How to use it
- Enter the deck length along the house and the depth out from the house. Add inches if you have them.
- Pick attached (a ledger board fastened to the house) or freestanding (a second beam near the house carries the joists).
- Pick the joist spacing, decking and board length. Boards run parallel to the house. The gap fills in for the material; match it to your board's instructions.
- Set the beam, post and footing numbers from your plans or your building department's deck guide.
- Turn on railing if you need it, then type prices in the cost section.
Results update as you type. Copy result gives you a plain-text material list, and the page address saves your numbers so you can bookmark the job.
The formula
Rows of boards = Depth in inches ÷ (Board width + Gap), rounded up
Boards = Rows ÷ rows each board covers, then × (1 + Waste % ÷ 100), rounded up
Joists = Length in inches ÷ Joist spacing, rounded up, + 1
Posts per beam = Length ÷ Post spacing, rounded up, + 1
Concrete per footing = 3.1416 × (Diameter ÷ 24)² × (Depth ÷ 12) cu ft
Screws = Rows × Joists × Screws per crossing × (1 + Waste %)
A board at least twice the deck length covers two rows. When the deck is longer than a board, the count is rows × length ÷ board length, assuming staggered joints and reused drops. Joists include both end joists. An attached deck gets a ledger plus an outer rim; a freestanding deck gets a rim at each side. Beam footage is beams × boards per beam × length. Waste applies only to deck boards and screws, because framing is counted piece by piece.
Worked example: a 14 ft × 12 ft attached deck
- Area: 14 × 12 = 168 sq ft
- Boards: 5/4 × 6 wood at 5.5 in plus a 1/8 in gap. 144 ÷ 5.625 = 25.6, so 26 rows. Each 16 ft board covers one row, and 26 × 1.10 = 28.6, so 29 boards (464 ft)
- Joists at 16 in: 168 ÷ 16 = 10.5, rounded up to 11, plus 1 = 12 joists, each 12 ft, and 12 hangers at the ledger
- Ledger and rim: 14 + 14 = 28 ft. Beam: 1 beam × 2 boards × 14 ft = 28 ft
- Posts at 6 ft: 14 ÷ 6 = 2.33, rounded up to 3, plus 1 = 4 posts and 4 footings
- Footings 12 in wide and 36 in deep: 2.36 cu ft each, 9.42 cu ft in all. 9.42 ÷ 0.60 = 15.7, so 16 bags of 80 lb
- Screws: 26 × 12 × 2 = 624, and with 10% waste 687 screws, or 2 boxes of 350
- Railing on the three open sides: 14 + 12 + 12 = 38 ft
Built freestanding, the same deck needs 2 beams (56 ft of beam lumber), 8 posts, 8 footings and 32 bags, and no ledger hangers.
A takeoff, not a structural design
This calculator counts material. It doesn't size anything. Joist and beam sizes and spans, post size, and footing size and depth come from your local code, the span tables in the residential building code or a set of plans. Many building departments hand out a deck guide with those tables, and most require a permit.
The ledger deserves the most care on an attached deck. It has to be bolted or screwed to the house framing the way your code spells out, never just nailed, and flashed so water can't get behind it. A leaky ledger rots the house framing it hangs on. Some walls, such as brick veneer, can't carry a ledger at all, and a freestanding deck is the answer there.
Wood or composite
Treated wood costs less up front, but it needs cleaning and sealing, and wet boards shrink as they dry. Composite costs more per board and needs less upkeep. Follow its maker's rules for joist spacing, gaps and fasteners, because the warranty usually depends on them. Most composite and 5/4 wood boards are rated for joists 16 in apart or closer, so the calculator warns you at 24 in. The framing under a composite deck is still treated wood.
Common mistakes
- Buying boards exactly as long as the deck. A 16 ft board on a 16 ft deck leaves nothing to trim. Go up a length if the ends overhang the rim.
- Driving deck screws into joist hangers. Hangers need the nails or connector screws their maker lists.
- Guessing at frost depth. Footings that sit too shallow can heave. Your building department knows the number.
- Forgetting the extras. Stairs, ledger bolts, flashing and ties aren't counted here.
Questions people ask
How many deck boards do I need for a 12 × 12 deck?
With 5.5 in boards and a 1/8 in gap, a 12 ft depth takes 26 rows. With 10% waste, that's 29 boards, one per row. Use 12 ft boards only if the ends won't overhang the rim; otherwise buy 16 ft and trim. Enter your own size above for an exact count.
How far apart should deck joists be?
Sixteen inches on center is the most common. Composite makers often call for 12 in when boards run on a diagonal. Two-inch-thick wood boards can span 24 in in many cases. Your board's instructions and local code decide.
How deep do deck footings go?
The residential code calls for at least 12 in below undisturbed ground, and below the frost line where frost applies. That ranges from almost nothing in warm climates to several feet in cold ones, so ask your building department.
How many screws per square foot?
In the worked example, 624 screws cover 168 sq ft, about 3.7 per square foot before waste. Closer joists push that up.
Why are hangers missing on a freestanding deck?
Here the joists sit on top of the beams, so there's no ledger to hang them from. They still need ties to the beam, which your plans or code will spell out.