Deck Joist Calculator

A deck joist calculator that counts joists and checks the size against the AWC DCA 6 exterior span tables, then sizes the beam, cantilever, hangers and ledger bolts.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To size deck joists, read the allowable span for your size, species and spacing from AWC DCA 6 Table 2, then divide the deck length by the spacing and add one for the count. A 16 ft long deck spanning 12 ft from ledger to beam, framed 16 in on center in southern pine #2, takes 13 joists of 2x10, which DCA 6 allows out to 14 ft, on a triple 2x10 beam with posts 8 ft apart.

Your measurements

Deck from above: boards run the length, joists run the width beneath them at a fixed spacing.

Deck length along the ledgerftThe direction the joists are spaced along. Lay the first joist out from one end so the decking and any hidden clip track land where you expect.Joist span, ledger to beamftDistance from the house ledger to the centerline of the beam. This is the span DCA 6 Table 2 checks, and it does not include any overhang past the beam.Joist spacing16 in suits 5/4 decking and every composite. 12 in is required for boards run at 45 degrees. 24 in works only with 2x nominal decking.Species and gradeDCA 6 groups Douglas fir-larch, hem-fir and spruce-pine-fir into one column because incising for treatment is assumed. Read the grade stamp at the yard.Joist sizeThe size you plan to buy. The result checks it against DCA 6 Table 2 and names the smallest size that carries your span if it comes up short.Cantilever past the beamftOverhang beyond the beam centerline. DCA 6 caps it at the tabulated overhang or one quarter of the joist span behind it, whichever is less.Beam post spacingftCenter to center along the beam. Wider posts mean a deeper or thicker beam and a bigger footing, so 6 to 8 ft is usually the cheapest combination.

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Results

Joists: 13 joists of 2x10; Southern pine #2 at 16 in on center spans up to 14 ft (DCA 6 Table 2, 40 psf live, 10 psf dead, wet service). 16 ft of deck divided by the spacing plus one
Cantilever allowed: 3 ft; The lesser of the Table 2 overhang for 2x10 at this spacing, 3 ft 5 in, and one quarter of the 12 ft backspan
Beam: triple 2x10; 3 posts at 8 ft on center carrying a 12 ft joist span. DCA 6 requires 6x6 nominal posts, notched or capped, and every 3-ply beam gets a post cap
Joist hangers and blocking: 13 hangers; Sized for 2x10 at 600 lb (2.7 kN) minimum vertical capacity (DCA 6), with a 10d common or 1.5 in N10 in every hole. Plus 12 blocks of full-depth blocking over the beam where the joists overhang
Ledger fasteners: 14 lag screws; Half-inch lags in two staggered rows at 15 in on center for a 12 ft joist span (DCA 6 Table 5). Ledger is 2x8 nominal minimum and no deeper than the house band joist
Planning estimate: Verify on site; DCA 6 is prescriptive and stops at an 18 ft joist span. Anything outside it, or a hot tub, planter or roof on the deck, needs an engineered design. The local code and the inspector govern

Deck Joist formula

Joists = Deck length (in) / spacing, rounded up, + 1
Joist size = smallest size whose DCA 6 Table 2 span >= ledger-to-beam distance
Cantilever = lesser of the Table 2 overhang and joist span / 4
Ledger lags = Deck length (in) / Table 5 spacing, rounded up, + 1

Worked example

A 20 ft long deck spanning 14 ft from the ledger to the beam, framed 16 in on center in spruce-pine-fir #2 with 2x10 joists. DCA 6 Table 2 gives that group 13 ft 7 in at 16 in on center, which is 5 inches short, so the joists go to 2x12 at 15 ft 9 in. Joists: 240 / 16 = 15, plus 1 = 16.

The 2x12 overhang for that group at 16 in on center is 3 ft 11 in, and a quarter of the 14 ft backspan is 3 ft 6 in, so the cantilever is capped at 3 ft 6 in. With posts 8 ft apart under a 14 ft joist span, Table 3A calls for a triple 2x12, which carries 8 ft 10 in. Ledger lags space at 13 in for a 14 ft joist span, so 240 / 13 = 18.5, rounded up to 19, plus 1 gives 20 lag screws.

