Deck Calculator

This deck calculator estimates the decking boards, joists and screws for a rectangular deck. Enter the deck length and width, pick your board width, gap and board length, and choose a joist spacing, and it works out how many rows of boards fit across the deck and how many boards to buy.

It also counts the joists at 12, 16 or 24 in on center and estimates face screws at two per board at every joist. It is a materials estimate for the deck surface and joists, not a structural design, so use it alongside span tables and your local code.

How to Use the Deck Calculator

1
Enter the deck size
Type the length (the direction the boards run, usually along the house) and the width (the direction the joists run) in feet.
2
Choose the decking
Pick the board width (6 in nominal boards are 5.5 in wide), the gap between boards, the board length you plan to buy, and an extra percentage for waste.
3
Pick the joist spacing
Choose 12, 16 or 24 in on center. Check the maximum spacing your decking allows.
4
Read the estimate
Results update as you type: boards to buy, rows, linear feet of decking, deck area, joist count and the number of deck screws.

How the deck calculator works

Decking boards run the length of the deck and sit on joists that run across it. The calculator first finds how many rows of boards fit across the width: each row takes one board width plus one gap, and the last row needs no gap. It rounds up so the deck is fully covered.

Each row then needs enough boards to cover the length. If your boards are longer than the deck, one board covers a row, and when a board is at least twice the deck length it yields two or more full pieces. If the deck is longer than your boards, each row needs several boards joined over joists. The waste percentage is added on top and rounded up.

Joists are the deck length in inches divided by the spacing, rounded up, plus one so both ends have a joist. Screws assume the usual pattern of two screws per board at every joist it crosses.

Rows = ⌈(width × 12 + gap) ÷ (board width + gap)⌉
Boards = rows × ⌈length ÷ board length⌉ × (1 + waste %), rounded up
Joists = ⌈length × 12 ÷ joist spacing⌉ + 1
Screws = rows × joists × 2

Worked example: a 16 x 12 ft deck

Take a deck 16 ft long and 12 ft wide, with 16 ft long 6 in boards (5.5 in actual), a 3/16 in gap, joists at 16 in on center and 5% extra for waste.

Deck area: 16 × 12 = 192 sq ft.
Rows: (144 + 0.1875) ÷ (5.5 + 0.1875) = 25.35, rounded up to 26 rows.
Boards: each 16 ft board covers a full row, so 26 boards, and 26 × 1.05 = 27.3, rounded up to 28 boards to buy. That is 416 linear feet of decking.
Joists: 16 × 12 ÷ 16 = 12, plus 1 = 13 joists, each spanning the 12 ft width.
Screws: 26 rows × 13 joists × 2 = 676 deck screws.

Deck materials for common sizes

All sizes use 5.5 in boards, a 3/16 in gap, joists at 16 in on center, boards as long as the deck, and 5% waste. Size is length (board direction) by width.

Deck size	Area	Rows	Boards to buy	Joists	Screws
10 × 10 ft	100 sq ft	22	24 (10 ft)	9	396
12 × 12 ft	144 sq ft	26	28 (12 ft)	10	520
16 × 12 ft	192 sq ft	26	28 (16 ft)	13	676
20 × 12 ft	240 sq ft	26	28 (20 ft)	16	832
16 × 16 ft	256 sq ft	34	36 (16 ft)	13	884

How joist spacing changes the count

Closer joists make a stiffer deck surface but add joists and screws. For the same 16 x 12 ft deck with 26 rows of boards:

Joist spacing	Joists	Screws
12 in on center	17	884
16 in on center	13	676
24 in on center	9	468

Tips and common mistakes

Match joist spacing to the decking. Many composite and PVC boards require 16 in on center or less for straight runs and 12 in for diagonal layouts; thinner wood boards also need closer joists. The manufacturer's instructions set the limit.
Gap boards to suit the material. Pressure-treated boards are often installed wet and tight, then shrink to leave a gap; kiln-dried wood and composites need a set gap, often 1/8 to 1/4 in.
Plan board lengths so butt joints land on joists and are staggered from row to row. Where two boards meet, you may need a doubled joist or blocking and extra screws.
Add 5 to 10% to the screw count for butt joints and dropped screws, and use fasteners rated for your decking and treated lumber.
This estimate covers boards, joists and face screws only. Ledger, rim joists, beams, posts, footings, joist hangers, stairs and railings are separate.

Structural safety: span tables and local code come first

A deck is a structure that carries people, furniture and sometimes snow, and deck failures cause serious injuries. Joist size and span, beam size, post and footing size, the ledger connection to the house, and guard height all have to meet your local building code. In the US, many jurisdictions base deck rules on the International Residential Code (section R507) and the American Wood Council's prescriptive deck guide (DCA 6).

Most areas require a permit and inspection for attached or raised decks. Use this calculator to estimate materials and compare options, then confirm the framing with your building department, a span table for your lumber species and grade, or a qualified deck builder or engineer.

Features

Deck area and rows of decking across the width
Board count for 8 to 20 ft boards, with adjustable waste
Joist count at 12, 16 or 24 in on center
Deck screw estimate (2 per board at each joist)
6 in or 4 in nominal boards and a choice of board gaps

Frequently Asked Questions

How many deck boards do I need for a 12x12 deck?

With 6 in boards (5.5 in actual) and a 3/16 in gap, a 12 ft wide deck needs 26 rows. Using 12 ft boards, each row takes one board, so you need 26 boards, or 28 with 5% extra for culls and cuts. That is 312 linear feet of decking. With no gap, as with wet treated lumber installed tight, you need 27 rows.

How many screws do I need per square foot of decking?

With 6 in boards and joists at 16 in on center, plan on about 3.5 screws per square foot, based on two screws per board at each joist. A 16 x 12 ft deck works out to 676 screws on 192 sq ft. At 12 in joist spacing it rises to about 4.6 per square foot (884 screws). Add 5 to 10% for butt joints and drops.

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

Divide the length in inches by the spacing and add one. For a 16 ft deck at 16 in on center, that is 192 ÷ 16 = 12, plus 1, for 13 joists. At 12 in on center you need 17, and at 24 in on center you need 9. The joist size needed for the span has to come from a span table and your local code.

What size gap should I leave between deck boards?

It depends on the material. Wet pressure-treated boards are often butted tight because they shrink as they dry. Kiln-dried wood and most composite boards need a gap, commonly 1/8 to 1/4 in, for drainage and movement, and composite makers usually specify it. A 3/16 in gap is a common middle value and the calculator's default.

Is 24 inch joist spacing OK for a deck?

Only if your decking is rated for it. Some thicker wood decking can span 24 in, but many composite, PVC and 5/4 wood boards are limited to 16 in on center, and diagonal layouts often need 12 in. Spacing joists too far apart makes boards feel bouncy and can void a warranty. Check the decking manufacturer's instructions and your local code.

Why is a 6 inch deck board only 5.5 inches wide?

Lumber is sold by nominal size, the rough size before drying and planing. A nominal 6 in deck board, such as a 5/4x6 or 2x6, is actually about 5.5 in wide, and a nominal 4 in board is about 3.5 in. Always calculate with the actual width, which the calculator does. Some composite boards differ slightly, so check the product sheet.

Last reviewed September 21, 2026