How it works
The calculator spaces the joists along the deck length, counts the boards you need for joists, rim joists and the ledger, and multiplies the joists by the hung ends to get the hangers.
Joists = ⌈ Deck length ÷ Spacing ⌉ + 1
Total joist length = Joists × Joist length
Joist boards = ⌈ ⌈ Joists ÷ ⌊ Board length ÷ Joist length ⌋ ⌉ × (1 + Extra ÷ 100) ⌉
Rim joist length = Rim rows × Deck length (1 on a ledger deck, 2 freestanding)
Joist hangers = Joists × Hung ends per joist
⌈ ⌉ means rounding up and ⌊ ⌋ rounding down. A deck needs one more joist than it has spaces, because there is a joist at both ends of the run. Rim and ledger boards are counted the same way as wall plates: a run no longer than a board comes from one board, and a longer run takes whole boards plus one cut piece.
Worked example in words
A 16 ft deck on a ledger with joists 16 in on center has 192 ÷ 16 = 12 spaces, so 13 joists. At 12 ft each that is 156 ft of joists, or 13 boards 12 ft long. The ledger and the rim joist are 16 ft each, so two 16 ft boards. If every joist hangs from the ledger, that is 13 joist hangers.
Spans are not checked
This page counts framing; it does not design it. The American Wood Council’s DCA 6, a prescriptive deck guide based on the IRC, lists maximum joist spans by species, size and spacing in Table 2 – at 12, 16 or 24 in on center – under stated loads and wet service, and limits joist length to 18 ft. Your local code may differ, and decks outside a prescriptive guide need a design. Choose joist size and spacing from those sources, then enter them here.
Hangers, rims and ledgers
DCA 6 attaches joists to a ledger or the side of a beam with joist hangers, chosen from an approved manufacturer’s product data for the joist size, at least 60 % as deep as the ledger or beam, and corrosion resistant. It calls for a continuous rim joist across the ends of the joists. A deck attached to a house hangs on a ledger board that must be fastened and flashed as the guide describes; a freestanding deck has its own lateral requirements. The calculator counts boards and hangers only – fasteners, flashing, blocking, beams, posts and footings are separate.
Standard US lumber lengths run from 8 to 20 ft in two-foot steps for Southern Pine (Southern Forest Products Association), so the filled-in board lengths are 12 ft for joists and 16 ft for rims. In metric no board length is filled in – enter the lengths your supplier stocks.
Worked example
Inputs
- Deck length (along the house)
- 16 ft
- Joist length
- 12 ft
- Joist spacing (on center)
- 16 in
- Deck type
- Attached to the house with a ledger
- Joist hangers
- At one end of each joist (e.g. the ledger)
- Joist board length
- 12 ft
- Rim and ledger board length
- 16 ft
- Extra lumber
- 0 %
Result
- Joists
- 13
- Total joist length
- 156 ft
- Joist boards
- 13
- Rim joist length
- 16 ft
- Ledger length
- 16 ft
- Rim and ledger boards
- 2
- Joist hangers
- 13
Common mistakes
- Forgetting the extra joist at the end of the run. A deck needs one more joist than it has spaces.
- Choosing joist spacing to save lumber. Spacing, joist size and span go together in span tables – changing one changes what the others may be.
- Using the deck’s width as the joist length on a deck with an overhang. Measure the joist as it will be cut, including any overhang past the beam.
- Counting hangers only for the middle joists. Every joist end that hangs from a ledger, beam or rim needs a hanger, including the outer joists unless the plans show another connection.
- Splicing joists to use shorter boards. A prescriptive deck uses full-length joists; buy boards at least as long as each joist.
Assumptions and limits
- Joists are laid out along the deck length at the spacing you enter, with a joist at both ends; the number of spaces is rounded up.
- Each joist is cut from one board. Two or more joists come out of one board only when they fit in its length.
- A ledger deck has one ledger and one rim joist along the deck length; a freestanding deck has a rim joist at both ends of the joists. Rim and ledger boards longer than the stock are pieced together, reusing offcuts where they fit.
- Hangers are joists × the number of hung ends you choose. Hanger models, nails and screws are not chosen.
- This is a lumber count, not a structural design. It does not size joists, beams, posts or footings or check any connection.
Frequently asked questions
How many joists do I need for a deck?
Divide the deck length by the joist spacing, round up and add one. A 16 ft deck with joists 16 in on center has 192 ÷ 16 = 12 spaces, so 13 joists. The spacing itself must come from your plans or a span table.
How far can deck joists span?
It depends on joist size, species, grade and spacing. AWC’s DCA 6 Table 2 lists maximum spans for 2x6 to 2x12 joists at 12, 16 and 24 in on center under its stated loads and assumptions, and limits joist length to 18 ft. Check your local code; this calculator does not check spans.
How many joist hangers do I need?
One for every joist end that hangs from a ledger, beam or rim. On a typical ledger deck where joists hang from the ledger and sit on a beam, that is one hanger per joist – 13 hangers for 13 joists.
What is a rim joist?
The board fastened across the ends of the joists. DCA 6 calls for a continuous rim joist at the joist ends. A deck on a ledger has a rim at the outer end; a freestanding deck has one at both ends.
Does this include beams and posts?
No. Beams, posts and footings depend on the joist span and loads and must come from span tables, local code or an engineer. Use the post hole and concrete column calculators once the footing size is known.
Sources
- DCA 6 – Prescriptive Residential Wood Deck Construction Guide (2015 IRC) — American Wood Council. Table 2: joist spans by size, species and spacing (12, 16, 24 in on center); joists hang from the ledger or beam on hangers chosen from the manufacturer’s product data, at least 60 % of the ledger or beam depth; a continuous rim joist at the joist ends; joist length limited to 18 ft.
- Specifying Southern Pine Lumber – Specification Guidelines — Southern Forest Products Association. Dimension lumber is specified in standard lengths from 8 to 20 ft in two-foot increments.
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 4, 2026