How to use the deck calculator
- Enter the deck's length (parallel to the house) and width (how far it sticks out). Deck boards run along the length, and joists run across the width.
- Choose the board size and the length you'll buy. Longer boards mean fewer butt joints but cost more per foot.
- Pick the joist spacing your decking requires. Check the manufacturer's spec for composite boards, especially on diagonal layouts.
Deck formulas
rows of boards = width × 12 ÷ (board width + gap)
boards per row = ⌈length ÷ board length⌉
joists = ⌈length × 12 ÷ spacing⌉ + 1
screws = rows × joists × 2
The calculator rounds each row up to whole boards, so any offcuts it counts can usually be reused on the next row. The waste allowance covers splits, warped boards and trimming the ends.
What this doesn't include
This estimates the decking surface and joist count. A full deck also needs beams, posts and footings, ledger board and flashing, joist hangers, rim joists, stairs and railings. Joist size depends on span: for example, in common US span tables 2×8 joists at 16 inches on center span roughly 12–13 ft in southern pine. Always check your local code and span tables, and pull a permit where required.
Deck board spacing
- Pressure-treated wood: often installed wet and tight (or with a nail-width gap), because it shrinks as it dries.
- Kiln-dried wood: about 1/8 inch.
- Composite and PVC: follow the maker's gap, often 3/16 inch, wider in heat.
Frequently asked questions
How many deck boards do I need for a 12x12 deck?
With 5.5 inch boards and 1/8 inch gaps, a 12 ft wide deck needs 26 rows. Using 12 ft boards, that is 26 boards, or about 29 with 10% extra.
How many screws do I need for a deck?
Two screws at every joist on every board. For a 12 × 12 deck with joists 16 inches apart, that is 26 rows × 10 joists × 2 = 520 screws, so about 350 screws per 100 sq ft.
What joist spacing do I need for a deck?
16 inches on center is standard for wood decking. Many composite boards require 12 or 16 inches, and diagonal decking usually needs 12 inches.