Joist Span Calculator
Depth cubed. Width barely matters.
Work out Joist Span. Depth cubed. Width barely matters. States the assumption instead of hiding it.
47 × 200 mm at 400 mm centres
Within span
3.5 m against an indicative maximum of 4.16 m
Joist depth matters far more than width, because bending stiffness goes as the cube of the depth. Doubling the depth makes a joist eight times stiffer; doubling the width only doubles it, which is why floor joists are tall and thin rather than square. Deflection usually governs before strength does. A floor can be perfectly safe and still feel unacceptably bouncy, and most span tables are set by a deflection limit — commonly span over 360 — rather than by anything breaking. Notching and drilling change the answer substantially. Notches are only permitted in specific zones near the supports and to limited depths, and a notch in the middle of a span can halve the joist's capacity.
How the Joist Span Calculator works
Indicative maximum spans for C16 softwood floor joists at domestic loading, with the joist count for a floor width. These are conservative table figures rather than a structural calculation.
Also known as: how far can a joist span · what size joists for a 4m span · floor joist span table · can i notch a joist
Frequently asked questions
How far can a floor joist span?
A 47 × 200 mm C16 joist at 400 mm centres spans about 4.1 m at domestic loading. Deeper joists span further, and wider spacing reduces the span.
Why does depth matter so much more than width?
Bending stiffness goes as the cube of the depth and only linearly with width. Doubling the depth makes a joist eight times stiffer; doubling the width only doubles it.
What actually limits the span?
Deflection, usually, rather than strength. A floor can be entirely safe and still feel unacceptably bouncy, and most span tables are set by a deflection limit of span over 360.
Can I notch or drill joists?
Only within specific zones and depths near the supports. A notch in the middle of a span can halve the joist's capacity, and it is the commonest way a compliant floor becomes non-compliant.
Are these figures safe to build to?
Treat them as indicative. Real capacity depends on species, strength class, moisture content, imposed load and whether there is a ceiling below — use the actual span tables or an engineer.
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