Quick reference

Maximum deck joist spans, No. 2 grade, 40 psf live and 10 psf dead load (AWC DCA 6-2015 Table 2)
Size	Spacing	Southern pine	DF-L, hem-fir, SPF
2x6	16 in	9 ft 0 in	8 ft 4 in
2x8	12 in	13 ft 1 in	12 ft 6 in
2x8	16 in	11 ft 10 in	11 ft 1 in
2x8	24 in	9 ft 8 in	9 ft 1 in
2x10	12 in	16 ft 2 in	15 ft 8 in
2x10	16 in	14 ft 0 in	13 ft 7 in
2x10	24 in	11 ft 5 in	11 ft 1 in
2x12	16 in	16 ft 6 in	15 ft 9 in
2x12	24 in	13 ft 6 in	12 ft 10 in

Why deck spans are shorter than floor spans

A deck joist carries the same 40 psf live load as a living room floor, but DCA 6 designs it for wet service and assumes incising on Douglas fir-larch, hem-fir and spruce-pine-fir. Those two adjustments take several inches off every number. A southern pine 2x8 at 16 inches on center spans 11 feet 10 inches outdoors and the same distance indoors, but the fir group drops to 11 feet 1 inch outdoors against 12 feet 9 inches inside.

Measure the span from the house ledger to the centerline of the beam. The overhang past the beam is a separate number and it does not count toward the span. That means a 13 foot wide deck can be framed as a 12 foot span with a 1 foot cantilever, which lands inside the 2x10 southern pine limit of 14 feet, instead of being pushed to 2x12 by measuring the full width.

Beam, posts and the ledger

The beam is sized from the joist span it carries and the distance between posts, not from the deck area. DCA 6 Table 3A pairs those two: at a 12 foot joist span, a triple 2x10 in southern pine reaches 8 feet 9 inches between posts, so 8 foot post spacing works and 10 foot does not. Posts are 6x6 nominal minimum, notched for the beam or fitted with an approved post cap, and every 3-ply beam gets a cap.

The ledger is the connection that fails when people get hurt. It is 2x8 nominal minimum, no deeper than the house band joist, and it bolts on with half-inch lag screws or through bolts in two staggered rows. The spacing comes from the joist span: 15 inches at 12 feet, 13 inches at 14 feet, 11 inches at 16 feet. Drill a half-inch clearance hole in the ledger and a 5/16 inch pilot in the band, and flash over the top edge with a drip edge before the decking goes down.

What this calculator leaves out

Footings. Size them from DCA 6 Table 4, which pairs beam span and joist span against a round diameter from 18 to 31 inches and a thickness from 7 to 13 inches, and they bear at least 12 inches below undisturbed grade and below the frost line. A footing within 5 feet of the house foundation has to bear at the house footing elevation. Also outside the count: the rim joist, hurricane clips at the beam, diagonal bracing on corner posts, lateral load connectors, stair framing and the decking itself.

It does not cover non-ledger decks, wood I-joists or metal plate connected truss floors at the house, brick or stone veneer, or attachment to a cantilevered bay. DCA 6 prohibits ledgers on all of those and asks for a freestanding deck or a full plan submission. It also stops at an 18 foot joist span and an 18 foot beam span, so anything longer, plus any hot tub, planter bed or roof, is an engineered design.

Code limits that apply

AWC DCA 6 Table 2checked
Southern pine 2x10 at 16 in OC spans 14 ft 0 in; DF-L, hem-fir and SPF reach 13 ft 7 in
If you miss it: A joist past the table bounces and gets tagged at framing inspection, and the fix is a mid-span beam with new footings
DCA 6 Table 2 note 3checked
Overhang is the lesser of the tabulated overhang and one quarter of the joist span
If you miss it: A deep overhang lifts the ledger end of the joist when someone stands on it and the decking springs at the house
AWC DCA 6 Table 3Achecked
Beam span set by joist span and ply; triple 2x10 southern pine reaches 8 ft 9 in at 12 ft
If you miss it: A doubled beam on 10 ft posts sags at mid-span and the deck drains toward the house instead of away
AWC DCA 6 Table 5
1/2 in lags or bolts in two staggered rows, 15 in on center at a 12 ft joist span
If you miss it: A ledger short of the bolt schedule pulls off the band joist under a crowd, which is the most common deck failure
IRC R507.9.1
Ledger 2x8 nominal min, no deeper than the band joist, with flashing and a drip edge
If you miss it: An unflashed ledger rots the house band joist from behind and nothing shows until the deck drops
AWC DCA 6 posts
Deck posts 6x6 nominal or larger, notched or capped, never bolted to the side of the post
If you miss it: A beam lag-bolted to an unnotched post relies on the bolts in shear and is rejected on sight

