Decks & Fencing
Deck Calculator
Work out the decking boards, joists and screws a rectangular deck needs, with an allowance for waste.
Calculate your decking
For one rectangular deck surface. Deck dimensions in feet; board width, gap and joist spacing in inches. This estimates materials from the measurements you enter — it doesn't size joists, beams or footings.
The method
- 01
Board rows
round up ((deck width + gap) ÷ (board width + gap))
Each row takes its own width plus one gap, except the last, which needs no gap after it. Rounds up: the final row is ripped narrower when it doesn't land flush, and it's still bought whole.
- 02
Decking run
board rows × deck length
The total linear feet of decking, before waste.
- 03
Add a waste allowance
decking run × (1 + waste allowance)
Covers offcuts, butt joints and the odd unusable board.
- 04
Boards to buy
round up (run with waste ÷ board stock length)
Divides the whole run by the stock length rather than rounding up row by row, which assumes offcuts get reused at butt joints. For unbroken rows with no joints, buy boards at least as long as the deck run instead.
- 05
Joists
floor (deck length ÷ joist spacing) + 1
Joists at your spacing across the deck's length, counting one at each end. This is a count at the spacing you entered — it isn't a judgement that the spacing or the joist size suits your span.
- 06
Screws
board rows × joists × 2
Two face screws per board at every joist it crosses. A hidden-fastener or clip system uses its own count — follow that system's instructions.
Worth knowing
- This is a materials estimate from geometry, not a structural design. Joist spacing and size, beams, posts, footings, ledger attachment and blocking are governed by load and local building code — check your local requirements, and involve a professional where the deck is raised, attached to the house, or carrying anything unusual.
- The joist count covers a plain rectangular frame at your spacing. Beams, posts, footings, rim joists, stair framing and blocking are not included.
- Use the board's actual face width, not the nominal size — a nominal 2×6 measures about 5.5 in, and using 6 in would under-count the rows.
- Gap spacing is product-specific: some pressure-treated boards are installed tight because they shrink as they dry, while dry or composite boards are spaced. Follow the instructions for the boards you're buying.
- This covers one rectangular deck surface. For an L-shaped or wrapped deck, run each rectangle separately and add the results; stairs, railings and fascia are separate estimates.
Deck calculator example
- Deck
- 16 × 12 ft
- Board
- 5.5 in wide, 12 ft long
- Gap
- 0.25 in
- Joist spacing
- 16 in
- Waste allowance
- 10%
- Deck area16 × 12 = 192 sq ft
- Board rows(144 + 0.25) ÷ (5.5 + 0.25) = 25.1 → 26
- Decking run26 × 16 = 416 linear ft
- After 10% waste416 × 1.10 = 457.6 ft
- 12 ft boards457.6 ÷ 12 = 38.13 → 39
- Joistsfloor (16 ft = 192 in ÷ 16 in) + 1 = 13
- Screws26 × 13 × 2 = 676
39 decking boards, 13 joists, about 676 screws.
26 rows at 5.75 in each slightly overshoots the 144 in width, so the last row gets ripped narrower — normal, and it's still bought as a whole board.
Use the actual board width, not the nominal size
This is the single easiest way to get a decking estimate wrong.
Lumber is sold by a nominal size that isn't what it measures. A nominal 2×6 is about 5.5 inches across the face, and a nominal 2×4 about 3.5 inches. Entering 6 instead of 5.5 on a 12 ft wide deck drops the row count by two, and two rows short is a second trip. Measure a board's face, or take the actual dimensions from the product listing, and enter that. Composite and hardwood decking vary more again between brands, so check rather than assume.
How big should the gap between boards be?
It depends on the board, and it's specified by whoever made it.
Gaps let water drain and air move, and they accommodate the way boards move with moisture. How much gap is right depends on the material and its condition when installed: wet pressure-treated boards shrink as they dry and are often installed tight for that reason, while dry timber and composite boards are spaced. Composite manufacturers also specify end gaps that change with temperature at installation. Follow the instructions for the boards you're buying — the calculator takes whatever figure you enter, including 0.
How much waste allowance to use
A waste allowance covers offcuts, butt joints and the occasional board you can't use.
| Situation | Typical waste allowance |
|---|---|
| Simple rectangle, boards run the full length | Around 5–10% may be a useful planning range |
| Rows made up from several boards with butt joints | Around 10% is a common planning starting point |
| Diagonal or picture-frame layout, or angled edges | 10–15% or more may be appropriate, depending on the layout |
Important: these are planning guidelines, not guarantees. The board count above divides the whole run by the stock length, which assumes offcuts get reused at joints. If you want unbroken rows with no butt joints, buy boards at least as long as the deck run instead of relying on the allowance.
What the joist and screw figures are — and aren't
Both come from geometry. Neither is a structural decision.
The joist count is simply how many joists fit across the deck's length at the spacing you entered, with one at each end. It excludes beams, posts, footings, rim joists, blocking, stair framing and fascia, and it is not a judgement that your spacing or joist size suits the span. Spacing and sizing depend on load, species, grade, span and your local building code — and on the decking itself, since some boards require closer spacing, and a diagonal layout requires closer spacing again. Check your local requirements, and involve a professional for a raised deck, one attached to the house, or anything carrying unusual load.
The screw figure assumes two face screws per board at each joist it crosses, which is the common convention for face-screwed decking. It doesn't include framing fasteners, joist hangers or structural connectors. A hidden-fastener or clip system works differently — use that system's own coverage figure.