Rough Sizing Calculator
Convert between finished (planed) and rough-sawn dimensions — before kiln drying.
Forward mode: enter the finished (planed, kiln-dried) dimension, planing allowance per face, saw kerf, species (or custom S_od + FSP), finished MC and the MC at which the timber is sawn, and the tool returns the rough-sawn dimension to cut. Reverse mode does the opposite — what finished size will I get from a rough blank? The math adds planing allowance and saw kerf to the green dimension and applies Wood Handbook 2021 Eq. 4-9 shrinkage between sawn MC and finished MC.
FAQ
Why add planing allowance and saw kerf separately?
They are different losses: kerf is wood turned to dust in the cut (it disappears), planing allowance is wood removed by the planer after drying (it also disappears, but later). The rough blank must be big enough that, after drying and planing, the final piece is still on size. The calculator sums both and then applies shrinkage, which depends on the species and the MC at the time of sawing.
What MC is the wood sawn at in practice?
For air-dried stock, sawn at 18–25% MC, the dimension barely changes by the time it reaches 12% MC. For green (freshly felled) tropical hardwoods at 60–100% MC, the math clamps to FSP ≈ 30% — the wood does not shrink until it crosses FSP. For the US softwood default (sawn at ~30% MC, dried to 8%) shrinkage is real and significant.
What kerf should I use for my saw?
Band saws 1.8–2.5 mm (use 2.2 as a reasonable default for wood); circular saws 3.2 mm for a standard blade, 2.4–2.8 mm for a thin-kerf blade; industrial gang rips 5.5 mm; primary breakdown rips (the first cut from a large log) 4–7 mm. Adding kerf incorrectly is the most common source of off-size stock — measure your actual kerf with calipers if you can.