Estimate joist count
This estimates joist lines only. It does not include noggings, trimming, double joists, blocking, waste or fixing requirements.
SpanFinderAustralian span tools
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Deck Joist Spacing Calculator
Estimate joist count and understand deck joist spacing before checking a joist span table or selecting a product in the live span finder.
450Covers common joist spacing terms such as 300 mm, 400 mm, 450 mm and 600 mm centres.
Estimated joist lines
9 joists
At 450 mm centres across 3600 mm, allow roughly 9 joist lines including the outside joist.
Check decking manufacturer requirements and the joist span table for the actual product and load case.
Related calculators and guides
Deck joist spacing FAQs
What is deck joist spacing?
Deck joist spacing is the centre-to-centre distance between adjacent joists under decking boards.
Is 450 mm joist spacing standard?
450 mm centres is common for many deck layouts, but the correct spacing depends on the decking board, joist product and span-table requirements.
How many joists do I need?
For preliminary estimating, divide the deck width by the joist spacing and add one outside joist. Allow for trimming, double joists and perimeter framing separately.
Does joist spacing affect span?
Yes. Closer spacing usually means each joist carries less load, but the correct maximum span still comes from the relevant span table.
Is this a material takeoff?
No. It is a quick spacing calculator only and does not include waste, blocking, trimming, fixing requirements or detailed framing rules.
SpanFinder toolkit
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Move between the live finder, load-width calculators and plain-English guide pages without losing the flow of the job.
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Calculation pathway
A cleaner way to work through a span check
Choose the member
Decide whether the check is for a joist, bearer, beam or related support member.
Confirm load width
Use FLW or spacing inputs to describe how much area the member is carrying.
Compare suitable spans
Use the live finder and guide pages to compare preliminary member options.
Verify final design
Confirm product data, site conditions, manufacturer requirements and engineering where required.