Floor Joist Calculator — Sizing, Spacing & Quantities

Our floor joist calculator determines the number of joists needed based on your room length and chosen spacing, then checks the span against UK Building Regulations span tables. We also calculate noggings and joist hangers.

The formula

Number of joists = (Room length ÷ Spacing) + 1

For example, a 4m long room at 400mm centres: 4.0 ÷ 0.4 + 1 = 11 joists.

Max span table (C16 timber, first-floor loading)

Maximum permissible clear span in metres. 47mm nominal sizes are shown — these machine down to 45mm finished (47×200 = 45×195 finished, etc.).

Span values are TradeCalculator's own calculations to BS EN 1995-1-1 (Eurocode 5) with UK National Annex parameters, cross-checked against published industry span tables, for an imposed load of 1.5 kN/m² plus a separate 2.0 kN concentrated-load check and a dead-load band of up to 0.50 kN/m² excluding joist self-weight, the typical first-floor band (chipboard deck plus plasterboard ceiling). No partition allowance is included: where any partition bears on or crosses the joists, add the BS EN 1991-1-1 allowance or have the floor checked by a structural engineer. Deflection is limited to the stricter of span/333 and 12 mm, a minimum 40 mm end bearing is assumed, and values are rounded down to 0.01 m. Heavier floors (over 0.5 kN/m² dead) need shorter spans. For higher loads, C24 timber or any structural decision, confirm sizing with a structural engineer.