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Floor Load Capacity Calculator — Joist Size & Safe Loads
Check C24 floor joists against both bending (Eurocode 5) and deflection (span/250), then compare the safe distributed and point loads with your planned use.
Clear span between supports (effective span)
Permanent floor self-weight (default 0.5)
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How We Calculate This
This calculator sizes C24 floor joists by the two checks that govern a domestic timber floor: the bending (strength) check to Eurocode 5 and the deflection (serviceability) check. It reports the safe loads and tells you which check is governing.
1. Bending capacity (ULS)
The design moment capacity of each joist is MRd = fm,k × kmod × ksys ÷ γM × (b × h² ÷ 6), using C24 fm,k = 24 N/mm² (BS EN 338), kmod = 0.8, ksys= 1.1 and γM = 1.3. For a 47 × 200 joist this gives MRd = 5.09 kNm.
The applied design moment is M = wULS × Spacing × Span² ÷ 8, where the ULS load wULS = 1.35 × dead + 1.5 × imposed(EN 1990, UK National Annex). The partial factor γM is already inside the capacity, so it is not applied again to the load.
2. Deflection (SLS)
The instantaneous deflection under each characteristic action is δ = 5 × w × L⁴ ÷ (384 × E × I)(unfactored load; E0,mean = 11 kN/mm²). Creep is then added to give the net final deflection wnet,fin = δdead(1 + kdef) + δimposed(1 + ψ2kdef), with kdef= 0.6 (solid timber, service class 1; EC5 Table 3.2) and ψ2 = 0.3 (Category A; BS EN 1990 UK NA).
That final deflection is checked against the span ÷ 250 limit for a floor with a plastered or plasterboard ceiling (NA to BS EN 1995-1-1 Table NA.5). On typical domestic spans deflection usually governs, so the safe distributed load shown is the lower of the bending and deflection checks.
Results are indicative; for final design always confirm against the current TDUK/TRADA span tables or a structural engineer’s calculation.
Frequently Asked Questions
Load capacity depends on joist size, spacing, span and timber grade. Per the current TDUK/TRADA (BS 5268-7.1 / Eurocode 5) span tables for C24 domestic joists at 400 mm centres with an imposed load of 1.5 kN/m² and dead load up to 0.25 kN/m², the maximum permissible clear spans are approximately: 47×145 ≈ 3.3 m, 47×170 ≈ 4.0 m, 47×195 ≈ 4.55 m, 47×220 ≈ 5.0 m. Increasing joist depth or reducing spacing (to 300 mm) raises the permissible span. Always verify against the current TDUK tables or a structural engineer's calculation.
UK Building Regulations require domestic floors to support a minimum imposed load of 1.5 kN/m² (approximately 150 kg/m²). This covers normal residential use including furniture and people.
A distributed load is spread evenly over the floor area (like furniture and people). A point load is concentrated at a single point (like a heavy appliance, piano or safe). Joists can carry a higher total distributed load than point load.
Dead load is the permanent weight of the floor structure itself: joists, boarding, plaster ceiling below, flooring finish. Typical dead load is 0.4–0.6 kN/m² for a standard domestic floor.
If your planned loading exceeds the joist capacity, if you’re removing walls, adding a bathroom (heavy point loads from bath), or converting a loft. Always consult a structural engineer for commercial or unusual loading.
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.