Understand · Going further · 4 min
How lengths to order are worked out
Why each drawing is rounded on its own, what a lap really costs, and why the program shows you what the other rule would give.
Ordering is arithmetic with a few choices in it. The choices are shown, not hidden, because people disagree about them and both views are reasonable.
Whole lengths
A 10 m run in 3 m stock is 3.33 lengths, so you order 4. It’s always rounded up: you can’t buy a third of a length. A run that’s exactly a whole number of lengths isn’t rounded up by rounding noise; 9 m in 3 m stock is 3, not 4.
Run by run, or once
A site orders for each place, and each place is cut and delivered for itself, so each drawing's run is rounded on its own and then added. Rounding the whole total once would give a smaller number that no site could actually deliver. The first can only ever be equal or higher. Choose the second only when offcuts really are reused across places.
Laps
A lapped piece covers the stock length less the lap. The cautious rule takes the lap off every piece, including the last; the between pieces rule takes it off only where two pieces meet. The cautious rule never gives fewer. When the two disagree for your run, the working says so, and shows the figure the other would give.
Butt straps
Straps join pieces end to end, so there’s one fewer than the pieces in each run. Laps and straps are alternatives for a flashing, never both.