Provision	Limit	If you miss it
AWC DCA 6 Table 2checked	Southern pine 2x10 at 16 in OC spans 14 ft 0 in; DF-L, hem-fir and SPF reach 13 ft 7 in	A joist past the table bounces and gets tagged at framing inspection, and the fix is a mid-span beam with new footings
DCA 6 Table 2 note 3checked	Overhang is the lesser of the tabulated overhang and one quarter of the joist span	A deep overhang lifts the ledger end of the joist when someone stands on it and the decking springs at the house
AWC DCA 6 Table 3Achecked	Beam span set by joist span and ply; triple 2x10 southern pine reaches 8 ft 9 in at 12 ft	A doubled beam on 10 ft posts sags at mid-span and the deck drains toward the house instead of away
AWC DCA 6 Table 5	1/2 in lags or bolts in two staggered rows, 15 in on center at a 12 ft joist span	A ledger short of the bolt schedule pulls off the band joist under a crowd, which is the most common deck failure
IRC R507.9.1	Ledger 2x8 nominal min, no deeper than the band joist, with flashing and a drip edge	An unflashed ledger rots the house band joist from behind and nothing shows until the deck drops
AWC DCA 6 posts	Deck posts 6x6 nominal or larger, notched or capped, never bolted to the side of the post	A beam lag-bolted to an unnotched post relies on the bolts in shear and is rejected on sight

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

Using an interior floor joist table outdoors: DCA 6 assumes wet service and incising, which cuts several inches off every span.
The prescriptive guide stops at an 18 ft joist span and an 18 ft beam span; past that the deck needs a stamped design.
Lag screws take a 1/2 in pilot in the ledger and a 5/16 in pilot in the band; driving them with a hammer voids the connection.
Joists may not frame in from both sides of one beam under DCA 6, so a wide deck gets two beams or a full plan submission.
Every deck post is 6x6 nominal or larger under DCA 6; a 4x4 is only allowed as a guard post, not as a beam post.
Corner posts over 2 ft tall need diagonal bracing parallel to the beam, and DCA 6 prohibits bracing on center posts.

Key facts

AWC DCA 6 Table 2 allows a southern pine #2 deck joist of 2x10 to span 14 ft 0 in at 16 in on center, and a 2x8 to span 11 ft 10 in.
DCA 6 caps a joist overhang at the lesser of the tabulated overhang and one quarter of the joist span behind it.
DCA 6 Table 2 assumes a 40 psf live load, a 10 psf dead load, No. 2 grade, wet service conditions and L/360 deflection.
DCA 6 groups Douglas fir-larch, hem-fir and spruce-pine-fir in one span column because incising for preservative treatment is assumed.
DCA 6 sets minimum joist hanger vertical capacity at 400 lb for a 2x6, 500 lb for a 2x8, 600 lb for a 2x10 and 700 lb for a 2x12.

Frequently asked questions

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

At 16 in on center, a 12 ft span from ledger to beam needs 2x10 joists. DCA 6 Table 2 allows 14 ft 0 in for southern pine #2 and 13 ft 7 in for the Douglas fir-larch, hem-fir and spruce-pine-fir group. A 2x8 reaches only 11 ft 10 in and 11 ft 1 in.

How far can deck joists span 16 inches on center?

Under DCA 6 Table 2 with No. 2 grade: southern pine spans 9 ft 0 in as a 2x6, 11 ft 10 in as a 2x8, 14 ft 0 in as a 2x10 and 16 ft 6 in as a 2x12. Douglas fir-larch, hem-fir and spruce-pine-fir come in about 5 percent shorter.

How far can deck joists cantilever past the beam?

The lesser of the DCA 6 tabulated overhang and one quarter of the joist span behind it. A 2x10 southern pine at 16 in on center over a 12 ft span is limited to 3 ft, because a quarter of 12 ft is smaller than the 3 ft 5 in the table allows.

Can I use 24 inch joist spacing on a deck?

Only with 2x nominal decking. No 5/4 board and no mainstream composite is rated for 24 in on center, and boards run at 45 degrees need 12 in. Going to 24 in also cuts the joist span hard: southern pine 2x10 drops from 14 ft 0 in to 11 ft 5 in.

Sources and references

DCA 6-2015 Prescriptive Residential Wood Deck Construction Guide, Tables 2, 3A and 5 (American Wood Council)
Chapter 5 Floors, Section R507 Decks, 2021 IRC (ICC)
2024 Span Tables for Joists and Rafters (American Wood Council)

Next steps for this project

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