Window Board Calculator

This calculator determines the correct window board dimensions, the number of boards needed, and fixings for your window installation.

Board width (depth)

Board width = Reveal depth + Front overhang. The reveal depth is measured from the window frame to the face of the plastered wall. The front overhang (nosing) is typically 25-30mm, projecting past the plaster line — so a 220mm reveal usually takes a 250mm-deep board.

Board length

Board length = Window width + (2 × Side overhang). The common UK convention is a 50mm horn each side (so roughly +100mm on the window width), projecting past the reveal and then trimmed back on site to suit the wall. Horn length is a matter of preference — keep both ends equal unless a corner or other obstruction gets in the way.

End caps

MDF boards need end caps (iron-on edging) on each exposed end to cover the raw MDF core. Each board typically needs 2 end caps. Timber and stone boards do not require end caps.

Fixing

• Expanding foam: Low-expansion PU foam supports and bonds the board
• Grab adhesive: Alternative to foam; apply in zigzag beads
• Weigh down the board while the adhesive cures (typically 1-2 hours)

Standards

Window boards should be fitted level and square. In bathrooms and kitchens, use moisture-resistant materials. The board should be fitted after plastering but before final decoration.