books are built for openings measured the way they are sold, so size-book prices and united-inch charges usually read the centerline size. Cut lengths, glass sizes and labels read the frame size.
Why do the frames plus posts come out slightly under the overall size?
Each frame is rounded down to the nearest 1/16" so the assembly can never come out oversize. The calculator shows the small difference that rounding leaves.
What mull post width should I use?
The width of the mullion your plant actually uses, from your own product specification. Use 0 if each opening is a separate frame with nothing between them.