Finds the volume of sawn timber from its dimensions and the number of pieces. As well as cubic metres it gives the sai and the koku, the units the timber trade still uses. One cubic metre is about 299.5 sai, and one koku is about 0.278 cubic metres.
Sawn timber is counted by volume. Multiply width by thickness by length, then by the number of pieces.
The arithmetic is trivial; the units are the trap. Width and thickness are normally quoted in millimetres while length is quoted in metres, so they must be reconciled before multiplying.
The Japanese timber trade still runs on traditional units.
One sai is the volume of a piece one sun square and twelve shaku long, which works out at about 0.003339 cubic metres. A cubic metre is therefore about 299.5 sai, and the trade rounds this to roughly 300.
One koku is ten cubic shaku, about 0.2783 cubic metres. It is used for logs and for large quantities.
The sai survives for a good reason. Counting small sections in cubic metres produces strings of decimals like 0.0033, whereas one sai, two sai can be counted without tracking zeros. It is a clear illustration that a unit should suit the size of the thing being measured.
The default input is ten pieces of 105 mm square section, 3 m long — the standard post size in Japanese conventional framing.
One piece is , about 0.033075 cubic metres. Ten come to about 0.33075.
In trade units that is about 99.05 sai, or about 1.1886 koku. Ten posts to roughly a hundred sai is a handy figure for estimating.
This handles sawn rectangular sections. Logs are round and taper from butt to tip, so they are measured differently, typically from the small-end diameter.
Nominal and actual dimensions can differ. Planed timber finishes smaller than its nominal size, so exact volumes should be computed from the actual section.