Deck Material Calculator
Enter the deck footprint, the joist spacing your plan calls for, and your board dimensions — the tool counts joists, rim boards, decking rows, whole boards from your stock length, and screws at your rate. Quantities only, loudly: joist sizing, spans, beams, footings, and ledgers are engineered, code-governed decisions this page refuses to make.
Deck footprint and boards
Example: a 16 × 12 ft deck at 16 in o.c. → 10 joists + 2 rim, 37 deck boards (5.5 in + ¼ in gap from 12 ft stock, 10% waste).
Structural boundary, in bold: this tool does NOT size joists, set allowable spans, place beams or footings, or design ledger connections. Those come from code span tables, your building department, or an engineer — a deck built to a quantity count instead of a design can fail. Quantities only.
Layout arithmetic, worked
The worked deck: 12 ft of width is 144 in, so at 16 in on-center the frame takes floor(144 ÷ 16) + 1 = 10 joists, each 16 ft, plus 2 rim boards at 12 ft. Decking: each 5.5 in board + ¼ in gap covers 5.75 in of the 16 ft length — 192 ÷ 5.75 = 33.39 rows, 10% waste → 37 rows × 1 board from 12 ft stock = 37 deck boards. Computed at build time by the same engine wired to the form.
What is sourced, what is yours
The 5.5 in board-width default is the PS 20 American Softwood Lumber Standard's dressed size for nominal 6 in stock (a 5/4×6 deck board is 1 × 5½ in dry) — cited, tested, and still editable because composite boards deviate. The joist spacings offered are the standard layouts, but which one applies is your plan's decision. Gap, stock length, waste, screw rate, and price are all yours, and no fastening schedule or span table is built in.
Framing lumber and the posts below
Pricing framing stock by volume? The board foot calculator turns your joist list into board feet at your price. The posts under the frame are set with the post hole calculator's concrete math, pier forms with the concrete column calculator, and the fence around the yard with the fence calculator.
Frequently asked questions
How are the joists counted?
The same end-plus-intervals formula as any layout: joists = floor(deck width ÷ spacing) + 1, running the deck length, spaced across the width — a 12 ft (144 in) width at 16 in on-center gives floor(144/16) + 1 = 10 joists — plus two rim boards capping the ends. The spacing is whatever your plan specifies (12, 16, and 24 in o.c. are the menu choices because they are the standard layouts); the tool never chooses it for you.
What size joists do I need?
That is a structural question this tool will not answer. Joist depth, species, span limits, beam placement, footings, and ledger attachment are governed by code span tables and your local requirements — get them from the IRC tables, your building department, or an engineer. This calculator counts pieces for a layout that is already designed.
Why does the board width default to 5.5 inches?
Because the most common deck board — nominal 5/4×6 wood decking — is dressed to 1 × 5½ in under PS 20, the American Softwood Lumber Standard, and most composite boards copy that 5.5 in face. It is a labeled, editable default: measure your actual product, especially for 5.5 in-nominal composites that run 5.36 or 5.25 actual.
How is the deck board count computed?
Boards run perpendicular to the joists. Each row of decking plus its gap covers (board width + gap) inches of deck length, so rows = length ÷ (width + gap), waste applied to that quantity before rounding up, and each row takes ceil(width ÷ stock length) boards. A 16 ft deck with 5.5 in boards and ¼ in gaps needs 192 ÷ 5.75 = 33.4 rows — 37 whole rows with 10% waste.
How many screws will I need?
Only you (or your fastener maker) can say: the rate depends on joist count and the manufacturer’s schedule — commonly two per board per joist for face-screwed decking, different for hidden clips. Enter your per-board rate and the tool multiplies; it deliberately ships no fastening schedule of its own.
Dimensions never leave your browser. Sources, conventions, and the estimating pipeline are documented on the methodology page.