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Carpentry · joists · sheets
How many floor joists do I need?
Joist count, noggin rows and floor sheets from the room size, with a rule-of-thumb span check that tells you when you are past what C16 timber will do.
1
Measure it
Every box has its own unit next to it.
Change the unit beside any box and the number converts. The (i) beside each question says what the box wants and where the figure usually comes from. Anything marked per cent is a percentage of the quantity above it.
2
Check the rates
Rates and assumptions used
These are the figures the calculator works to. Change any of them and the order list updates — your merchant’s pack sizes and your own waste allowances beat the defaults every time.
How this one works
Joists = floor width ÷ centres + 1
Joist length = clear span + a bearing each end
Noggin rows = one every 2.5 m of span
Span check is a rule of thumb for C16: depth (mm) × 0.019, adjusted for centres. It is not a span table
The figures the calculator works to. Every one can be changed in step 2, and your merchant’s pack sizes and your own waste allowances beat these defaults every time.
Rate
Default
Why
Floor sheet
2.88 m²
2400 × 1200 T&G chipboard or ply.
Noggin row every
2.5 m
Strutting to stop the joists twisting.
Hangers per joist
2 no.
Only if they are hung rather than built in.
Restraint straps every
2 m
To external walls.
Span factor
0.019
Rule-of-thumb metres of span per mm of joist depth, C16.
A worked example
With the defaults the page opens on:
How far do the joists span? 3.8 metres
How wide is the floor? 4.6 metres
What centres? 400 mm
Which joist size? 47 × 145
How much bearing at each end? 100 millimetres
Spare for cuts 7.5 per cent
Joists
Joists: 13 no.— 4.6 m ÷ 400 mm + 1
Length each: 4 m — span + 100 mm bearing each end
Joist timber: 52 m
Joist hangers: 26 no. — if hung rather than built in
Restraint straps: 3 no. — at 2 m centres
Noggins & decking
Noggin rows: 1 rows — one every 2.5 m of span
Noggin timber: 4.6 m
Floor area: 17.48 m²
Sheet waste: 7.5 %
Floor sheets: 7 sheets— at 2.88 m² a sheet
Span check
47x145 at 400 mm: 2.89 m— rule-of-thumb maximum for C16
Change any figure above and every line recalculates; the small print under each row is the sum that produced it.
Questions people ask
How many joists do I need?
Floor width ÷ centres + 1. A 4.6 m wide floor at 400 mm centres needs 13 joists, each the clear span plus a bearing at each end.
Is the span check a span table?
No. It is a rule of thumb for C16 timber - depth in mm × 0.019, adjusted for centres - and the page says so on the row. If your span is past it, the page tells you to get a span table or an engineer. If it is inside, it still tells you to check a proper table before you order.
How many noggin rows?
One row of strutting every 2.5 m of span, to stop the joists twisting.
Read before you order. These work out quantities, not structural design — spans, lintels, rebar and foundation depths need an engineer and Building Control sign-off. Waste allowances are defaults, not promises: awkward cuts, long runs and bad weather all eat more than 5%. Where a merchant sells whole packs, the figure is rounded up to whole packs.
