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Deck Calculator
Building a deck starts with two numbers: how many boards to cover the surface, and how many joists to hold them up. Enter the length and width of your deck and this free deck calculator gives you the deck area, the linear feet of decking, the number of 12-foot boards, and the joist count at the standard 16-inch spacing.
Joists run along the length and are spaced across the width, so the width sets the joist count. Length is the direction the boards run.
| Deck area | 192 sq ft |
| Decking needed | 419 linear ft |
| Boards | 35 × 12 ft |
| Joists | 13 |
Results are estimates for planning only. Posts, beams, footings, stairs, and railings are not included — check local building rules before you build.
Common deck sizes
| Deck size | Area | Decking | 12 ft boards | Joists (16 in) |
|---|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 218 lin ft | 19 | 9 |
| 12 × 12 ft | 144 sq ft | 314 lin ft | 27 | 10 |
| 12 × 16 ft | 192 sq ft | 419 lin ft | 35 | 13 |
| 16 × 20 ft | 320 sq ft | 698 lin ft | 59 | 16 |
| 20 × 20 ft | 400 sq ft | 873 lin ft | 73 | 16 |
How it works
A deck is two systems working together: the boards you walk on, and the joists underneath that carry them. The calculator works out each one separately, because they are measured in different ways. Boards are measured in linear feet — the total length of board laid end to end — while joists are counted one by one across the width of the deck.
The board calculation starts with the area. Length times width gives the square footage: a 12 by 16 foot deck is 192 square feet. Then the area is divided by the face width of one board. A deck board sold as 6 inches wide actually measures about 5.5 inches once it has been milled smooth, and 5.5 inches is 0.458 feet. Dividing 192 by 0.458 gives 419 linear feet of board. That is the total length of decking if every board were laid end to end in one long strip. Divide that by the length of the boards you are buying — 12 feet, in the default — and round up: 419 ÷ 12 = 34.9, so 35 boards. The rounding up matters, because the last board of a row is usually cut, and the offcut rarely fits anywhere else.
Joists work differently. They run in one direction — along the length of the deck — and they are spaced evenly across the width. The standard spacing is 16 inches from the center of one joist to the center of the next, written as 16 inches on center. To count them, convert the width to inches (16 feet × 12 = 192 inches), divide by the spacing (192 ÷ 16 = 12 spaces), and add one: 13 joists. The extra one is the joist at the starting edge — 12 gaps between joists need 13 joists to close them, just as a fence with 12 gaps needs 13 posts. Closer spacing, such as 12 inches on center, gives a stiffer deck and is often used for diagonal board patterns or heavy loads; wider spacing, such as 24 inches, is only right for thicker boards and lighter use, and many local rules do not allow it for a home deck.
Two details catch people out. The first is the gap between boards. Decking boards are laid with a small gap — usually about an eighth to a quarter of an inch — so rain can drain through and the wood can swell and shrink with the weather. The calculator ignores that gap, which makes its board count very slightly generous; that is the safe direction to be wrong in. The second is board length versus deck length. If your deck is 16 feet long and you buy 12-foot boards, every row needs a join, and the joins should be staggered from row to row so they do not line up. Buying 16-foot boards for a 16-foot deck avoids joins entirely, but long boards cost more per foot and are harder to carry and handle alone. Choose the board length in the calculator and watch the count change before you decide.
What this calculator does not cover is the frame that holds the joists up: posts, beams, footings, and the ledger board that fixes the deck to the house. Those depend on how high the deck sits, how far the joists span between supports, the ground underneath, and the building rules where you live. A low deck a foot off the ground needs far less structure than a deck ten feet up with stairs and a railing. Use this page for the boards and joists, then get the structure checked against local rules before you order timber or dig a single footing.
Worked example
Alex is building a deck 12 feet long and 16 feet wide behind his house. He plans to use standard 5.5-inch boards in 12-foot lengths, with joists at 16 inches on center.
Step 1: Find the area: 12 × 16 = 192 square feet.
Step 2: Find the linear feet of decking: 192 ÷ (5.5 ÷ 12) = 192 ÷ 0.458 = 418.9, rounded to 419 linear feet.
Step 3: Find the board count: 419 ÷ 12 = 34.9, rounded up to 35 boards.
Step 4: Find the joist count: 16 feet × 12 = 192 inches; 192 ÷ 16 = 12 spaces; 12 + 1 = 13 joists.
Alex orders 35 twelve-foot boards and 13 joists. If he decides to switch to 16-foot boards instead, the same 419 linear feet divided by 16 gives 26.2, rounded up to 27 boards — fewer boards, no joins, but each board costs more and is heavier to carry.
FAQ
How many decking boards do I need for a 12x16 foot deck?
A 12x16 foot deck is 192 square feet. With 5.5 inch wide boards you need 419 linear feet of decking, which is 35 boards of 12 feet each, rounded up.
How many joists does a deck need?
At 16 inches on center, a deck 16 feet wide needs 13 joists: 16 feet x 12 inches divided by 16 inches is 12 spaces, plus 1 joist for the starting edge, which makes 13 joists.
What does 16 inches on center mean?
It means the distance from the center of one joist to the center of the next is 16 inches. It is the standard spacing for most home decks, and it keeps the decking boards from flexing underfoot.
Why is a 5.5 inch board used in the calculation?
A common deck board is sold as 6 inches wide but measures about 5.5 inches once it is milled. Dividing the deck area by the true 5.5 inch face width gives the real length of board you need.
Does this calculator include posts and beams?
No. This calculator covers decking boards and joists only. Posts, beams, footings, stairs, and railings depend on the deck height, local rules, and the design, so they need a separate estimate.