Window Board Calculator
How much window board do you need? Buying a length for each window usually buys more than you need, because two short sills can come out of one long board. Enter your reveals and get each board sized with its horns and overhang, then cut from the fewest metres of stock.
Last reviewed: 30 September 2026
Board
Enter each reveal: its width between the plastered sides, its depth from the frame to the face of the wall, and how many windows are that size.
How Much Window Board Do I Need?
The usual way, in Skirting World's buying guide, is to measure each reveal and pick a length for that window. It works, but it buys more than you need. For reveals of 1200mm (two windows), 900mm (two) and 600mm, one board per window is 5 x 2100mm, 10.50m in all. Cut from the lengths, it is 1 x 2100mm + 1 x 3660mm, 5.76m. In the table below, the cutting plan buys less in 5 of 6 jobs.
Each window board is the reveal width plus a horn at each end, so a 900mm reveal takes a 1000mm board, and a 2100mm length holds two with a saw cut between. This calculator sizes every board, then cuts them from the mix of long and short lengths that buys the least.
How the Plan Works
- Horns. Skirting 4 U: horns of "around 50mm on each side". Change it if yours differ.
- Depth. The reveal plus a front overhang the same as the board's thickness (Skirting 4 U's rule of thumb). The calculator warns if the board is too shallow.
- One piece per window. A board longer than the longest length is flagged, not joined.
- Cut from the fewest metres. Boards longest first, each into the length with least room left, with a 3mm saw cut, trying every mix of 2100 and 3660mm lengths (our working).
What it does not do: bay windows cut on an angle, notching round the plaster, and uPVC window boards, which come in other sizes.
Window Board Sizes, With Sources
Every board in the dropdown has a Wickes product page, in both lengths.
The horns and overhang are from Skirting 4 U's guide to measuring a window board, and the one-length-per-window method from Skirting World's buying guide. The packing and the saw cut are our own working.
Common Jobs: Lengths Needed
Generated from the calculator above, for 25 x 219mm board with 50mm horns.
| Job | Cutting Plan | One Per Window |
|---|---|---|
| One window, 1000mm reveal | 1 x 2100mm (2.10m) | 1 x 2100mm (2.10m) |
| Two windows, 1500mm | 1 x 3660mm (3.66m) | 2 x 2100mm (4.20m) |
| Three windows, 1200mm | 1 x 2100mm + 1 x 3660mm (5.76m) | 3 x 2100mm (6.30m) |
| Four windows, 900mm | 2 x 2100mm (4.20m) | 4 x 2100mm (8.40m) |
| Bay, 1800mm + two 600mm sides | 1 x 3660mm (3.66m) | 3 x 2100mm (6.30m) |
| House: 1200mm x2, 900mm x2, 600mm | 1 x 2100mm + 1 x 3660mm (5.76m) | 5 x 2100mm (10.50m) |
Frequently Asked Questions
How much window board do I need?
Measure each reveal and add a horn of about 50mm at each end, as Skirting 4 U's guide suggests. The usual way, in Skirting World's buying guide, is then to pick a length for each window. But window board is sold in long lengths, and two short sills can come out of one. For reveals of 1200mm (two windows), 900mm (two) and 600mm, one board per window is 5 x 2100mm, 10.50m in all. Cut from the lengths, it is 1 x 2100mm + 1 x 3660mm, 5.76m. In the table below, the cutting plan buys less in 5 of 6 jobs.
How long should the horns be?
Skirting 4 U says many people choose horns of around 50mm on each side, adding 100mm to the board. The horn runs past the reveal onto the wall and is notched round the plaster. You can change it.
How deep should a window board be?
The reveal depth plus the front overhang. Skirting 4 U's rule of thumb is to make the overhang the same as the board's thickness, so 25mm on a 25mm board. A 150mm reveal needs a board at least 175mm deep, and the calculator warns if the board you picked is not deep enough.
Can I get two window boards out of one length?
Yes, if both fit with a saw cut between them. Four 900mm reveals need 1000mm boards with their horns, and a 2100mm length holds two, so 2 x 2100mm will do, not four lengths. The calculator tries every mix of long and short lengths and keeps the one that buys the least.
Should I join window board?
Window boards are normally one piece, so the calculator warns if a board with its horns is longer than the longest length. A single window gets its own length: a 1000mm reveal needs 1 x 2100mm.