Floor Joist Span Calculator — m, mm to joist size
How to use this calculator
Enter the clear span, joist spacing (300, 400, or 600 mm centres), timber strength class, and loading conditions. The calculator determines the minimum joist size per Eurocode 5 (EN 1995-1-1) and shows a full span table with pass/fail for all common sizes.
Live load: 1.5 kN/m² residential (EN 1991-1-1 Category A), 2.0 kN/m² offices (Cat. B). Dead load: 0.5-1.0 kN/m² typical. Deflection is checked at L/300 for instantaneous live load (Eurocode serviceability).
| Joist Size | b × d (mm) | Max Span | Deflection | Status |
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Saved Calculations
| Time | Span | Spacing | Class | Load | Recommended | Status |
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How to Size Floor Joists (Eurocode 5)
This floor joist span calculator determines the minimum joist size for a given clear span per EN 1995-1-1 (Eurocode 5). Enter the span, joist spacing, timber strength class (C16, C24, C30, or GL24h glulam), and loading in kN/m² to find the required joist dimension.
Span Calculation Method
Maximum spans are determined from characteristic bending strength (fm,k) and mean modulus of elasticity (E0,mean) for each strength class per EN 338. Both ULS bending capacity (with kmod and γM = 1.3) and SLS deflection at L/300 are checked. The more restrictive limit governs.
European Timber Sizes
Common metric joist sizes per EN 336: 45×145, 45×170, 45×195, 45×220, 45×245, and 63×245 mm. Regularised timber (C-graded) is available in standard cross sections. C24 is the most widely specified strength class for structural floor joists.
Load Categories (EN 1991-1-1)
Category A (residential): 1.5 kN/m² imposed. Category B (offices): 2.0 kN/m². Dead load typically 0.5-1.0 kN/m² for timber floor construction with plasterboard ceiling below. Partial factors: γG = 1.35 (permanent), γQ = 1.5 (imposed).
