Floor Joist Span Calculator — m, mm to joist size
How to use this calculator
Enter the clear span face-to-face between supports (subtract bearing — not the room dimension), spacing (400 mm centres is the EU residential default), strength class, and the imposed/permanent loads. The first section passing both ULS bending and SLS deflection is the recommended joist. Use it to estimate volume with the lumber volume calculator.
Imposed load: 1.5 kN/m² residential (EN 1991-1-1 Category A), 2.0 kN/m² offices (Cat. B), 2.5–3.0 kN/m² assembly/public. Permanent load: 0.5–1.0 kN/m² for timber floors with plasterboard ceiling. SLS deflection is L/300 by default; use L/250 for utility/deck and L/350 for tile or screed finishes that crack at higher bounce.
| Joist Size | b × d (mm) | Max Span | Deflection | Status |
|---|
Saved Calculations
| Time | Span | Spacing | Class | Load | Recommended | Status |
|---|
Floor Joist Sizing Notes (Eurocode 5)
Spans are computed to EN 1995-1-1: ULS bending using design strength fm,d = kmod·fm,k/γM and SLS deflection limited to L/300 under the characteristic imposed load, with characteristic strengths from EN 338. Use the recommended section to estimate timber volume with the lumber volume calculator, or design a deck floor with the metric deck calculator.
How to use
Enter the clear span between supports, joist spacing (400 mm centres is typical), strength class, and the imposed/permanent loads. The first section passing both ULS bending and SLS deflection is highlighted. Size the floor sheathing with the plywood & OSB calculator and the bearing wall studs with the stud wall calculator.
Formulas
Bending: MEd = wL²/8 ≤ MRd = fm,d·W with W = bd²/6, so Lbend = √(8·fm,d·W / wULS). Deflection: Δ = 5wL⁴/(384·E0,mean·I) with I = bd³/12, limited to L/300, so Ldefl = ∛(384·E0,mean·I / (5·wSLS·300)). The governing span is the smaller. ULS line load wULS = (γG·g + γQ·q)·s with γG = 1.35, γQ = 1.5; kmod = 0.8 (medium-term, service class 1–2), γM = 1.3.
Loads & sizes
EN 1991-1-1 Category A (residential): 1.5 kN/m² imposed; Category B (offices): 2.0 kN/m². Permanent load 0.5–1.0 kN/m² for timber floors with a plasterboard ceiling. Regularised C-graded sections per EN 336 are 45×145, 45×170, 45×195, 45×220, 45×245 and 63×245 mm; C24 is the most specified class. Add guardrails with the metric railing calculator.
On install day
Three things estimators miss on joist orders. Bearing length — EN 1995-1-1 §6.1.5 typically wants 38 mm minimum on timber and 75 mm on masonry; long runs need a built-up beam or LVL, not a deeper joist. Strutting / herringbone bracing — required at mid-span for joists deeper than 220 mm and where span exceeds 2.5 m, without it a passing calc still bounces. Stock length and waste — kiln-dried C24 is commonly sold in 600 mm increments up to 6.0 m; add 10–15% framing waste plus trimmers and end joists. Typical price in 2025 EU is roughly €4–7/m for 45×195 C24 and €6–9/m for 45×245 C24. Price the takeoff with the lumber volume calculator and design any deck floor with the metric deck calculator.
FAQ
Is creep included? The SLS check uses the instantaneous deflection winst. For final deflection apply kdef (service class 1: 0.6 for solid timber) to the quasi-permanent part — do this in a full design for long spans.
Why does 600 mm spacing fail earlier than 400 mm? The line load on each joist scales with spacing, so deflection governs at a shorter span. Reduce spacing or increase depth.
Is this an engineered design? No — it is a prescriptive estimate for simple single spans. Vibration, notching, and multi-span continuity need a full Eurocode 5 design.
