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Deck joist spacing calculator
Pick the country and the joist centres follow its practice: 16 in in the US and Canada, 400 mm in the UK and Ireland. Enter the deck size and you get the joist count, the span checked against the section, and the beam and posts when the run is too long. Free, no sign-up, no invented prices.
The rule in three lines
1. Take the centres from the country
16 in on centre under IRC / DCA 6 practice; 400 mm on centre under BS 8579 practice. Close them up for composite boards or a diagonal board layout.
2. Divide the length, round up, add one
The extra joist is the one that closes the last bay. Missing it is the most common reason a hand count comes out one short.
3. Check the span, not just the count
A joist has a maximum run for its section. Past it you either go deeper or drop in a beam with posts and footings — both change the list.
The same deck, two countries
- Deck
- 16 ft x 12 ft
- US / Canada — centres
- 16 in
- US / Canada — joists
- 13
- UK / Ireland — centres
- 400 mm
- UK / Ireland — joists
- 14
- Beam and posts
- Added when the joist run passes its span
Common questions
What spacing should deck joists be at?
16 in on centre is standard practice in the US and Canada; 400 mm on centre is standard in the UK and Ireland. Composite boards and boards laid at 45 degrees usually need closer centres, because the board itself spans less.
How many joists do I need?
Deck length divided by the joist centres, rounded up, plus one for the closing joist. A 16 ft deck at 16 in centres comes out at 13 joists; the same deck at 400 mm centres comes out at 14.
How far can a joist span?
It depends on the section, the timber and the centres. A 2x8 southern pine at 16 in centres runs to roughly 13 ft; a 47 x 150 mm joist at 400 mm centres runs to roughly 3.0 m. When your deck is longer than that, the answer is a beam and posts in the middle, or a deeper joist.
What happens if I go over the span?
The check tab flags it and names the next section up that carries the measurement, rather than just telling you it failed. You can then add an intermediate beam instead, and the posts and footings update with it.
Is this a structural design?
No. It is a take-off with the spans checked against published practice so you can see them. Ground conditions, loads, ledger fixing and building control are still yours to satisfy.
See the frame, not just a number
The designer draws the joists, beams, posts and footings in their real positions, and the list beside them tells you what to buy by length.
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