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Deck joist span calculator

Pick the wood, joist size and spacing to get the longest span allowed. Enter the span you need to see which sizes work.

Measurements

Printed on the grade stamp

Span you need

Face of the ledger to face of the beam. Enter 0 to skip.

Results

Longest span allowed
14' 0"
Does your span work?
Yes. 12' 0" is within the limit.
Sizes that carry your span
2x8 at 12 in, 2x10 at 16 in or closer, 2x12 at 24 in or closer
Longest overhang past the beam
3' 0"
House12' 0"2x10 at 16 in on center: 14' 0" at most

How it works

This is a lookup, not a formula. The spans come from Table 2 of DCA 6, the deck guide published by the American Wood Council. The 2021 International Residential Code prints the same spans in Table R507.6 for a 40 psf live load.

Example: a deck 12 feet deep with one beam, so the joists span 12 feet. In Southern Pine at 16 inches on center, a 2x8 is allowed 11 ft 10 in. That is 2 inches short. A 2x10 is allowed 14 ft, so it works. A 2x8 also works if you move the joists to 12 inches on center, where it is allowed 13 ft 1 in. For the 2x10, the overhang table gives 3 ft 5 in, but a quarter of the 12 foot span is 3 feet, so 3 feet is the limit.

Deck joist span table

SpeciesJoist12 in on center16 in on center24 in on center
Southern Pine2x69 ft 11 in9 ft7 ft 7 in
2x813 ft 1 in11 ft 10 in9 ft 8 in
2x1016 ft 2 in14 ft11 ft 5 in
2x1218 ft16 ft 6 in13 ft 6 in
Douglas Fir-Larch, Hem-Fir or Spruce-Pine-Fir2x69 ft 6 in8 ft 4 in6 ft 10 in
2x812 ft 6 in11 ft 1 in9 ft 1 in
2x1015 ft 8 in13 ft 7 in11 ft 1 in
2x1218 ft15 ft 9 in12 ft 10 in
Redwood, Western Cedar, Ponderosa Pine or Red Pine2x68 ft 10 in8 ft6 ft 10 in
2x811 ft 8 in10 ft 7 in8 ft 8 in
2x1014 ft 11 in13 ft10 ft 7 in
2x1217 ft 5 in15 ft 1 in12 ft 4 in

Spans are the longest allowed, measured between supports.

Overhang table

This is how far a joist may run past the beam. The limit is the number below or a quarter of the joist span, whichever is less.

SpeciesJoist12 in on center16 in on center24 in on center
Southern Pine2x61 ft1 ft 1 in1 ft 3 in
2x81 ft 10 in2 ft2 ft 4 in
2x103 ft 1 in3 ft 5 in2 ft 10 in
2x124 ft 6 in4 ft 2 in3 ft 4 in
Douglas Fir-Larch, Hem-Fir or Spruce-Pine-Fir2x611 in1 ft1 ft 2 in
2x81 ft 8 in1 ft 10 in2 ft 2 in
2x102 ft 10 in3 ft 2 in2 ft 9 in
2x124 ft 4 in3 ft 11 in3 ft 3 in
Redwood, Western Cedar, Ponderosa Pine or Red Pine2x69 in10 in11 in
2x81 ft 5 in1 ft 7 in1 ft 9 in
2x102 ft 5 in2 ft 7 in2 ft 8 in
2x123 ft 7 in3 ft 9 in3 ft 1 in

What the table assumes

The guide does not cover decks with a snow load over 40 psf, or heavy point loads such as a hot tub. Snow shortens the spans. At a 70 psf ground snow load, the 2021 code cuts a Southern Pine 2x8 at 16 inches from 11 ft 10 in to 9 ft 10 in. Your building department can tell you which load applies.

Measuring the span

Span is the clear distance between supports, from the face of the ledger board on the house to the face of the beam. It does not include any overhang past the beam. If the joists sit on top of the beam and run past it, that extra length is the overhang, and it has its own limit. The code also wants each joist end to rest on at least 1.5 inches of wood or metal, or 3 inches of concrete or masonry.

Other limits to check

Decking sets the joist spacing too. Under the International Residential Code, 5/4 wood deck boards need joists no more than 16 inches on center, and 2 inch thick boards can span 24 inches. Composite decking follows the maker instructions.

A 2x6 can be a joist on a small deck, but the ledger board has to be at least a 2x8, and so does any joist or rim board that a guard post bolts to. The guide stops joist length at 18 feet, which is why two 2x12 entries read 18 ft. This page covers joists only. Beams, posts and footings have their own tables in the same guide.

Common questions

How far can a 2x8 deck joist span?

In Southern Pine, 13 ft 1 in at 12 inches on center, 11 ft 10 in at 16 inches and 9 ft 8 in at 24 inches. In Douglas Fir-Larch, Hem-Fir or Spruce-Pine-Fir it is 12 ft 6 in, 11 ft 1 in and 9 ft 1 in.

How far can a 2x10 deck joist span?

In Southern Pine, 16 ft 2 in at 12 inches on center, 14 ft at 16 inches and 11 ft 5 in at 24 inches. Other species span a little less.

How far can a 2x6 deck joist span?

In Southern Pine, 9 ft 11 in at 12 inches on center, 9 ft at 16 inches and 7 ft 7 in at 24 inches. The ledger board and any joist that holds a guard post still need to be 2x8 or larger.

What size joists do I need for a 12 foot span?

In Southern Pine, a 2x10 at 16 inches on center, a 2x8 at 12 inches or a 2x12 at 24 inches. The same three choices work in Douglas Fir-Larch, Hem-Fir and Spruce-Pine-Fir. In redwood or cedar a 2x8 falls short, so use a 2x10 at 16 inches or a 2x12 at 24 inches.

Can deck joists be 24 inches on center?

Yes, if the joist is big enough for the span and the decking can bridge 24 inches. Wood deck boards 2 inches thick can. Boards 5/4 thick need joists 16 inches on center or closer, and composite decking often does too.

How far can a deck joist overhang the beam?

No more than the value in the overhang table, and no more than a quarter of the joist span. For a Southern Pine 2x10 at 16 inches on center the table allows 3 ft 5 in, so a 12 foot span can overhang 3 feet.

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Sources

Facts on this page were last checked against these sources in October 2026.

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