Decks and fences are the two projects on this site where the estimate is a list of distinct parts instead of a quantity of one thing. The arithmetic is more involved and the caveats matter more.
Deck boards: it starts with a course width
A nominal “5/4 × 6” deck board is not 6 inches wide. It measures 5.5 inches, and boards are laid with a gap between them. The gap is typically 1/8 inch for pressure-treated lumber, which shrinks as it dries, and closer to 1/4 inch for composite and kiln-dried material, which does not.
The effective width each board occupies, the course width, is therefore 5.625 inches at a 1/8-inch gap.
Rows = ⌈ (deck width in inches) ÷ 5.625 ⌉
The gap lets water drain, lets air circulate underneath, and gives wood somewhere to go when it swells. Butt the boards tight and a pressure-treated deck will cup and buckle within two seasons.
Boards run across the joists, which in a typical framing layout means across the deck’s shorter dimension. A 12 × 16 deck therefore has rows running the 16-foot direction, and:
- Rows: 12 ft = 144 in, ÷ 5.625 = 25.6, so 26 rows
- Linear feet: 26 rows × 16 ft = 416 linear feet
- Boards at 12 ft each: 416 ÷ 12 = 34.7, so 35 boards
Decking is commonly sold in 8, 12 and 16-foot lengths. Match the length to your deck. Sixteen-foot boards on a 16-foot run mean no butt joints at all, which looks better and is structurally cleaner than 12-foot boards with a seam in every row.
Deck screws: two per board per joist
Joists sit at 16 inches on centre in standard residential framing. Over a 16-foot run that gives:
- (16 ft × 12) ÷ 16 = 12, plus one for the closing joist = 13 joists
Each board is fastened at every joist it crosses with two screws, one near each edge, to hold the board flat and stop it cupping.
Screws = rows × joists × 2
For our deck: 26 × 13 × 2 = 676 screws. Buy a box of 750 or 1,000. You will drop some, strip some, and want spares.
Hidden fastener systems change this calculation entirely and come with their own per-square-foot coverage figures on the packaging.
Fence posts: sections, not spacing
People get the post count wrong by one, every time.
Posts go at 8-foot spacing for a typical wood fence, because rails come in 8-foot lengths. A 100-foot run is 12.5 sections, which rounds up to 13 sections.
Thirteen sections need 14 posts, because both ends are closed. This is the fencepost problem in its original form: each section has a post at each end, adjacent sections share one, so you always need one more post than sections.
Sections = ⌈ linear feet ÷ spacing ⌉, Posts = sections + 1
Add a post for every corner and every gate side. A gate needs heavier posts than a run does, since it carries a swinging load that a line post never sees.
Rails and pickets
Rails are the horizontals. Three levels (top, middle, bottom) is standard for a 6-foot privacy fence, and two is common for a 4-foot fence.
Rails = sections × rail levels
Our 13-section run at three levels needs 39 rails.
Pickets are the vertical boards. A standard picket is 5.5 inches wide. A shadowbox or spaced picket style leaves a gap of around 1.75 inches, while a true privacy fence butts them nearly tight.
Pickets = ⌊ (linear feet × 12) ÷ (picket width + gap) ⌋
At 5.5 + 1.75 = 7.25 inches per picket, a 100-foot run takes 1200 ÷ 7.25 = 165 pickets.
Adjust the gap to the style you want. A gap of zero gives full privacy and the highest picket count. A 3-inch gap gives an open picket fence with considerably less material.
What a material list cannot tell you
This is why the fence and deck calculators produce a shopping list and not a plan.
Post depth and footings depend on your frost line and your soil. The common guidance is a third of the post length in the ground, below the frost line, in concrete. It varies enormously by region, and in a cold climate it can mean 42 inches or more.
Span tables govern joist size and spacing on a deck, based on the species, the grade, the load and the span. A 16-inch joist spacing is the common case, not a universal one, and the framing under a deck is a structural question.
Permits: most jurisdictions require one for a deck above a certain height and for fences above a certain height. Many have setback rules controlling how close to a property line a fence can sit, and some say which side the “good” face must point.
Property lines: establish yours before you dig. A fence built a foot over a boundary is a legal problem that outlives the fence.
Utilities: call the national dig line before putting a post in the ground. In the United States that is 811, it is free, and most states require it by law.
Gates, stairs, railings and hardware are not in these estimates. Stairs in particular are their own calculation and their own code section.
Buying notes
Buy long, cut short. Where a run is close to a standard length, taking the longer board and trimming beats seaming two shorter ones.
Order 5-10% over on lumber. Boards warp, split at the end, and occasionally arrive with a knot exactly where you needed to fasten. Lumber has no dye lot problem the way flooring does, so topping up later is easy, but a trip back mid-job still costs an afternoon.
Let pressure-treated lumber dry before staining or sealing. Fresh treated wood is wet with preservative and will not take a finish.
Check every board at the yard. Sight down its length for bow and crown. You are allowed to reject the bad ones, and the difference in a finished deck shows.
Formulas at a glance
| Quantity | Formula |
|---|---|
| Deck rows | width in inches ÷ 5.625 |
| Deck boards | (rows × run length) ÷ board length |
| Deck screws | rows × joists × 2 |
| Fence sections | linear feet ÷ 8 |
| Fence posts | sections + 1 |
| Fence rails | sections × rail levels |
| Fence pickets | (linear feet × 12) ÷ (picket width + gap) |
The deck board calculator and fence calculator run these and print the assumptions (board width, gap, spacing, rail count), so you can set them to whatever your lumber yard actually stocks.