Garden Bench Calculator — Timber Quantities & Dimensions

This calculator estimates the timber needed for a traditional garden bench with seat slats, optional backrest, legs, and structural braces.

Slat calculation

Seat slats = floor((Seat depth + Slat gap) / (Slat width + Slat gap))

Back slats = floor((Backrest height + Slat gap) / (Slat width + Slat gap))

Slats sit flush at both ends with a drainage gap between each, so a run of n slats has only n−1 internal gaps. This fence-post rule (and the physical limit that the solid slat material alone must fit the span) is why the count rounds down rather than up.

Slat gap (default 5mm) and backrest height (default 400mm) are editable under Advanced Options, so the formula uses your values, not fixed constants.

Timber total

Total timber includes: all slats at bench length, 4 legs, 2 armrests at seat depth, and 3 cross braces at bench length. All measurements are converted to linear metres, then multiplied by your waste factor (default 1.1 = 10%).

For a conservative order, every leg is costed at full back-leg height (seat height + backrest height + 50mm clearance). On a bench with a backrest only the two rear legs actually reach the top of the back, so this slightly over-orders leg timber — useful headroom for cuts and waste rather than a shortfall.