Floor Joist Calculator
Works out how many floor joists a floor needs and how long they are, with doublers and blocking. It deliberately stops short of the joist size, which comes from a span table for your timber and code.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
joists = floor(floor length ÷ spacing) + 1 + doublers joist length = span + 2 × bearing
The count depends only on the dimension across the joists and the spacing. The span sets their length — and, separately, their depth, which this page leaves to the span table because it depends on the grade, the load and the code edition.
What to watch out for
Count and size are different questions:
- Count is geometry — this page.
- Size is a span table. For a given span, spacing, timber grade and load, the table gives the minimum depth. The tables differ between countries and code editions, and they are the thing that keeps a floor from sagging.
Closer spacing lets you use a shallower joist for the same span. 300 mm centres with 150 mm joists can span what 400 mm centres needs 175 mm for — which matters when the floor depth is fixed.
Double the joists under any partition that runs parallel to them, and at the trimmers round a stairwell or chimney.
Frequently Asked Questions
How many floor joists do I need?
Divide the floor length (measured across the joists) by the spacing and add one. A 6 m floor at 400 mm centres needs 16, plus doublers under partitions and at openings.
What size joist do I need for a 4 m span?
That comes from a span table, not a calculator — it depends on the timber grade, the spacing and the loading in your building code. As a rough indication, C24 at 400 mm centres is often around 47 × 195 mm for that span, but check the table that applies to you.
Do I need blocking between joists?
Yes on any span over about 2.5 m — one row of solid blocking or herringbone strutting at mid-span, two rows over about 4.5 m. It stops the joists rolling and shares load between them.