Floor Span CalculatorFloor Joist Span & Size Calculator
Check which standard joist sizes safely span your floor at a given spacing and load. A clear pass/fail table compares deflection and bending stress across sizes, highlighting the smallest economical joist.
Why this calculation matters
Joist size decides whether the floor bounces. Timber floors are almost always governed by deflection, not strength — a floor can be safe and still feel wrong.
Floor Span Calculator
Floor Joist Span & Size Calculator
Distance between supports.
Dead + live. Domestic floor ~2.5.
| Joist Size (b x d) | Deflection | Stress N/mm2 | Result |
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Where these numbers come from. Stiffness (E) is the mean modulus of elasticity parallel to grain, E0,mean, from EN 338:2016 Table 1. The permissible grade stresses are BS 5268-2 values. Those two are compatible for this purpose: stiffness does not carry the permissible-stress reduction that strength does, and BS 5268 and EN 338 agree on E to within about 2%. This sizes a member; it is not a design check. For anything you are building, have the section verified against EN 1995 (Eurocode 5) by someone who will sign it off.
About Floor Span Calculator
Floor joists must be deep enough not to sag (deflection) or break (bending stress) under load. This calculator checks a range of standard joist sizes against your span, spacing and load, and shows in a clear pass/fail table which sizes work - highlighting the smallest economical joist. It uses the classic beam formulas behind Eurocode 5 and IS 883 timber design. Use it in place of a generic joist span table — as a floor joist size calculator it checks your exact section, spacing and load instead of the nearest table row.
Where Is This Used?
Formulas Used
Line load w = Total load (kN/m2) x Spacing (m)Second moment of area I = b x d^3 / 12Deflection (UDL) = 5 w L^4 / (384 E I)Bending moment M = w L^2 / 8, stress = M / Z (Z = b d^2/6)Pass if deflection <= L/300 AND stress <= grade strengthHow the calculation works
Two separate checks run on every joist size, and the joist has to pass both. The stiffness check asks how far it sags; the strength check asks whether the timber is overstressed. On normal domestic floors the sag check almost always decides the size, which is why a joist that is nowhere near breaking can still be too small.
Show the formula
Line load w = area load x joist spacing. Second moment I = b d^3 / 12. Section modulus Z = b d^2 / 6. Deflection = 5 w L^4 / (384 E I). Bending moment M = w L^2 / 8, so stress = M / Z. A size passes only if deflection <= L/300 AND stress <= the allowable bending stress for the grade.E values are EN 338 mean moduli (C16 8.0, C24 11.0, C30 12.0, D30 11.0, D50 14.0, D70 20.0 GPa). The bending figures are ALLOWABLE working stresses, not EN 338 characteristic values, so this is permissible-stress sizing rather than a limit-state design. Uniformly distributed load, simply supported, laterally restrained.
References and what each one provides
- EN 338CEN (European Committee for Standardization)Characteristic bending strength and mean modulus for the C and D strength classes.
- EN 1995-1-1 (Eurocode 5)CENDesign of timber structures: fastener spacing and edge distances, connection capacity, and deflection limits.
- IS 883Bureau of Indian StandardsIndian permissible stresses and the L/300 deflection limit.
Unverified values are marked as such rather than presented as sourced.
Limitations of this calculation
Preliminary sizing from a uniformly distributed load on laterally restrained, defect-free members. It is not a structural design. It does not account for point loads, walls or tanks over the span, notches, holes, bearing, vibration, fire resistance, or long-term creep.
Do not use this for Structural design approval, building control submission, or certification of an existing floor. Sizing and comparison only.
Figures are engineering estimates from the inputs and assumptions shown. Verify against the actual material and, where the result affects structure or safety, against a qualified professional.
- EN 338 strength classes — E₀,mean and bending strength
- Deflection limit L/300 (IS 883, Eurocode 5)
Formula checked against the sources above by an automated regression test (tests/test-calculators.js) that derives each expected value independently of this page. Last reviewed .
How to use this tool
- Enter the job details below, then press Calculate.
- Pick your strength grade from the dropdown.
- Fill in clear span, joist spacing, total load.
- Press Calculate.
Worked example
Common mistakes to avoid
- Using clear-wood strength values without a safety factor. Real joists have knots; codes reduce allowable stress a lot.
- Measuring span to the wall face instead of bearing centres. It changes the calculation.
- Ignoring deflection limits. A joist can be strong enough yet still bounce annoyingly — check L/360, not just breakage.
Frequently Asked Questions
Wood species data
Density, hardness and movement for 60 timbers
Full listing in the timber strength table.