Deck Calculator

Deck boards, joists, posts, footings and screws in one schedule

Give the deck’s footprint and this calculator builds the material schedule in build order: surface boards counted across the deck at their width plus gap, joists at your spacing underneath them, a post-and-beam layout on stated maximum spacings, footings figured from the holes you will dig, a screw count from the real board-to-joist intersections, and railing parts if you give it a railing length. What it will not do is certify a span: joist and beam sizes depend on species, grade and load tables, so this page counts members and leaves their sizing to the span tables and your building department, and says so rather than guessing.

Disclaimer: This calculator gives estimates for general information only. It is not financial, medical or other professional advice. It counts materials and does not certify spans, loads or code compliance; size framing from published span tables for your species and grade and confirm permits locally.

What this calculator does

A deck is four purchases pretending to be one: the surface, the frame, the foundation and the fasteners, plus a railing if the drop demands one. Estimators go wrong by treating it as a square-foot price, because boards run in one direction (so the count lives across the other dimension), joists multiply by spacing rather than area, and footings are holes whose concrete has nothing to do with the deck’s size and everything to do with its post layout. This schedule keeps the four apart, each on its own arithmetic, with the layout conventions printed in the method section where they can be argued with before money moves.

How to use it

  • Boards run along the length you enter. If your boards will run the other way, swap the two dimensions when you type them: the count follows the direction, not the compass.
  • Diagonal layouts change two fields. Boards at 45 degrees need joists at 12 in on centre and waste nearer 15 percent, and the board count here (which assumes straight runs) will under-read the diagonal’s true linear footage; treat a diagonal result as a floor, not an order.
  • Spacing follows the board, not habit. Sixteen inches suits straight wood decking; many composite boards require 12, especially diagonally, and the manufacturer’s sheet outranks any default on this page.
  • Railing length counts open edges only. The side against the house needs no rail; stairs need their own rail and are not in this length. Measure the edges a person could fall from, since that is the railing’s actual job description.

The method and layout conventions

  • Deck boards = width across the boards in inches / (board width + gap), rounded up, then waste added and rounded up again. Linear feet = boards × board length.
  • Joists run along the board direction beneath them: count = (width in inches / spacing) rounded down, plus 1. Rim joists and the ledger run the perimeter and are listed as its full linear footage.
  • Posts and beams (stated convention): beam lines run along the length, spaced across the width at most 8 ft apart (lines = width / 8 rounded up, plus 1); posts along each line at most 6 ft apart (posts per line = length / 6 rounded up, plus 1). Real layouts move posts for stairs, trees and views; the grid is the honest starting count.
  • Footing concrete = π × (diameter/2)² × depth per footing, times the post count; bags at about 0.45 cu ft per 60 lb bag.
  • Screws = 2 per board-to-joist intersection: boards × joists × 2. Hidden-fastener systems replace the count with their own clip spacing, sold by the box against coverage.
  • Railing: balusters at 5.5 in of coverage each (a 1.5 in baluster with the common 4 in maximum gap), railing posts at most 6 ft apart along the entered length.

Worked examples

The 16 by 12 deck. Boards of 5.5 in at a quarter-inch gap cover 5.75 in each: across 144 in that is 144 / 5.75 = 25.04, so 26 boards, 416 linear feet, and 29 boards with 10 percent waste. Joists at 16 in on centre: 10 joists. The layout grid gives 3 beam lines and 4 posts per line, 12 posts, and 12 footings at 12 in diameter by 36 in deep hold 2.36 cu ft each: 28.27 cu ft of concrete, 63 bags of 60 lb. Screws: 26 boards × 10 joists × 2 = 520 screws, a number that surprises everyone who budgeted “a box”.

The railing. Three open sides of that deck total 44 ft of railing: balusters at 5.5 in of coverage come to 96 balusters, with 8 railing posts at the 6 ft maximum spacing. The railing is routinely a fifth of a deck’s material bill, and it is the part a square-foot estimate forgets entirely.

Reading your results

Read the board count against available lengths: the schedule assumes full-length boards exist for your run (stock runs to 16 or 20 ft in most yards), and a run longer than stock becomes butt joints, doubled boards per row, and a different conversation with the yard. Read the post grid as a plan to adjust, not a drawing: moving one post for a stair opening is normal, and the count barely moves with it. Read the footing bags as the deep breath of the project: footings are where deck budgets go quiet, and 63 bags is what 12 honest holes cost.

Limitations

  • No span is certified here. Whether a 2×8 joist spans your width depends on species, grade, load and local code tables. This page counts joists at your spacing; it does not size them, by the brief’s own rule, and a deck is not the place to improvise the difference.
  • Replacement decks are a different estimate. Reusing a sound frame drops the joist, post and footing lines to inspection and repair quantities; the board and fastener counts carry over, the rest is a judgement made standing on the old frame, not in a calculator.
  • Stairs, landings and skirting are their own schedules. A deck’s stairs are a small staircase project (the stairs calculator on this site sizes their rise and run) and are excluded here rather than averaged in.
  • Hardware beyond screws is job-specific. Joist hangers, post bases, ledger flashing and structural screws follow the framing plan and local amendments; the fastener line here covers the deck boards’ screws and says so.

Frequently asked questions

Should joists be 12 or 16 inches apart?

Sixteen inches on centre is the standard for straight-laid wood decking and feels solid underfoot with 2× or 5/4 boards. Twelve inches is specified by many composite manufacturers (their boards flex more), required in effect for diagonal layouts because the diagonal board crosses joists at a longer effective span, and chosen by anyone who wants a noticeably stiffer floor. Twenty-four inches exists in some tables for thick wood boards and feels like it. The field here takes the board manufacturer’s number first, habit second.

How much gap should deck boards have?

Enough for drainage and seasonal movement: about a quarter inch is the common wood-deck gap as laid (dry wood boards are often gapped by an 8d nail’s width and swell toward each other), while composites follow their specification, commonly three sixteenths to a quarter inch, because composite moves lengthwise with temperature instead. The gap is an input rather than a constant because it also changes the board count slightly, and on a wide deck slightly becomes a board.

Why so many screws?

Because a deck board is fastened at every joist it crosses, twice, and the crossings multiply: the worked example’s modest deck has 260 intersections. The count is worth having before the build rather than during it, since running out of the right coated or stainless screws mid-deck is a classic Saturday stall; hidden fastener systems spend the same money as clips instead, counted by coverage on their boxes.

Can I skip footings and rest the deck on blocks?

For a genuinely ground-level, freestanding platform, some jurisdictions allow deck blocks without dug footings, under conditions about height, attachment and frost that vary locally. Anything attached to the house, raised, or holding a hot tub belongs on real footings below frost. The calculator’s footing section assumes the dug kind; if your project lawfully uses blocks, the post count still serves you and the concrete lines are simply not your purchase.

Related calculators

Method: boards by coverage across the deck with waste, joists at the entered on-centre spacing, posts on the stated 8 ft beam-line and 6 ft post-spacing grid, footing concrete from hole dimensions, and screws at two per board-to-joist crossing; no spans certified. Last reviewed: 15 October 2026.