A deck cost calculator counts the deck boards and joists for a rectangular deck and multiplies them by your lumber prices. Formula: Boards = width × 12 ÷ board width, rounded up; joists = length × 12 ÷ joist spacing, rounded up, + 1; cost = boards × board price + joists × joist price. A 16 by 12 foot deck with 5.5-inch boards and joists 16 inches apart needs 27 boards and 13 joists. At $18 a board and $14 a joist, the lumber costs $668.
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How to use the deck cost calculator
- Enter the deck length, the side the boards run along, in feet.
- Enter the deck width in feet.
- Enter the board width. A nominal 5/4×6 or 2×6 deck board is about 5.5 inches wide.
- Enter the joist spacing on center, often 12 or 16 inches.
- Enter your price for one board long enough to span the deck, and one joist.
The deck cost calculator gives the board count, the joist count and the lumber total.
What formula does the deck cost calculator use?
Boards run the length of the deck, so the number of rows is the width in inches divided by the board width, rounded up. Joists run across the width and are spaced along the length, so their count is the length in inches divided by the spacing, rounded up, plus one for the end. The cost is each count times its price.
The tool does not count beams, posts, footings, ledger board, rim joists, hangers, screws or railings. Those can cost as much as the boards, so price them before you set a budget.
Deck cost calculator worked example
Results from the deck cost calculator:
| Deck | Spacing | Boards | Joists | Lumber cost |
|---|---|---|---|---|
| 16 × 12 ft ($18 board, $14 joist) | 16 in | 27 | 13 | $668.00 |
| 20 × 14 ft ($22 board, $18 joist) | 12 in | 31 | 21 | $1,060.00 |
| 12 × 10 ft ($15 board, $12 joist) | 16 in | 22 | 10 | $450.00 |
Closer joists add lumber. On the 20 by 14 foot deck, 12-inch spacing needs 21 joists; at 16 inches it would need 16.
Joist spacing and span limits
The American Wood Council’s prescriptive deck guide, DCA 6, lists the longest span for each joist size and spacing. For Southern Pine at 16 inches on center, a 2×8 can span 11 feet 10 inches and a 2×10 can span 14 feet. At 24 inches the 2×8 drops to 9 feet 8 inches. The guide also says each piece of decking must bear on at least 3 joists, and boards should be spaced about 1/8 inch apart.
Many composite boards need joists at 16 inches or closer, so check the maker’s instructions before you enter a spacing in the deck cost calculator. Your local building code and inspector have the final say.
Buying the right board lengths
The calculator assumes each row is one board that runs the full length. If your deck is longer than the boards you can buy, each row needs two pieces with a joint over a joist. Enter the price of the boards you will actually use, or double the count for rows that need two pieces. Planning the joints to land on doubled joists makes a stronger, neater deck.
Turning the count into a full budget
Start with the lumber total from the deck cost calculator. Then list the parts it leaves out: footings and concrete, posts, beams, a ledger board and flashing if the deck attaches to the house, joist hangers, structural screws or bolts, deck screws, stairs and railings. Get a price for each. Many people find the framing hardware and railings add more than they expected, especially on a raised deck.
Finally, ask your local building department whether a permit and inspection are needed. Many areas require one for decks attached to a house or built above a set height.
Common mistakes to avoid
A common mistake is budgeting only for boards and joists. Footings, hardware and railings often double the cost. Another is choosing a joist spacing wider than the decking allows. A third is forgetting a few spare boards for bad pieces and miscuts. Use the deck cost calculator as the lumber part of a full budget.
Prices change from store to store and week to week, so price the exact items you plan to buy on the same day, and keep the receipt for anything you might return.
Estimates for planning only. Prices, codes and site conditions vary; get a written quote and check local building rules before you buy materials.
Related converters and calculators
Related calculators:
- Concrete calculator: footings
- Square footage calculator: deck area
- Lumber volume converter: board feet
Sources
American Wood Council: Prescriptive Residential Wood Deck Construction Guide (DCA 6).
Questions about this calculator
How many deck boards do I need?
Divide the deck width in inches by the board width and round up. A 12-foot width with 5.5-inch boards needs 27 rows.
How many joists for a 16-foot deck?
At 16 inches on center, 192 ÷ 16 = 12 spaces, so 13 joists.
What joist spacing should I use?
Twelve or 16 inches on center is common. Check the decking maker’s rules and the AWC DCA 6 span tables.
Does the deck cost calculator include footings and railings?
No. It counts boards and joists only. Price footings, beams, hardware and railings separately.
How far apart should deck boards be?
The AWC DCA 6 guide says about 1/8 inch for lumber decking. Composite makers set their own gaps.
Check the assumptions and units before relying on a result. Financial, tax and health estimates do not replace advice for your individual circumstances.
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