Rough Sizing: How Big to Saw Before Kiln and Planer
Combine planing allowance, saw kerf and Wood Handbook shrinkage to size rough stock for a finished dimension.
Two allowances and one shrinkage
To go from a finished dimension back to the rough-sawn blank, you add: (1) planing allowance on both faces, (2) saw kerf if the blank is ripped from a larger piece, and (3) the shrinkage that happens between sawn MC and finished MC. The math is green = finished / (1 − S_end/100), and rough = green + planing + kerf. The Rough sizing tool does the three-part sum; pick a species (16 verified presets) or enter a custom S_od and FSP.
US softwood defaults: 2 × 3/16″ planing, 2 mm band kerf
The American Woodworker convention is 3/16″ (4.76 mm) per face, so 9.5 mm of planing allowance on the cross-section. With 2 mm of band-saw kerf on a 3/4″ (19 mm) finished board sawn at 30% MC, the rough blank comes out about 31 mm. For thicker stock (8/4 = 1″ nominal), the rough blank is 35–40 mm. Vietnamese shops typically use less planing (2–3 mm/face) since planer accuracy is improving — the Rough sizing tool lets you tune the allowance per face.
Green tropical hardwoods do not shrink until they cross FSP
Freshly sawn acacia at 60–80% MC has not crossed FSP (≈30%), so its dimension is the same as the sawn blank. The Rough sizing tool clamps the sawn MC to FSP automatically. If you saw an acacia board at 60% MC to 28 mm rough, the green dimension is also 28 mm, and the finished 8% MC dimension is 28 − shrinkage. For oak (S_t 10.5%) the shrinkage from FSP to 8% MC is 7.7%, so a 28 mm rough blank ends up at 25.5 mm after drying and planing — close to the 25 mm finished target.
Measure your saw kerf, do not trust the label
A 'thin-kerf' circular saw blade (2.4 mm) really does cut at 2.4 mm, but only when new. After 20 sharpenings the kerf is closer to 2.6 mm. A band saw at 1.8 mm new widens to 2.0–2.2 mm after a few hours of cutting oak. Measure the actual kerf by cutting a test piece of known length and measuring the gap: rough-cut 100 mm of wood, the saw kerf is (cut + waste − 100) / number of cuts. The Rough sizing tool has presets for band, circular, thin-kerf, gang and primary breakdown saws — pick the closest, then tweak if you have measured your actual kerf.
Plan forward, not just the current board
If you are ripping 50 mm acacia boards down to 20 mm finished for a furniture run, work out the rough size once (say 28 mm) and saw the whole lot to that dimension before the kiln. Sawing each board to its final size, drying, then planing individually wastes time, planer life, and 5–10% extra material from mis-cut boards. The Rough sizing tool keeps the same answer visible while you run the numbers for different finished dimensions or different species.