Lumber Size Discrepancies

Put a tape measure along a sheet of plywood and prepare to be surprised. At least then you'll know why you always had trouble fitting them :)

Cool project. You really put some planning into it.As others have mentioned S4S lumber in the US is not standardized in size. It can vary. If your using a softwood like Pine, it will also not be strait. This is going to cause your tight tolerances to be off. You should build this with lumber that has been fully dried, and fully straitened and taken to a lesser tolerance, something that can be plained and jointed down to.

It's just a matter of working with base elements that are divisible by 3 and 4 really.So instead of buying 100cm planks, buy 120cm planks?

Metric woodworking just uses integer millimeters for everything.

Haven't built a house, but some indoor construction notes:Measurements are specified in mm, yes. How well they're adhered to depends on the application.Factory-made kitchen cabinets, for example, are usually 600 mm wide, and this is pretty spot on (and since it's chipboard it won't warp much). The doors are specified a few mm under, so 597 mm for example.Drywall comes in at 1200 mm; again, this is quite accurate. The studs then go on a convenient fraction, usually ei

Carpentry takes imperial measurements to a whole new level - calling a piece of wood that is 1.5x3.5 a “2x4”.

I always figured it was due to milling to finished dimensions from raw lumber. You can buy unmilled lumber by the board-foot (basically 1”x12”x12”), and you get exact pay for dimensions, but when you’re buying finished, milled lumber the only thing that you can trust is that it’s been milled to a standard.In other words, they start with ~2”x~4” and mill to exactly 1.5”x3.5”Don’t ask me why that all changes at 1/2” dimensions.

For carpentry, 12-inches-to-a-foot isn't actually that useful, since wood is never the nominal size. For lumber, nominal sizes are measured assuming that sawing and planing remove no wood, but sawing and planing will remove about 1/4 to 1/2 an inch in each direction. That means that you can't stack two 2x4 to get an exact 4x4, and you can't use dimensional lumber to fill a nominal 1 foot gap without cutting.Carpentry, both the house-building and furniture making kinds

The stuff on the shelves is rarely sized precisely enough for the units to matter. I've bought 1/4" ply that measured much closer to 6.0 mm than to 1/4".

Cutting your own pine boards to replace the particleboard shelves and side panels in your Billy bookcase is likely to give you ok-but-also-not-exactly-straight-and-parallel results without a lot of specialized tools and techniques.