Wall Protection Zone Calculator
Surrounds and backboards are sold by their own outside dimensions, which tells you nothing about whether they reach as far as your darts do. Measure the furthest mark you have put in the wall, and this converts it into the one figure worth comparing: how far the protection has to extend past the edge of the board, measured out from the bull.
451 mm is the bristle standard. Measure yours — electronic boards run wider.
Log a few sessions and measure the furthest mark from the bull centre.
Headroom for the throw you have not had yet.
The overall outside measurement, not the cut-out.