Joist Spacing Calculator
Depth beats width — stiffness goes with the cube.
Depth beats width — stiffness goes with the cube. Joists across a floor or deck at a given centre spacing, with the count, linear metres and cost.
Set by the span table for your load, timber grade and section
Across 4 m at 400 mm centres
11 joists
10 gaps, so 11 members
This counts joists at a spacing you supply; it does not tell you what spacing is safe. That comes from a span table for your timber grade, section depth and the load the floor carries, and those tables are set by local building codes. Halving the spacing roughly halves the load on each joist, and increasing depth helps far more than increasing width — stiffness goes with the cube of depth.
How the Joist Spacing Calculator works
Joists across a floor or deck at a given centre spacing, with the count, linear metres and cost. This counts joists at a spacing you supply — what spacing is safe comes from a span table, not from arithmetic.
Also known as: how many joists do i need · floor joist count calculator · deck joist spacing calculator · joists at 400 centres
Counting joists and choosing spacing are different questions
This counts joists at a spacing you supply — gaps plus one, the same arithmetic as studs. What spacing is safe comes from a span table for the timber grade, joist section and the load carried, and those tables are set by building code.
The relationship is not intuitive. Halving the spacing roughly halves the load on each joist, which allows either a longer span or a smaller section. Which trade to make depends on headroom, cost and what depth the construction can accommodate.
Codes differ between jurisdictions in both the tables and the load assumptions, so a span figure taken from one country's table is not transferable to another. The table that applies where you are building is the one that matters.
Depth beats width by a large margin
Bending stiffness goes with the cube of depth and only linearly with width. A 47 × 220 mm joist is dramatically stiffer than a 94 × 110 mm one, despite having the same cross-sectional area and costing roughly the same.
Which is why joists are tall and thin rather than square. Material at the top and bottom of the section does almost all the work; material near the neutral axis in the middle contributes very little.
That also explains the rules about notching and drilling. Notches near the top or bottom remove exactly the material carrying the load; holes on the neutral axis remove material that was doing little. Codes permit the second within limits and restrict the first sharply.
Why a compliant floor can still feel wrong
Span tables are set by strength and a deflection limit (commonly span over 360) rather than by perceived comfort. A floor can pass both and still feel lively underfoot, particularly at longer spans.
The fix is closer spacing, deeper joists, or blocking and strutting between them. Blocking stops joists twisting and spreads a point load across several, which makes a measurable difference to how a floor responds to footfall.
Squeaks are a separate problem: movement between the subfloor and the joist rather than deflection. Screwing and gluing the subfloor down is the durable fix, and adding more nails to a squeaking floor almost never works.
Where to go next
The Joist Spacing question rarely arrives on its own. These are the ones that usually come with it:
- Stud Spacing Calculator — Twelve gaps need thirteen studs.
- Deck Board Calculator — The gap between boards is not optional.
- Rafter Length Calculator — The run is half the span less half the ridge.
- Ring Size Calculator — Finger measurement to US, UK, EU and Japanese sizes.
Frequently asked questions
How do I calculate the number of joists?
Divide the span by the spacing to get gaps and add one for the joist at the far end. A 4 m floor at 400 mm centres needs eleven joists.
What spacing should floor joists be?
Commonly 400 or 600 mm, and the correct figure comes from a span table for the timber grade, joist section and the load carried. It is set by building code rather than preference.
Does deeper or wider timber help more?
Deeper, by a wide margin. Bending stiffness goes with the cube of depth and only linearly with width, so a 47 × 220 joist is dramatically stiffer than a 94 × 110 of the same cross-sectional area.
Why does my floor feel bouncy if it passes the span table?
Span tables are usually set by strength and a deflection limit, not by perceived comfort. A floor can be entirely safe and still feel lively — closer spacing or deeper joists fixes it.
What is blocking or strutting between joists?
Cross-members that stop joists twisting and spread point loads between them. Most codes require it at intervals on longer spans, and it makes a noticeable difference to how a floor feels.
Can I notch a joist for pipes?
Only within limits set by code — typically shallow notches in defined zones near supports, and holes on the neutral axis. Notching in the wrong place removes exactly the material carrying the load.
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Related calculators
Stud Spacing Calculator
Twelve gaps need thirteen studs.
OpenDeck Board Calculator
The gap between boards is not optional.
OpenRafter Length Calculator
The run is half the span less half the ridge.
OpenRing Size Calculator
Finger measurement to US, UK, EU and Japanese sizes.
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