What size floor joists do I need for my span?

Joist size depends on span and spacing. As a general guide for C16 timber at 400mm centres: up to 2.7m span use 47x150mm, up to 3.7m use 47x200mm, up to 4.1m use 47x225mm. This floor joist span calculator checks your dimensions against UK Building Regs Approved Document A span tables. For longer spans, consider engineered joists (I-joists) or steel beams with shorter timber joists.

What spacing should floor joists be?

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 — chipboard decking (18mm) needs 400mm maximum centres, while 22mm chipboard can span 600mm. Once the joists are in, lay your floor covering — see our Laminate Flooring Calculator (/flooring/laminate-flooring-calculator) or Carpet Calculator (/flooring/carpet-calculator).

Do I need noggings between floor joists?

Yes, noggings (also called strutting, blocking or bridging) prevent joists from twisting and distribute loads between joists. UK Building Regulations require noggings where joist spans exceed 2.5m. Herringbone strutting (timber or metal) or solid timber noggings should be installed at mid-span or at ~1.2m intervals for longer spans. This joist quantity calculator includes nogging pieces in its output.

What are joist hangers and do I need them?

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.

What do I do after installing floor joists?

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

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Floor Joist Calculator — Sizing, Spacing & Quantities

Calculate floor joist quantities, check span tables against UK Building Regs, and work out noggings and joist hangers.

Joist Span

Distance joists need to span (wall to wall)

Room Length

Length of room (parallel to wall joists sit on)

Joist Spacing

400mm is standard for most domestic floors

Timber Size

Select based on span — see span table below

Price Per Joist (optional)

Price per joist length from your timber merchant

How We Calculate This

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, domestic loading)

Timber Size	400mm	450mm	600mm
47 × 100 mm	1.72 m	1.63 m	1.44 m
47 × 150 mm	2.76 m	2.62 m	2.35 m
47 × 175 mm	3.25 m	3.09 m	2.78 m
47 × 200 mm	3.72 m	3.54 m	3.18 m
47 × 225 mm	4.16 m	3.96 m	3.58 m

Span values are based on Approved Document A (Structure) and TRADA span tables for C16 grade timber with standard domestic floor loading (1.5 kN/m² imposed load). For higher loads or C24 timber, consult the full TRADA tables or a structural engineer.

Frequently Asked Questions

Related Calculators

Screed Calculator

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Self-Levelling Compound Calculator

Work out how many bags of self-levelling compound you need.

Floor Tile Calculator

Calculate tiles, adhesive and grout needed for floor tiling.

Laminate Flooring Calculator

Work out how many packs of laminate or engineered flooring you need.

Carpet Calculator

Calculate carpet area, underlay and gripper rod quantities.

Last updated: February 2026

All calculations are estimates. Verify with your supplier.