Sawhorse Calc

Deck calculator

Enter the deck size, board width and joist spacing. You get the deck boards to buy, the number of joists, the screws, and the gap to leave between boards.

Deck, boards and joists

Starting figures are typical. Type the ones on your product or quote if you have them.

Cost (optional)

29 deck boards

Deck area, sq ft
192.0
Rows of boards
26
Joists
13 at 12.0 ft
Deck screws
676
Gap between boards
3/16 in

How to work it out

  1. Measure the deck's length along the house and its depth out from the house. The boards run along the house, across the joists.
  2. Enter the real board width. A 5/4 by 6 or a 2 by 6 deck board is 5.5 in wide.
  3. Set the gap. Wood is often gapped about 1/8 to 1/4 in, depending on how wet it is; composite boards have a gap set by the maker, often 3/16 in.
  4. Enter the joist spacing. 16 in is common; some composite boards and diagonal layouts need 12 in.

Quick reference

ItemTypical
Deck board width, 5/4 × 6 or 2 × 65.5 in
Gap between boards1/8 to 1/4 in, or as the maker says
Joist spacing16 in, or 12 in for diagonal or some composites
Screws2 per board at every joist

These are common starting points. Your site, climate and supplier may call for something different.

A worked example

A deck 16 ft along the house and 12 ft deep is 144 in from the house out. With 5.5 in boards and a 3/16 in gap, that is 144 ÷ 5.6875 = 26 rows of 16 ft boards, or 29 boards with 10% extra. Joists 16 in apart across 16 ft come to 13, and two screws at each crossing make 676 screws.

What this leaves out

This counts the deck surface and joists only. Beams, posts, footings, the ledger board and its flashing, rim joists, railings and stairs depend on span tables and local code. Most places need a permit for a deck attached to a house or above a set height, so check with your building department and follow the board maker's install guide.

Common questions

How many deck boards do I need?

Divide the deck depth by the board width plus the gap to get the rows, multiply by the boards per row, then add about 10%. A 16 by 12 ft deck of 16 ft boards needs 26 rows, so about 29 boards.

How many joists do I need for a deck?

Divide the deck length by the joist spacing and add one. A 16 ft deck at 16 in spacing needs 13 joists, not counting rim joists.

How many screws do I need for a deck?

Two screws at every joist for every board. A 16 by 12 ft deck with 26 rows and 13 joists needs about 676 screws, so a box of 750 or more.

What gap should I leave between deck boards?

About 1/8 to 1/4 in for dry wood. Wet pressure-treated boards are often laid tight because they shrink. For composite, use the gap in the maker's guide.

Should deck boards be the full length of the deck?

It looks best and lasts longer if they are. When boards must join, stagger the joints and make each one land on a joist, often with a doubled joist under it.

Doing this job for a customer? Turn your numbers into a free written estimate on QuoteToPaid.

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