Construction

Joist Count Calculator

Count floor or ceiling joists along a run at a given spacing, including both end joists.

Length of run (m)
Joist spacing (cm)

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This result is the number of joists to buy for the run, including one joist at each end.

Formula

Number of joists = (run length in centimetres divided by spacing in centimetres) plus one, rounded up, so both ends of the run have a joist.

How it works

Enter the length the joists must span across and their centre-to-centre spacing. The result includes a joist at each end. Joist size, grade and maximum span must still be checked against span tables or a professional design.

Worked example

For example, a run of 5 metres is 500 centimetres, so 500 ÷ 40 = 12.5, plus 1 gives 13.5, which rounds up to 14 joists.

FAQs

What spacing is common?
Four hundred millimetres is common for many floors, but the correct spacing depends on joist size, load and local practice.

Does this count blocking or noggins?
No, only the joists themselves. Blocking, rim boards and fixings are extra.

Can I stretch spacing to save money?
Only if the design allows it. Wider spacing can cause bouncy floors or overload, so check first.

Rough estimate only — consult a licensed professional for structural work before building.