Enter your deck size, board width, gap and joist spacing. You get the number of deck boards with waste, how many joists to frame, how many deck screws to buy and the deck area.
2 screws per board at every joist. Check the decking maker’s span limit for your spacing.
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This deck calculator is for homeowners and builders planning a wood or composite deck. It works out how many rows of boards fit across the deck once the gaps are counted, how many boards each row needs, how many joists sit under them at your chosen spacing, and how many screws hold it all down.
How Do You Calculate How Many Deck Boards You Need?
Divide the deck width by the board width plus the gap to get the number of rows, then multiply by the boards per row and add about 10% waste. A 16 by 12 ft deck with 5.5 inch boards and 3/16 inch gaps needs 26 rows of 16 ft boards, or 29 boards with waste.
What Is the Formula for Deck Boards?
The deck board formula is rows = deck width ÷ (board width + gap), rounded up, then boards = rows × boards per row × (1 + waste).
boards per row = deck length ÷ board length, rounded up
joists = deck length (in) ÷ spacing + 1
screws = rows × joists × 2
Use the real width of the board, not its name. A 5/4 × 6 deck board, said “five quarter by six”, is actually about 1 × 5.5 inches, and a 2 × 6 is about 1.5 × 5.5 inches. This difference between the name and real size is called the nominal size versus the actual size. Many composite boards are also 5.5 inches wide. To get the deck area for pricing or for stain, the square footage calculator also handles L shapes.
What Joist Spacing Do I Need for Decking?
Most wood decks use joists 16 inches on center, which suits both 5/4 and 2 inch boards laid straight across the joists. The International Residential Code, in Table R507.7, sets these limits for wood decking:
| Decking | Boards square to joists | Boards at a diagonal |
|---|---|---|
| 1 1/4 in wood (5/4) | 16 in max | 12 in max |
| 2 in wood (2 × 6) | 24 in max | 16 in max |
| Composite or plastic | Per the maker’s rating | Per the maker’s rating |
On center means measured from the middle of one joist to the middle of the next. Many composite boards need 16 inches, or 12 inches when laid at an angle, so check the label. Metric decks commonly use 400 mm or 450 mm joist centers. The code also caps diagonal boards at 45 degrees.
How Big Should the Gap Between Deck Boards Be?
A gap of about 1/8 to 1/4 inch (3 to 6 mm) between boards is common, and 3/16 inch is a popular middle value. The gap lets rain drain, lets air dry the boards and leaves room for wood to swell. Boards that are still wet from pressure treating shrink as they dry, so many builders butt them tight and let the gap open up. Composite boards often need a set gap from the maker. Always follow the board maker’s instructions over a rule of thumb.
How Many Screws Do I Need for a Deck?
Plan on 2 screws per board at every joist, which is about 350 screws per 100 ft² with 5.5 inch boards on joists 16 inches apart. A 16 by 12 ft deck works out to 676 screws. Use coated or stainless deck screws rated for treated lumber, or hidden fasteners for composite. If the deck needs steps down to the yard, the stair calculator sizes the stringers, and the concrete calculator works out the footings.
Worked Example
Dana is building a 16 ft long, 12 ft wide deck with 5.5 inch boards, 3/16 inch gaps and joists 16 inches on center. The width of 144 inches divided by 5.6875 inches gives 25.3, so the tool rounds up to 26 rows. Each row takes one 16 ft board, and with 10% waste that is 29 boards (464 ft in total). The frame needs 13 joists, and 26 rows × 13 joists × 2 gives 676 screws for 192 ft² of deck. At 12 inch spacing it needs 17 joists and 884 screws. A 20 ft deck with 12 ft boards needs 2 boards per row, so 58 boards with waste.
How Do You Use the Deck Calculator?
Pick your units
Tap Feet, inches for US lumber or Meters, mm for metric boards.
Enter the deck size
Type the length along the boards, the width across them and the joist spacing.
Read the board count
Add board width, gap, length and waste. The big number is the boards to buy.
Check joists and screws
The boxes show deck area, rows, joists, screws and total board length.
Frequently Asked Questions
How many deck boards do I need for a 12 × 12 deck?
With 5.5 inch boards and 3/16 inch gaps, a 12 ft wide deck takes 26 rows. Using 12 ft boards, that is 26 boards, or 29 with 10% waste. Using 16 ft boards and cutting them down wastes 4 ft per board.
Should deck joists be 12 or 16 inches apart?
16 inches on center is the standard for most decks and for 5/4 wood decking laid square to the joists. Use 12 inches for 5/4 boards laid at a diagonal, for some composites, or for a stiffer feel. 2 inch boards can span 24 inches.
How many screws per deck board?
Use 2 screws at every joist the board crosses. A 16 ft board on joists 16 inches apart crosses 13 joists, so it takes 26 screws. Keep screws about 3/4 inch from the board edges to avoid splitting.
What is the actual width of a 2 × 6 deck board?
A 2 × 6 is about 1.5 × 5.5 inches. A 5/4 × 6 deck board is about 1 × 5.5 inches. Always enter the actual width in a deck calculator, or the board count will be about 9% too low.
Which way should deck boards run?
Deck boards usually run parallel to the house, square to the joists. Running them at a diagonal looks good but needs closer joists, 12 inches for 5/4 boards under the IRC, and creates more waste, often 15% instead of 10%.
How much waste should I add for decking?
Add about 10% for a simple rectangle with boards that fit the length. Add 15% or more for diagonal boards, picture frame borders or decks with many cuts. Use board lengths that divide your deck length to cut waste.
Sources and Further Reading
- City of New Prague, MN: IRC Section R507 Decks, with Table R507.7 decking spans
- Journal of Light Construction: Highlights from the American Wood Council DCA 6 deck guide
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Last reviewed: October 2026