Decking Calculator

Decking Calculator

Plan the materials for a timber or composite deck. Enter the deck size, the board width and gap, the length of boards you can buy and the joist spacing, and get the number of deck boards, joists and screws.

Boards, joists & screwsTimber or compositeMetric & imperialLast updated: November 3, 2026

Calculate decking

m
m
mm
Actual width: 120/145 mm · US 5/4×6 = 5½ in
mm
mm
400 mm / 16 in standard · 300 mm / 12 in for composite or diagonal
%

Decking formulas

Rows of boards

Rows = ⌈deck width ÷ (board width + gap)⌉

Boards

Boards = ⌈rows × deck length × (1 + waste%) ÷ board length⌉

End joints must land on a joist, so offcuts are often unusable; 10% waste is typical for a straight lay and 15% for diagonal.

Joists and screws

Joists = ⌊deck length ÷ spacing⌋ + 1  ·  Screws = rows × joists × 2

Joists run across the boards, so each is as long as the deck is wide. Two screws (or hidden clips) go into every joist a board crosses.

Worked examples

4 m × 3 m deck, 145 mm boards, 5 mm gap, 3.6 m lengths
  1. Rows = ⌈3 ÷ 0.150⌉ = 20; run = 20 × 4 = 80 m
  2. +10% = 88 m ÷ 3.6 = 24.4 → 25 boards
  3. Joists = ⌊4 ÷ 0.4⌋ + 1 = 11 joists × 3 m; screws = 20 × 11 × 2 = 440
16 × 12 ft deck, 5½ in boards, ⅛ in gap, 12 ft lengths
  1. Rows = ⌈144 ÷ 5.625⌉ = 26; run = 416 ft
  2. +10% = 457.6 ft ÷ 12 = 39 boards
  3. Joists at 16 in = 13 × 12 ft; screws = 676

Joist spacing guide

DeckingMax joist spacing
Softwood 28 mm / 5/4 in boards400–450 mm (16 in)
Softwood 32–38 mm / 2× boards500–600 mm (24 in)
Composite boards300–400 mm (12–16 in), check the brand
Diagonal boards (any)300 mm (12 in)

Building tips

  • Leave a 5 mm (⅛–¼ in) gap between boards for drainage and expansion.
  • Use stainless or coated screws rated for treated timber; ordinary screws corrode.
  • Stagger end joints across different joists so they don’t line up.
  • Add a membrane and gravel under ground-level decks to stop weeds.

Frequently asked questions

How many deck boards do I need?

Divide the deck width by the board width plus gap to get rows, multiply by the deck length for total run, add 10% and divide by the board length. A 4 × 3 m deck with 145 mm boards needs 25 boards of 3.6 m.

How many screws per deck board?

Two per joist. A 3.6 m board over 400 mm joists crosses 10 joists, so it needs 20 screws.

What joist spacing do I need?

400 mm (16 in) centres is standard for timber boards; composite and diagonal layouts usually need 300 mm (12 in).

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Results are estimates. Beam, joist and post sizes for raised decks must meet local building rules.

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