Counting joists is easy and it is the least important part. What decides whether the floor works is the joist size for the span — and that is a span table and a building inspector, not a web page. This does the count and points you at the rest.
Floor and ceiling joists. Enter the length of the wall the joists sit on and the spacing — not the span they cross.
Works in metric and US imperial — you choose at the start and can switch at any time. Every figure this calculator uses is published with its source, and the result comes with a printable order sheet. No account, no email, and nothing you type leaves your browser.
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Enter the length of the wall the joists sit on and the spacing — not the span they cross. The count follows the same positions-not-divisions rule as any run. What actually decides whether the floor works is the joist size for the span, and that comes from a span table and a building inspector rather than from a calculator.
Sight along each joist and fit it with the bow upward. Loaded, it settles towards flat; fitted the other way the dip is permanent and visible in the finished floor. Anything over about 2.5 m of span wants a row of herringbone strutting or solid blocking at mid-span to stop the joists twisting under load — it is cheap, quick, and the difference between a solid floor and a bouncy one.
Notching and drilling rules are not advisory. For solid timber joists, notches go only in the top edge within the outer quarters of the span, and holes have their own permitted zone. Joist hangers must be the right hanger for the joist size and fully nailed with the specified nails — not screws, not partly filled. And plan the services before the joists go in: deciding a pipe route afterwards is how joists end up notched in the middle of the span, which is exactly where they must not be.
That comes from a span table and your building inspector, not from a quantity calculator. Joist size depends on span, spacing, grade of timber and the load above. This page counts how many you need once the size has been decided.
Crown up — sight along the edge and fit it with the bow upward. Under load it settles towards flat. Fitted the other way, the dip becomes permanent and shows in the finished floor.
Notches only in the top edge and only within the outer quarters of the span; holes have their own permitted zone along the neutral axis. These are rules rather than guidance, and notching mid-span is a common and serious mistake.
Over about 2.5 m of span, yes. A row of herringbone strutting or solid blocking at mid-span stops the joists twisting under load, and it is the cheapest thing you can do to stop a floor feeling bouncy.