Deck joist calculator
This tool counts and lays out joists. It does not size them — and that distinction matters, because a joist that is the right number and the wrong depth is a failed deck.
Your framing
How this is calculated
- Joist count = ⌊(span direction in inches) ÷ spacing⌋ + 1. The +1 is the far end joist.
- Actual spacing = (span in inches) ÷ (count − 1). This is the number to mark on your rim, because the nominal spacing rarely divides evenly.
- Linear feet = count × joist length.
- Blocking = one row every 4 ft of span, across every joist bay, for lateral stability.
Worked example
A 16 × 12 ft deck, joists running the 12 ft direction at 16″ o.c.:
- Count = ⌊(192) ÷ 16⌋ + 1 = 12 + 1 = 13 joists
- Actual spacing = 192 ÷ 12 = 16.0″ — divides evenly, which is unusual
- Linear feet = 13 × 12 = 156 lf
- Rim = 2(16) + 2(12) = 56 lf
Try the same deck at 14 ft and the arithmetic stops being tidy: ⌊168 ÷ 16⌋ + 1 = 11 joists, actual spacing 16.8″. Still inside a 16″ nominal layout, but you must mark to 16.8″, not 16″ — mark every 16″ and the last bay comes up short.
Why sizing is not in this tool
Joist depth is a function of five things: species and grade, nominal size, on-centre spacing, live load (40 psf for a residential deck, 50–60 for a deck used as a workshop or with a hot tub), and the deflection limit (L/360 for most decks, L/720 for tile or stone). Change the species from SPF to Douglas Fir and the allowable span moves by more than a foot. No single table covers it, and a table that pretends to is worse than no table.
What you need is the span table from your lumber supplier or the AWC's Wood Construction Data No. 1, read for your species, grade and spacing. This tool gives you the count and the layout so that when you open that table you already know what you are looking up.
What this calculator will not approve
- It does not approve a span. See above. Undersized joists bounce, and a bouncy deck is a structural warning, not an aesthetic one.
- It does not size beams, posts or the ledger. Those carry the joist loads and are a separate calculation.
- It does not handle cantilevers. A cantilever is limited to 1/4 of the adjacent span and needs its own check.
- It does not account for stairs, hot tubs or heavy equipment. Point loads change everything and need an engineer.
Questions people actually ask
How many joists do I need for a 16×12 deck?
Thirteen at 16″ on centre, running the 12 ft direction: eleven interior bays plus both end joists. That is 156 linear feet of joist material, before rim joists.
What spacing should deck joists be?
It is set by the decking, not by preference. Composite decking typically requires 16″ or closer; some diagonal layouts require 12″; 5/4×6 pressure-treated wood is usually fine at 16″. Check the decking manufacturer — closer spacing than required is wasted money, wider voids the warranty.
Can I use 2×6 joists for a deck?
Only for short spans. A 2×6 at 16″ on centre spans considerably less than a 2×8, and the limit depends on species and load. Look it up in a span table for your exact combination rather than assuming.
Why does the actual spacing differ from what I entered?
Because the joist count is rounded to a whole number. On a span that does not divide evenly by the spacing, the real gap between joists is slightly wider or narrower. The tool recalculates and shows you the number to mark.
Do I need blocking between joists?
It is good practice on spans over 8 ft and required by some inspectors. One row every 4 ft keeps the joists from twisting and stiffens the floor. It is cheap insurance — a few short offcuts.