How many deck boards do I need?

Deck width in inches ÷ (board width + gap) gives the rows; rows × deck length is the linear feet; divide by the board length you buy. A 16 × 12 ft deck with 5.5 in boards needs about 26 rows, 416 linear feet, 26 boards of 16 ft.

How far apart should joists be?

Sixteen inches on center for wood decking laid square, twelve inches for most composite boards and for any decking laid diagonally. The manufacturer's instructions govern.

One under each post, with posts every 8 ft along each beam. This sheet estimates from the deck size; the actual count follows the beam span table and your local code, and a ledger-attached deck needs fewer.

Deck calculator

Form 56 · Materials estimateFigures checked 2026-09-04

US customary

Measurements

Deck length(ft)Along the boards

Deck width(ft)Across the boards, from the house out

What you need

Quantity · Amount

01Deck area16 × 12 ft192 sq ft
02Board rows at 5.5 in + 0.19 in gap26
03Decking, linear feet26 rows × 16 ft416 ft
04Boards of 16 ft$22 each26$572
05Joists at 16 in on centerspanning the 12 ft width13$260
06Footingsposts every 8 ft under 2 beams6$150

Total, structure$982

Ledger board, joist hangers, flashing, screws, railing and stairs are extra; screws run about 350 per 100 sq ft of decking.
Joist size and span, footing depth and ledger attachment are code matters: check your local requirements and pull the permit.

The material

Board width(in)A 5/4×6 or 2×6 board is 5.5 in actual

Gap between boards(in)3/16 in for wet treated lumber; 1/4 for dry

Board length sold(ft)Boards come 8, 10, 12, 16, 20 ft

Joist spacing(in)16 in for wood; 12 in for most composite

Prices — put in what you were quoted

Deck board($ per each)Per board of the length above

Joist($ per each)2×8 PT of the deck's width

Footing($ per each)Tube, concrete and hardware per footing

Total, structure26 deck boards$982

Where these figures come from

Gap 3/16 in for wet pressure-treated boards; joist spacing 16 in wood / 12 in composite: standard practice and manufacturer instructions.
Prices are typical US lumber-yard figures at the date on this sheet.

Type the deck's size, the width of the decking board and the joist spacing (16 in for wood, 12 for most composite). The sheet counts linear feet of decking and how many boards of your length that is, the joists, the beams and the footings, and prices them.

Worked examples

Deck 16 × 12 ft, 5/4×6 boards: 26 — boards of 16 ft, about $982 in structure.
Composite 20 × 14 ft, 12 in joists: 38 — boards of 16 ft, about $1,456 in structure.

How it is worked out

Rows = deck width (in) ÷ (board width + gap), rounded up. Linear feet = rows × deck length. Boards = linear feet ÷ board length, rounded up. Joists = deck length (in) ÷ spacing + 1. Footings = posts every 8 ft under each beam.

The figures behind it

Gap 3/16 in for wet pressure-treated boards; joist spacing 16 in wood / 12 in composite: standard practice and manufacturer instructions.
Prices are typical US lumber-yard figures at the date on this sheet.

Every figure here is an estimate. Materials vary with moisture, compaction and product; order a little extra and confirm quantities with your supplier.

Questions people ask

How many deck boards do I need?: Deck width in inches ÷ (board width + gap) gives the rows; rows × deck length is the linear feet; divide by the board length you buy. A 16 × 12 ft deck with 5.5 in boards needs about 26 rows, 416 linear feet, 26 boards of 16 ft.
How far apart should joists be?: Sixteen inches on center for wood decking laid square, twelve inches for most composite boards and for any decking laid diagonally. The manufacturer's instructions govern.
How many footings?: One under each post, with posts every 8 ft along each beam. This sheet estimates from the deck size; the actual count follows the beam span table and your local code, and a ledger-attached deck needs fewer.

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