Floor Joist Calculator — Sizing, Spacing & Quantities

Joist size depends on span and spacing. As a general guide for C16 timber at 400mm centres (first-floor loading: 1.5 kN/m² imposed plus a separate 2.0 kN concentrated-load check, dead-load band up to 0.5 kN/m²): up to 2.62m span use 47x150mm, up to 3.07m use 47x175mm, up to 3.52m use 47x200mm, up to 3.97m use 47x225mm. Remember 47mm nominal timber machines down to 45mm finished. This floor joist span calculator checks your dimensions against TradeCalculator's own C16 span table, calculated to Eurocode 5 (BS EN 1995-1-1) with UK National Annex parameters and cross-checked against published industry span tables. For longer spans, consider engineered joists (I-joists) or steel beams with shorter timber joists.

400mm or 450mm centres are standard for domestic floor joists in the UK. 400mm is strongest and allows the longest spans. 600mm centres are used for ceiling joists or very lightly loaded floors. The spacing must suit your flooring material — 18mm P5 (V313) chipboard decking is rated for a maximum of 450mm joist centres, while 22mm P5 chipboard spans up to 600mm (per BS EN 312 manufacturer data, e.g. Caberfloor/Egger/Kronospan). Once the joists are in, lay your floor covering — see our Laminate Flooring Calculator or Carpet Calculator.

Yes, noggings (also called strutting, blocking or bridging) prevent joists from twisting and distribute loads between joists. Strutting is required where joist spans exceed 2.5m. The standard rule (Approved Document A / NHBC 6.4.15 / TRADA) is one row at mid-span for spans of 2.5m to 4.5m, and two rows at the third-points for spans over 4.5m. Use herringbone strutting (timber minimum 38×38mm, or proprietary metal) or solid timber blocking at least 38mm thick and 0.75× the joist depth. This joist quantity calculator includes the strutting/nogging pieces in its output.

Joist hangers are galvanised metal brackets that support floor joists where they meet a wall or beam. They are required by Building Regs when joists are built into masonry walls. Simpson Strong-Tie and Expamet are common UK brands. Each joist needs a hanger at each end where it meets a supporting wall or beam.

After the joists and chipboard decking are fitted, you may need to level the surface before installing floor coverings. Use our Screed Calculator for sand and cement screed, or our Floor Levelling Calculator for self-levelling compound. Then choose your finish — tiles, laminate, vinyl or carpet — and use our dedicated calculators for each.