A deck is a simple structure to describe — a frame of joists covered by boards — but it adds up to a long shopping list. This calculator takes the deck’s size, joist spacing and the boards you plan to use and works out the deck boards, joists, rim and ledger, hangers and fasteners. It assumes the common layout: a rectangular deck attached to the house, joists running out from the ledger, and deck boards running parallel to the house.
How to use the deck calculator
- Choose US or Metric units.
- Enter the deck length along the house and the depth out from the house.
- Pick the joist spacing — 16 in on center unless your decking calls for less.
- Enter the actual board width and the gap between boards.
- Choose the board length you will buy.
- Set the extra percentage for culls and miscuts and the screws per joist.
Deck formulas
When a row is shorter than one board, whole rows are cut from each board. When rows are longer than a board, the joints are staggered and offcuts reused, so boards are counted by total length.
Worked example: a 16 × 12 ft deck
Joists at 16 in: ⌈ 192 ÷ 16 ⌉ + 1 = 13 joists, each 12 ft long, plus a 16 ft rim and a 16 ft ledger.
Board rows: ⌈ (144 + 0.125) ÷ 5.625 ⌉ = 26 rows of 16 ft.
Using 16 ft boards: 26 boards; with 10% extra → 29 boards.
Screws: 26 × 13 × 2 = 676, so buy about 750. Joist hangers: 13.
If the same deck were 20 feet long and you bought 12-foot boards, each row would need a joint. The 26 rows total 520 feet, or 44 boards before waste and 49 with it.
Choosing decking and spacing
| Decking | Actual width | Typical joist spacing |
|---|---|---|
| 5/4 × 6 treated or cedar | 5½ in | 16 in on center |
| 2 × 6 treated | 5½ in | 16–24 in on center |
| Composite or PVC | about 5½ in | 16 in (12 in diagonal) |
| Hardwood (ipe, etc.) | 3½–5½ in | 16 in on center |
Board width is the actual measured width, not the name: a “2 × 6” is 1½ × 5½ inches. Diagonal decking needs closer joist spacing and adds about 5–10% more waste.
Planning the frame
This calculator counts framing members but does not size them. Joist size depends on the span, the spacing, the lumber species and grade, and the loads in your area; beams, posts and footings depend on where you put them. Many jurisdictions follow the deck provisions of the International Residential Code (section R507), which include span tables and the rules for attaching a ledger to a house. Check those tables, or the prescriptive deck guide your building department hands out, before buying framing lumber, and get a permit where one is required.
A few practical points:
- Ledger flashing keeps water out of the house rim joist; it is one of the most important details on any attached deck.
- Butt joints in decking must land on a joist. Many builders add a second joist at each joint line so both board ends get full bearing.
- Fasteners must be rated for treated lumber or the decking maker’s system.
For stairs off the deck, the stair calculator works out risers, treads and stringers. Concrete for post footings comes from the concrete calculator, and the board foot calculator helps compare lumber prices quoted per board foot.
Frequently asked questions
How many deck boards do I need for a 12 × 16 deck?
With boards running parallel to the 16-foot side, 5½-inch boards with ⅛-inch gaps need 26 rows across the 12-foot depth. Each row is one 16-foot board, so 26 boards plus 10% extra means buying 29.
What joist spacing should I use for a deck?
Sixteen inches on center is the most common spacing and suits most wood and composite decking laid square to the joists. Many composite and PVC boards require 12 inches when laid diagonally, and some thinner boards always need 12. Use the maximum span printed in your decking's installation guide.
How many screws do I need for a deck?
Two screws per board at every joist is standard. On joists 16 inches apart, that works out to about 3½ screws per square foot of deck, so a 12 × 16 ft deck takes roughly 700 screws. Hidden clip systems use one clip per joist crossing instead.
How big a gap should I leave between deck boards?
Kiln-dried wood and most composites are spaced about ⅛ inch apart, though some composite makers ask for up to ¼ inch. Wet pressure-treated boards are often butted together because they shrink as they dry and open their own gaps.
Do I need joist hangers on a deck?
Joists that frame into a ledger or a flush beam must be supported by hangers or another approved connection; they are not just nailed through the face of the ledger. Use hangers rated for exterior, treated-lumber use, with the fasteners the hanger maker specifies.