Metric To Imperial Lumber Conversion

North American lumber is priced, cut, and specified in inches and board feet; most of the rest of the world works in millimeters and cubic meters — and the conversion between the two systems is exact arithmetic, but it's easy to apply it to the wrong number if you convert a nominal size instead of an actual one.

Linear conversion: exact, no ambiguity

One inch equals exactly 25.4mm — this is a defined constant, not a measured approximation, so converting a real dimension is always exact once you have the actual (not nominal) figure in hand. An actual 2x4, 1.5" x 3.5", converts to 38.1mm x 88.9mm. Compare that to a European or Baltic-sourced sheet good like Baltic birch plywood, which is specified in true metric thickness from the start (18mm converts to 0.709" — see /reference/plywood-sheet-sizes-and-thickness/ for the full Baltic birch thickness table) rather than being metric-labeled dimensional lumber pretending to follow the North American nominal system.

Volume conversion: board feet to cubic meters

One board foot is exactly 1/12 cubic foot, and one cubic foot is 0.0283168 cubic meters, so one board foot equals 0.00236 cubic meters. Inverted, one cubic meter is about 423.8 board feet. This conversion is what you need when comparing a North American board-foot lumber quote against an international or metric-sourced quote given in cubic meters — species like teak, sapele, or Honduran mahogany are frequently sourced internationally and may arrive quoted either way depending on the supplier.

Where the conversion goes wrong

The arithmetic above is exact; the mistake isn't in the conversion, it's in converting the nominal North American size instead of the actual one. A nominal "2x4" is not 2" x 4" (50.8mm x 101.6mm) — it's 1.5" x 3.5" (38.1mm x 88.9mm) actual, and converting the nominal figure to metric produces a number that's wrong by the same margin the nominal-vs-actual gap always produces, just expressed in millimeters instead of inches. See /reference/nominal-vs-actual-2x4/ for the full actual-size reasoning before converting any North American dimensional lumber figure to metric.

A worked comparison table

Dimension	Actual (in)	Actual (mm)
2x4 (actual)	1.5 x 3.5	38.1 x 88.9
Baltic birch 18mm	0.709 (converted from mm)	18.0 (native)
Baltic birch 12mm	0.472 (converted from mm)	12.0 (native)

Notice the direction of conversion matters for which figure is "native" — North American dimensional lumber's native unit is inches, converted to millimeters for comparison; Baltic birch plywood's native unit is millimeters, converted to inches for comparison. Neither figure is more "real" than the other, but knowing which one is the manufacturer's actual specified target (rather than a rounded conversion) tells you which number to trust if the two don't quite line up after rounding.

Practical use

If you're specifying hardware, jigs, or fixtures that need to work with material sourced from either system — a router base plate designed to register off an 18mm Baltic birch panel but potentially built from 3/4" (0.719" actual, per /reference/plywood-sheet-sizes-and-thickness/) domestic plywood instead — check the real thickness difference (0.719" is 18.26mm, close to but not identical to true 18mm) before assuming the two are interchangeable in a precision fit. A quarter-millimeter difference is invisible on a shelf; it's not invisible in a router jig or a drawer slide channel.

Weight and density conversions follow their own factors

Density figures published for species (this site's own data included) are commonly given in both pounds per cubic foot and kilograms per cubic meter, and the two convert via a fixed factor: 1 lb/ft³ equals about 16.02 kg/m³. This matters when comparing a species sourced from a supplier that quotes metric density (common for internationally traded tropical hardwoods) against domestic species quoted in the more familiar lb/ft³ figure — converting both to the same unit before comparing avoids the easy mistake of reading a metric density figure as if it were already in pounds per cubic foot, which would make a species look far lighter or heavier than it actually is.

Moisture content and shrinkage percentages don't need conversion at all

One thing that does NOT need a unit conversion between systems: moisture content and shrinkage figures are already expressed as percentages of the wood's own oven-dry weight or dimension, which is a ratio, not a length or volume — a species' published shrinkage percentage means the same thing whether the underlying board is measured in inches or millimeters. Only the resulting dimensional change (see /reference/moisture-content-targets-by-region/ and the wood movement calculator) needs a unit conversion once you're translating that percentage into an actual movement figure for a board of a known size.