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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.

Joist Size TableDeflection + StressC16 to D70 GradesPass/Fail ComparePDF Report
FS

Floor Span Calculator

Floor Joist Span & Size Calculator

Span & Load
mm

Distance between supports.

mm
kN/m2

Dead + live. Domestic floor ~2.5.

Timber
OK
Floor Span Results
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mm
Smallest Joist
-
mm
Deflection Limit
-
kN/m
Line Load
-
grade
Strength Grade
Joist Size (b x d)DeflectionStress N/mm2Result
Span Calculation
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.
Where Is This Used?
Floor FramingJoist SizingStructural EstimatingRenovation + ExtensionsMezzanine DesignBuilding Quotes
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 strength
Frequently Asked Questions
What does the L/300 deflection limit mean?
It limits how much a joist may sag under full load to its span divided by 300 - so a 3600mm span may deflect at most 12mm. This keeps floors feeling solid and stops cracking of finishes above. Some codes use L/360 for stiffer floors or where brittle finishes (tile, plaster) are present.
What is the difference between C and D grades?
C grades are softwoods (C16, C24, C30) and D grades are hardwoods (D30 to D70), with the number indicating bending strength in N/mm2. Higher numbers are stronger and stiffer, allowing smaller or more widely spaced joists. Dense tropical hardwoods like teak fall in the upper D classes.
Can I rely on this for actual construction?
Use it for sizing, estimating and comparing options - it implements the correct beam theory. But final structural design must be signed off by a qualified engineer against your local code (Eurocode 5, IS 883, or national annex), accounting for load duration, notches, holes, bearing and lateral support that a quick calculator cannot fully capture.
ResourcesView the formulasFormula Library
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