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The rule of thumb

The thinner the board, the shorter the supported span. A shelf twice as wide needs roughly 8 times the bending stiffness, so thickness grows fast with span.

Common thicknesses

• 4–6 mm: drawer bottoms, backs, panel infills
• 12–16 mm: drawer sides, light cabinet sides
• 18 mm: standard furniture carcase, shelves up to ~600 mm span
• 25–30 mm: worktops, heavy shelves, bench tops
• 38 mm+: structural beams, load-bearing frames

Deflection check

For a shelf loaded with books:

Max span ≈ thickness × 33 (for solid wood shelves with moderate load)

An 18 mm shelf supports roughly 600 mm; a 25 mm shelf roughly 800 mm.

Practical tips

• Load matters more than looks — verify spans for heavy storage.
• Plywood is stiffer than softwood of equal thickness.

Plan your panels with the furniture plan generator.