tigtools

Deck Material Calculator

Enter your deck size and board dimensions to get a shopping list: boards, joists, beam lines, posts and boxes of screws. Spans are inputs, so use the limits from your local code or the timber supplier.

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Units
m

The direction the deck boards run.

m

The direction the joists run.

mm
mm
m
mm
m
m
%
17.28 m²
Deck area
28
Deck boards
132 m
Board length needed
13
Joists
6
Posts
715
Screws
  • 25 rows of boards × 1 board(s) per row = 28 boards of 4.8 m, waste included.
  • 13 joists, each 3.6 m long, plus 9.6 m of rim board along the two long sides.
  • 2 beam line(s) with 3 posts each: 6 posts.
  • 325 board-to-joist crossings: 715 screws, about 3 box(es).
This is a material estimate for a simple rectangular deck, not a structural design. Joist sizes, spans, post footings and fixings must follow your local building code and the manufacturer's instructions.

How to use it

  1. Choose metric or imperial units.
  2. Enter the deck length (along the boards) and width (along the joists).
  3. Enter the board width, gap and stock length you plan to buy.
  4. Set joist spacing, the maximum joist span and post spacing for your timber and local rules.
  5. Adjust screws per crossing and the waste allowance, then read the shopping list.

How it works

Boards run along the deck's length. The number of board rows is the smallest number of boards plus gaps that covers the width: rows = ⌈(width + gap) ÷ (board width + gap)⌉. Each row needs ⌈length ÷ board length⌉ boards; the waste allowance is added to the total and to the board length.

Joists run across the width at the chosen spacing, with one at each end: joists = ⌈length ÷ spacing⌉ + 1, each as long as the deck is wide. Beam lines are added so that no joist spans further than the maximum span; a ledger on a building replaces one of them. Each beam needs a post at each end and at least every maximum post spacing.

Screws are counted per crossing of a board over a joist (two per crossing is common for face screws), plus waste, and rounded up to whole boxes. Hidden fastening systems use clips instead; enter their count per crossing. The estimate ignores stairs, railings, picture framing, blocking, footings and hardware such as joist hangers.

Examples

4.8 × 3.6 m deck

140 mm boards with 5 mm gaps, 4.8 m stock, joists at 400 mm, 10 % waste: 25 rows, 28 boards, 13 joists of 3.6 m and 715 screws (3 boxes of 350). Freestanding with a 3 m joist span it needs 3 beam lines and 9 posts.

16 × 12 ft deck on a ledger

5.5 in boards, 3/16 in gaps, joists 16 in on centre and a 12 ft span: 26 board rows, 13 joists, one beam line with 3 posts at 8 ft spacing.

Frequently asked questions

Which way do deck boards run?

Usually at right angles to the joists. In this calculator the boards run along the deck length and the joists across the width; swap the two numbers if your layout is the other way round.

How much waste should I allow?

5–10 % is common for a straight layout with boards that match the deck length; allow 15 % or more for diagonal boards, many cut ends or a lot of defects in the timber.

Does the calculator check if my joists are strong enough?

No. It uses the spans you enter. Allowed spans depend on timber species, grade, size, loads and local code, so take them from span tables or your building authority.

Why do I get more joists than length ÷ spacing?

Because a joist is needed at both ends. A 4.8 m deck with joists every 400 mm has 12 spaces and therefore 13 joists.

Privacy

All calculations run in your browser. Nothing you enter is sent anywhere.