Deck Calculator

How to calculate deck boards

Deck boards go down in parallel rows, so the count comes from two things: how many rows fit across the deck, and how many boards it takes to fill each row. The calculator does it in four steps, and you can read each one in the result.

1. Rows. Take the depth the rows must cover, in inches, add one gap, and divide by the board width plus one gap. Round up. Rows are the number of boards side by side.
2. Linear feet. Multiply the rows by the length of one row, which is the side the boards run along.
3. Boards. Add your waste, then divide the linear feet by the length of the boards you buy. Round up.
4. Joists and screws. Joists sit across the boards at your spacing, one at each end. Every board gets 2 screws at every joist it crosses.

If a row is longer than one board, the boards meet end to end. The calculator assumes every butt joint lands on a joist. TimberTech’s installation guide says the ends of its boards must fall on a joist, and that joists should be doubled at butt seams. TimberTech installation guide.

Nominal and actual board width

The size on the label is not the size of the board. A “2×6” is a name. The width you must use in the math is the actual one, and for softwood lumber it is set by the American Softwood Lumber Standard, PS 20-25 (January 2025, checked October 2026). These are the minimum dressed sizes for dry lumber.

The standard says its widths apply to every thickness. Thickness does not change the count, but for the record a nominal 1 inch board is 3/4 inch thick, a 5/4 board is 1 inch and a 2× board is 1-1/2 inches. Lumber that is still green is cut a little bigger, 5-5/8 inches for a nominal 6. Composite boards have their own sizes. Trex lists its boards as 5.5 inches wide, so choose the 6 inch option for them or type the width printed on your boards under “Other”.

Example: a 12 ft by 16 ft deck

The boards are 2×6 (5.5 inches actual), the gap is 1/8 inch, the boards are 16 ft long and run along the 16 ft side, the joists are 16 inches on center and you add 10% waste.

Now see what the nominal width does. With 6 inches instead of 5.5, the rows become (144 + 0.125) ÷ 6.125 = 23.53, which is 24 rows, and the order drops to 27 boards of 16 ft. You would be 2 boards short, and about 9 inches of the deck would be left uncovered.

Common deck sizes

Here are some common sizes with 5.5 inch boards, a 1/8 inch gap, 16 ft boards running along the longer side, offcuts reused, joists 16 inches on center and no waste. Waste goes on top.

Trex gives its own count for a 12 ft by 12 ft deck: 27 boards of 12 ft, 21 of 16 ft or 17 of 20 ft (Trex, June 2023). Trex says its figures include waste from cutting. Set the gap to 0 and leave waste empty and this calculator returns the same three numbers, with no waste added. Trex’s method does not count the gap, so it asks for one board more in each case than the 26, 20 and 16 boards you get with a 1/8 inch gap.

Joist spacing

Joists carry the boards, and the closer they are, the firmer the deck feels. The spacing you can use depends on the board. TimberTech says that for its residential decking the joists should never be more than 16 inches on center, that 12 inches gives a firmer feel, and that boards laid on a diagonal need 12 inches at most (TimberTech, checked October 2026). Wood boards follow the International Residential Code, which sets the maximum by board thickness and by whether the boards run square or on a diagonal. The calculator offers 12, 16 and 24 inches, and it is up to you to pick a spacing your boards and your local code allow.

The joist count is the length the joists are spread along, divided by the spacing, rounded up, plus one for the last joist. It does not include the rim joists, the ledger, beams, posts, blocking or the doubled joists at butt seams.

Screws, gap and waste

• Screws. The International Residential Code asks for at least two fasteners where wood decking meets each supporting member. TimberTech asks for two screws per joist, including at end joints. That is why the calculator counts 2 screws for every place a board crosses a joist, and 4 at a butt joint, where two boards meet on the same joist. It does not count extras or hidden clips.
• Gap. TimberTech requires 1/8 to 3/16 inch between its boards. For wood, Decks.com says to keep it to 1/8 inch or less when the boards are saturated with preservative, and that it normally opens to about 1/4 inch as the wood dries.
• Waste. Trex says 5%. TimberTech says roughly 10%. For boards laid at 45 degrees, Decks.com says to add another 10% to 15% on top. The box starts empty so that you choose.
• Board lengths. Trex says deck boards usually run from 8 to 20 feet and that its own come in 12, 16 and 20 foot lengths. Check which lengths your supplier stocks.

Offcuts and the last row

“Reuse offcuts” divides the total linear feet by the board length, which is the usual way to price a deck, and it works when you can use the leftover piece of a board somewhere else in the deck. “Do not reuse” buys every row from whole boards, so a 16 ft deck built with 12 ft boards needs two boards per row. The second option always asks for the same number of boards or more.

The last row usually has to be ripped narrower than a full board. The calculator tells you how wide it will be. Rows are counted as a full board each, which is the safe way to order.

Before you build

A deck is a structure that carries people, and its frame is a matter of safety. This calculator does not give joist spans, beam sizes or footing sizes on purpose. The International Residential Code covers them in section R507, your city or county may have its own rules, and your building department can tell you whether you need a permit, plans and inspections. A county guide based on the 2021 IRC says it is for reference only and that final approval comes after plan review and field inspections (Anne Arundel County deck guide). Prices are not shown here because they change by region and supplier. Take your numbers to your supplier and ask for a quote.

Frequently asked questions