Deck Board Calculator

Background.

A 16-by-12-foot deck has 192 square feet of surface, but nobody sells decking by the square foot. You buy boards of a fixed length, lay them side by side with a gap between each, and screw them down wherever they cross a joist. The order depends on how many rows of boards fit across the deck, how many feet of board those rows add up to, and how many stock lengths cover that with a margin for offcuts and the odd bad board.

With 5.5-inch boards and a quarter-inch gap, each row takes 5.75 inches of the deck's width. Twelve feet is 144 inches, and 144 ÷ 5.75 = 25.04, so the deck needs 26 rows, called courses, each 16 feet long: 416 lineal feet. Allow 10% for waste and that comes to 29 boards in 16-foot lengths, or $696 at $24 a board. Two screws at each of the 13 joists every course crosses add up to 676 fasteners.

Small choices move those numbers more than you might expect. Lay the same boards tight together and the deck needs 27 courses; open the gap to 3/8 inch and it needs 25. Turn the boards to run the 12-foot way and there are 34 shorter courses, each leaving a 4-foot offcut if you cut it from a 16-foot board. Set the joists 12 inches apart instead of 16 and the screw count rises to 884. Trying a few combinations before ordering is cheaper than a second trip to the lumber yard, or a pile of unused boards.

What is deck board calculator?

A deck board estimate turns the plan of a rectangular deck into a purchase list for its walking surface. A course is one row of boards running the length of the deck. The module is the width each course takes up, board plus gap. Lineal feet is the total length of board needed, whatever its width, and the stock length is the length boards are sold in, usually even numbers of feet for lumber. Board width means the actual width: a nominal 2×6 measures 1.5 by 5.5 inches, and 5/4×6 decking is also 5.5 inches wide. The estimate covers decking only. Joists, beams, posts, footings, ledger attachment, guards and stairs are structural, and the American Wood Council's DCA 6 guide sets out prescriptive rules for building them under the International Residential Code.

How to use this calculator.

1. 01Enter the Deck length in the direction the boards will run, in feet (16.5 for 16 ft 6 in). On a deck built off a ledger, the joists usually run out from the house, so the boards run parallel to the wall and this is the side along the house.
2. 02Enter the Deck width, the dimension across the courses.
3. 03Enter the Board actual width as measured, not the nominal size, and the Gap between boards. Composite makers specify their own gap; wet pressure-treated boards shrink as they dry, so they are often laid with little or no gap.
4. 04Enter the Board stock length you will buy and a Waste allowance. Ten percent suits a straight layout; raise it for diagonal boards, borders or many short courses.
5. 05Enter the Joist spacing on centre and the Fasteners per board per joist crossing: two face screws is the usual pattern, while hidden clip systems follow their maker's count.
6. 06Enter the Price per deck board for the stock length chosen, then read the courses, lineal feet, boards to buy, fasteners and cost.

The formula.

The module comes first: m = (board width + gap) ÷ 12, in feet. The number of courses is the deck width divided by the module, rounded up, N = ⌈W ÷ m⌉, because a part course still needs a board, even if it is ripped narrower. Lineal feet is N × L, where L is the deck length the boards run along. Waste is added to that length, lineal feet × (1 + w), and the result is divided by the stock length and rounded up to whole boards: Boards = ⌈N × L × (1 + w) ÷ Lₛ⌉. Cost is the number of boards times the price of one.

Fasteners are counted at joist crossings. A course of length L crosses ⌈L ÷ s⌉ + 1 joists, where s is the joist spacing in feet: one joist at the start, one every s, and one at the far end. When the length is not an exact multiple of the spacing, the last bay is shorter but the far rim joist still takes its screws, so the count rounds up: a 15-foot deck at 16-inch spacing has 13 crossings per course, not 12, and 26 courses at 2 screws each give 676 screws. Multiply the crossings by the number of courses and by the fasteners at each crossing.

Three simplifications are worth knowing. The module counts a gap after every board, including the last, so a width that comes out just over a whole number of courses gets an extra course that may only need to cover a fraction of an inch. The board count pools all the length together, as if offcuts will be joined end to end over joists; if every course must be one unbroken board, you need at least one board of sufficient length per course, which can be more. And the fastener count has nothing extra for butt joints, borders, fascia or stairs.

The deck is treated as a plain rectangle with straight courses running across the joists. Diagonal and herringbone layouts, picture-frame borders, breaker boards, curves and cut-outs around posts all need a separate allowance.

A worked example.

Frequently asked questions.

References& sources.

1. [1]American Wood Council (2018). Prescriptive Residential Wood Deck Construction Guide, DCA 6, 2015 IRC Version. AWC.
2. [2]American Wood Council (2024). 2024 Wood Frame Construction Manual. AWC.
3. [3]National Institute of Standards and Technology (2026). NIST Handbook 133, Appendix E: General Tables of Units of Measurement. U.S. Department of Commerce.
4. [4]National Institute of Standards and Technology (2008). Guide for the Use of the International System of Units (SI), NIST Special Publication 811. U.S. Department of Commerce.
5. [5]Bureau International des Poids et Mesures (2019). The International System of Units (SI Brochure), 9th edition. BIPM.