Find the maximum allowable span for floor and ceiling joists from the joist size, spacing and timber grade.
The result is the maximum clear span for a typical domestic floor. These figures are indicative simplifications of published span tables. Always confirm the final span against the official span tables (TRADA / Eurocode 5 in the UK, IRC in the US) and your local building control before building.
The span is the clear distance a joist bridges between its supports (walls or beams). Beyond the maximum span the joist will sag or fail, so a deeper or more closely spaced joist is needed.
Yes. Moving from 600 mm to 400 mm centres (or 24" to 16") shares the load across more joists, allowing a slightly longer span or a smaller section for the same span.
These are UK strength grades for softwood. C24 is stronger and stiffer than C16, so it can span a little further for the same size – the calculator adjusts for this.
They are indicative only. Building control and structural designers work from full span tables that account for load, deflection and notching. Always verify your final design against the official tables.
The Joist calculator works out how many joists and hangers you need for a given area. This Joist Span calculator tells you how far a given joist size can safely span.
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