Screw spacing calculator
Divides a run into a whole number of equal gaps as near your target spacing as the length allows. With fixings at both ends the count is gaps plus one, so 2400 mm at 300 mm spacing is eight gaps and nine screws rather than eight.
How to use the screw spacing calculator
This is the fencepost problem, and it costs a second trip to the merchant rather than a mistake in the timber. Eight gaps need nine fixings; the same run fixed only between the ends needs seven. Being out by one is the commonest error in setting out a batten or a rail, and it reliably shows up when you are already up a ladder with the box open.
The other half is the remainder. A 2400 mm run at a 300 mm target divides evenly, which is a coincidence of the numbers people usually try. Most runs do not: 2150 mm at 300 mm gives 7.17 gaps, and the real choice is eight gaps at 268.8 mm or seven at 307.1 mm. This rounds to a whole number of gaps and reports how far that pushed you off target, because that difference is the entire decision. Seven millimetres along a fence rail is invisible; the same seven across a row of exposed cabinet screws is the first thing anyone notices.
Edge distance is the constraint the arithmetic cannot see. A fixing set too near the end of a board splits it, and the working rule in solid timber is two to three fastener diameters in from the edge and around five from the end. If the first-fixing figure comes out under roughly 20 mm for a 4 mm screw, move the run rather than trusting the spacing, or drill a pilot hole and accept the risk knowingly.
What people use it for
- Marking out fixings along a fence rail or a batten
- Avoiding a short last gap on a run that will be seen
- Buying the right number of fixings before starting
- Setting out shelf brackets evenly across a wall
- Spacing deck board fixings along a joist
Questions
Nine, with fixings at both ends: eight gaps and nine fixings. Seven if the ends are left unfixed.
Because the run is divided into a whole number of equal gaps. A 2150 mm run at 300 mm becomes eight gaps of 268.8 or seven of 307.1; the alternative is a short gap at one end.
Commonly 400 to 600 mm for joists, with two fixings per board per joist. Check the rules that apply where you are building.
Almost always, and it is the default here. Boards and battens lift at an unfixed end.
Far enough not to split the board: around five fastener diameters from the end and two to three from the edge in solid timber.