Calculators / Deck calculator

Deck calculator

Enter the deck size to get the deck boards, joists, hangers and screws to buy, with the math shown.

US units (feet, inches, pounds); metric inputs accepted · Planning and purchasing estimate · Reviewed October 20, 2026

Start from a common job: 12 × 16 ft deck 10 × 10 ft deck Composite 20 × 14 ft, 12 in joists

Before you build: Decks usually need a permit. Footings, beams, ledger attachment, joist spans and stairs must follow your local code (IRC section R507 in most areas), and decks more than 30 in above the ground need guards.

Supplier or contractor? Put this calculator on your own website with your logo, and get the customer’s quantities with each quote request. See how it works.

The formula

Rows    = deck depth (in) ÷ (board width + gap), rounded up
Boards  = rows × (1, or deck width ÷ board length if wider) × (1 + extra %)
Joists  = deck width (in) ÷ joist spacing, rounded up, + 1
Screws  = rows × joists × 2, plus 5%

Example: a deck 16 ft wide and 12 ft deep, with 5½ inch boards and a 3/16 inch gap running along the house, has 144 ÷ 5.6875 = 25.3, so 26 rows. One 16 ft board covers each row, so you need 29 boards with 10% extra. Joists 16 inches apart: 192 ÷ 16 = 12 spaces, so 13 joists, each 12 ft long. Two screws at every joist give about 710 screws.

Quick answers for common deck sizes

5½ in boards with a 3/16 in gap, 10% extra boards, joists 16 in apart. Decks wider than 16 ft need a joint in each row.
Deck (width × depth)	16 ft deck boards	Joists	Deck screws
10 × 10 ft	25	9 × 10 ft	416
12 × 12 ft	29	10 × 12 ft	546
16 × 12 ft	29	13 × 12 ft	710
16 × 16 ft	38	13 × 16 ft	929
20 × 12 ft	36	16 × 12 ft	874
20 × 16 ft	47	16 × 16 ft	1,143

Planning tips

Pick boards at least as long as the deck is wide and there are no joints to line up. Wider decks need a butt joint in each row, centered on a joist and staggered from row to row.
Joist spacing: 16 inches is standard. Many composite boards require 16 inches or less, and 12 inches when boards run at an angle; check the board maker’s guide.
Gaps: composite and dry lumber need the gap the maker specifies, often 3/16 to 1/4 inch. Wet pressure-treated boards are often laid tight because they shrink as they dry.
Not included: footings, posts, beams, ledger bolts, stairs and railings. Their sizes depend on the deck height, span and local code, so plan them with your building department.

Sources and assumptions

Board coverage uses the actual width of the board plus the gap: a 5/4×6 or 2×6 deck board is 5½ inches wide. Two screws per board at every joist is the usual face-fastening pattern. Lumber lengths round up to 8, 10, 12, 14, 16 or 20 ft.

Questions

How many deck boards do I need for a 12 × 16 deck?

With 5½ inch boards and a 3/16 inch gap running along the 16 ft side: 26 rows of 16 ft boards, or 29 boards with 10% extra.

How many joists do I need for a 16 ft deck?

13 joists at 16 inches on center: 16 ft is 192 inches, which is 12 spaces, plus one joist to close the last space.

How many screws per deck board?

Two at every joist the board crosses. A 16 ft board over joists 16 inches apart crosses 13 joists, so about 26 screws.

Which way should deck boards run?

Usually parallel to the house, across the joists. The joists then run out from the house, hung from the ledger.

References and review

Units convert with exact definitions: 1 ft = 12 in = 0.3048 m, 1 yd³ = 27 ft³. Results round up to whole bags, sheets and pieces so you don’t come up short. Last reviewed October 20, 2026. If your bag, pallet or supplier lists a different figure, use theirs: change the matching field above.

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