Deck Calculator

By Stephen Morrison, Editor Last reviewed October 30, 2026

Enter your deck's size and board details. You get the number of deck boards, joists and screws, for boards laid straight or on the diagonal.

Your deck

Length (along the house)

Boards run this way when laid straight

Width (out from the house)

Joists run this way

Board layout

StraightDiagonal

Board length

12 ft16 ft20 ft

More options: board width, gap, joists, price

Board width

5/4×6 and 2×6 are 5½ in actual

Gap between boards

From your decking's instructions

Joist spacing

in o.c.

Set by the layout; edit to override

Extra for waste

Price per board

You need

– deck boards, 16 ft

– sq ft · – linear ft of decking

Rows of boards–
Joists at 16 in o.c.– × – ft
Deck screws (about)–
Decking cost–

Shopping list

Joists only. Beams, posts, footings and ledger attachment depend on spans and local code, so they aren't calculated here.

Quick answer: a 16 × 12 ft deck with 5½ in boards laid straight needs 26 rows. That's 29 boards of 16 ft with 10% extra,13 joists at 16 in on center, and about 676 screws.

How the calculator works

Rows: width (in) ÷ (board width + gap), rounded up
Decking: rows × length (diagonal: area ÷ (board width + gap))
Boards: decking × (1 + waste%) ÷ board length, rounded up
Joists: length (in) ÷ spacing + 1, each as long as the width
Screws: 2 wherever a board crosses a joist

Diagonal decking uses closer joists (12 in) and more waste (15%) for the angled cuts. A diagonal board crosses a joist every spacing × √2 along its length.

Boards for common deck sizes

Straight layout, 5½ in boards, 3/16 in gap, 16 ft boards, 10% extra, joists 16 in on center.

Deck	Area	Boards	Joists	Screws
10 × 10 ft	100 sq ft	16	8	352
12 × 12 ft	144 sq ft	22	10	520
16 × 12 ft	192 sq ft	29	13	676
20 × 12 ft	240 sq ft	36	16	832
20 × 16 ft	320 sq ft	47	16	1,088

Tips for ordering decking

Pick a board length that matches your deck, so joints land on joists and offcuts are short. A 16 ft deck suits 16 ft boards.
For decks longer than your boards, stagger the butt joints across different joists.
Buy fascia and stair treads separately. They're often a matching board from the same line.
Check the decking's warranty conditions. Wrong joist spacing or fasteners can void it.

Common questions

How many deck boards for a 12 × 16 deck?

With 5½ in wide boards, a 3/16 in gap and 16 ft boards running the 16 ft side, you need 26 rows. That's 416 linear ft, or 29 boards with 10% extra.

What joist spacing do I need?

Decking makers such as ArmorGuard and Fiberon specify 16 in on center for residential decks with boards laid straight across the joists, and ArmorGuard says to reduce it to 12 in for diagonal decking. Always use your decking's own spec, and your local code for joist size and span.

How many screws per deck board?

2 at every joist the board crosses, which is what ArmorGuard requires and what this calculator counts. Hidden fastener systems have their own counts on the package.

How big a gap between deck boards?

It depends on the product and the temperature when you install, so check your decking’s instructions. Enter its gap under "More options". The calculator accepts fractions like 3/16.

Does this size the beams, posts and footings?

No. Those depend on spans, loads, soil and local code, and usually need a permit and inspection. Use your local building department’s span tables or a deck design guide for the framing below the joists.

Sources

Formulas and figures last checked against these sources onOctober 30, 2026. Spotted an error? Tell us and we'll fix it.

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