Deck board and joist calculator

Boards, joists and fasteners for a rectangular deck. Board counts are shown for every stock length, both ways of cutting them, and whether the butt joints will land on a joist.

Decking

26 boards

16 ft (4.9 m) long, one per row across 26 rows

A 16 ft (4.88 m) × 12 ft (3.66 m) deck, 192.0 sq ft (17.84 m²). 13 joists at 16 in (406 mm) centres, each 12 ft (3.66 m) long. 676 deck screws at 2 per board per joist, or about 338 hidden clips.

The last row is only 1 13/16 in (46 mm) wide. Adjust the gap slightly, let the boards overhang the rim, or rip the first and last rows to share the difference.

Boards by stock length

"One row at a time" cuts every row from its own boards and throws the offcuts away. "Offcuts reused" lets the leftover from one row start another, plus the waste allowance. Real decks fall between.

Stock length	Per row	One row at a time	Offcuts reused	Joints on joists
8 ft (2.4 m)	2	52	52	Yes
10 ft (3.0 m)	2	52	46	No
12 ft (3.7 m)	2	52	39	Yes
14 ft (4.3 m)	2	52	33	No
16 ft (4.9 m)	1	26	26	Yes
20 ft (6.1 m)	1	26	23	Yes

How to use it

Enter the deck's size with the length measured along the boards and the width across them, so the width is the direction the rows stack in and the joists run. Enter the board's actual width and the gap you will leave, then the joist spacing from your framing plan. The switch at the top changes every field between feet and inches and metric, and the stock lengths change with it: even feet from 8 to 20 ft, or 2.4 to 5.4 m.

The method

Rows: n boards and n − 1 gaps must cover the width, so n = ⌈(width + gap) ÷ (board + gap)⌉. The last row is whatever width is left.
Joists: one at each end of the length and one at every spacing between, ⌈length ÷ spacing⌉ + 1. If the length is not a whole number of spacings, the last bay is shorter.
Boards, one row at a time: ⌈length ÷ stock⌉ × rows.
Boards, offcuts reused: ⌈rows × length × (1 + waste) ÷ stock⌉, never more than the one-row-at-a-time figure.
Screws: rows × joists × screws per crossing. Hidden clips: roughly rows × joists, one per board edge at each joist.

Butt joints between boards must land on a joist, usually on a doubled joist or a wide one so each board end has bearing. A stock length that is a whole number of joist spacings (8 ft at 16 in centres is six spacings) makes that easy. A 10 ft board at 16 in centres is 7.5 spacings, so each one needs trimming to land its end on a joist.

A worked example

The default deck is 16 × 12 ft with 5 1/2 in boards and 3/16 in gaps. One board plus one gap is 5.6875 in. (144 + 0.1875) ÷ 5.6875 = 25.35, so 26 rows, and the last row is 144 − 25 × 5.6875 = 1 13/16 in wide. That thin last row is the first thing to fix: tweaking the gap, letting the outer board overhang the rim joist, or ripping both edge rows gives a better edge.

The boards run 16 ft, so 16 ft boards need one per row: 26 boards. With 12 ft stock, each row needs two boards and wastes 8 ft, so 52 boards cut row by row, or 39 if every 8 ft offcut is used to start another row. Joists at 16 in centres along 16 ft: 12 bays and 13 joists, each 12 ft long. Screws: 26 rows × 13 joists × 2 = 676.

Common sizes

Board widths: nominal 6 in lumber (5/4 × 6 or 2 × 6) is 5 1/2 in actual; nominal 4 in is 3 1/2 in. Metric decking is often 90, 120, 140 or 145 mm.
Joist spacing: 16 in (406 mm) or 12 in (305 mm) on centre in the US; 400 or 450 mm is common in metric countries.
Stock lengths: even feet from 8 to 20 ft; metric yards commonly carry 2.4 to 5.4 m in 0.6 m steps.

Where this stops being reliable

Structure. This is a count of materials, not a design. It says nothing about whether a joist size will span, how beams, posts and footings are sized, or how the deck attaches to the house.
Picture frames and borders. A border board around the edge changes the field size and adds mitred boards. Count the border separately and reduce the deck size accordingly.
Diagonal decking. Boards at 45° need closer joist spacing and produce much more waste; add 15% or more.
Blocking, rim joists and ledgers. Only the common joists are counted. The rim or band joists, ledger, blocking and hardware come from the framing plan.
Stairs and railings. Not included.

Common questions

How far apart should deck joists be?

16 in (406 mm) on centre is the most common spacing for decking laid square to the joists, and 12 in (305 mm) is typical for diagonal decking and for many composite and PVC boards. Some thicker lumber allows 24 in. The limit comes from the decking, not the joists: in the US, the International Residential Code sets maximum joist spacing by decking type and angle, and composite boards carry their own limit in the manufacturer’s instructions.

What gap should I leave between deck boards?

It depends on the material. Kiln-dried wood is commonly gapped around 1/8 in (3 mm) or a little more. Wet pressure-treated boards are often laid tight or nearly so, because they shrink as they dry. Composite and PVC boards have a gap specified by the maker, sometimes varying with the temperature at installation. Hidden-fastener systems set the gap for you.

What is the actual width of a 5/4 × 6 or 2 × 6 deck board?

Both are nominally 6 in but actually 5 1/2 in (about 140 mm) wide in US dimensional lumber. Composite boards are commonly around 5 1/2 in as well, but check the product sheet. Enter the actual width, not the nominal size, or the row count will be wrong.

How many screws per deck board?

Two per board at every joist is the normal pattern for boards up to about 6 in wide, with three for wider boards. That is what this page counts. Use screws rated for the decking and for contact with treated lumber, and add a box for drops and ends.

Can this calculator size my joists and beams?

No, deliberately. Joist and beam sizes, spans, post spacing, footings and the ledger connection to the house are structural decisions governed by local building codes, the species and grade of lumber and the loads involved. Use your local code’s span tables or a designer. This page counts the materials once the framing layout is known.