Lumber: nominal vs actual, board feet, cuts, and defects

One-line orientation

Lumber is labeled at its nominal (pre-surfaced) size but used at its smaller actual size — and board feet are still figured from the nominal number; that mismatch plus the named defects are the test material.

Key points

• Nominal vs actual dimension:
  • Nominal — the labeling size (e.g., 2×4, 1×6).
  • Actual — the true size after the wood is dried and planed; a 2×4 is actually 1½″ × 3½″.
• Board feet (lumber volume):
  • (thickness in × width in × length ft) ÷ 12 = board feet, using nominal sizes.
• Cut types:
  • Plainsawn — more cost-effective, varied grain appearance, more movement.
  • Quartersawn — pricier (more involved manufacturing), but enhanced quality and dimensional stability.
• Common defects / warps:
  • Bow — curving along the length.
  • Crook — curving along the edge.
  • Kink — a sharp bend.
  • Cup — concave / hollow across the width.
  • Twist — spiral distortion along the length.
  • Check / split — cracks (checks from uneven drying; splits run through).
  • Knot — where a branch met the trunk.
  • Wane — bark or missing material along an edge/corner.

A “2 × 4” is a name, not its finished dimensions

The end section is drawn at true relative scale; the board-foot lane deliberately uses the nominal envelope.

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Key

• dashed = nominal label size (2 × 4)
• blue solid = actual finished size (1½ × 3½)

Board feet = nominal thickness (in) × nominal width (in) × length (ft) ÷ 12.

Confusions / comparison

	Nominal	Actual
What it is	Labeling size (2×4)	Finished size after dry + plane
Example	2×4	1½″ × 3½″
Used for board feet?	Yes	No

Cut	Cost	Movement / stability	Grain
Plainsawn	Lower	More movement	Varied / cathedral
Quartersawn	Higher	More stable	Straight, uniform

Defect	What it is
Bow	Curving along the length
Crook	Curving along the edge
Cup	Concave / hollow across the width
Twist	Spiral distortion along the length
Check	Crack from uneven drying (split runs through)
Knot	Where a branch met the trunk
Wane	Bark or missing material along an edge/corner

Related

→ Wood moisture and seasoning (this topic): drying is why nominal shrinks to actual, and why warps form · Wood structure and shrinkage (this topic): the directional movement behind cup/bow/check · Engineered wood products (this topic): how manufactured members sidestep solid-lumber defects.