How to use this deck calculator
- Measure the deck length in the direction the boards will run, and the width.
- Choose the board size and the length you will buy.
- Enter the joist spacing: 16 in on center is standard, 12 in is often required for composite or diagonal decking.
- Keep 10% extra for cuts and defects.
Example
A 20 × 12 ft deck with 5.5 inch boards needs 572 ft of decking, which is 36 boards (16 ft). It sits on 16 joists, 12 ft 0 in (3.66 m) long and needs about 832 deck screws.
What to know before you buy
Wood or composite?
Pressure-treated pine is the most affordable decking but needs sealing and can warp or crack. Cedar and redwood are naturally rot-resistant and attractive. Composite and PVC decking cost more up front but need very little maintenance and do not splinter.
Framing basics
Joists commonly run 16 inches on center and rest on beams supported by posts on footings. Joist size depends on the span: 2×8 joists typically span around 12 ft and 2×10 around 14–16 ft, depending on wood species and local code. Always check span tables for your area.
Deck boards (5.5 in wide, 16 ft long, 10% waste)
| Deck size | Deck boards | Linear feet of decking |
|---|---|---|
| 12 × 12 ft | 22 boards (16 ft) | 343 ft |
| 16 × 12 ft | 29 boards (16 ft) | 458 ft |
| 16 × 16 ft | 39 boards (16 ft) | 616 ft |
| 20 × 12 ft | 36 boards (16 ft) | 572 ft |
| 20 × 16 ft | 49 boards (16 ft) | 770 ft |
Formula
Rows of boards = width ÷ (board width + 1/8 in gap). Linear feet = rows × deck length. Joists = length ÷ spacing + 1. Screws ≈ 2 per board per joist.
Tips from the pros
- Choose board lengths that match the deck length (or half of it) to avoid waste and joints.
- Stagger butt joints over different joists and pre-drill near board ends.
- Use hidden fasteners or color-matched screws for composite decking.
Common mistakes to avoid
- Using the wrong joist spacing for composite boards, which causes sagging.
- Installing wet pressure-treated boards tight together; they shrink and leave wide gaps.
Frequently asked questions
How far apart should deck joists be?
16 inches on center for most wood decking; check the manufacturer for composite boards, which often require 12 inches.
How many screws per square foot of deck?
Roughly 350 screws per 100 sq ft with 16 in joist spacing.
Does this include framing like beams and posts?
No. Beams, posts and footings depend on the span tables in your local building code.
How many deck boards do I need for a 12x12 deck?
With 5.5 in wide boards, about 26 rows 12 ft long; with 10% waste that is 22 boards of 16 ft.
What size deck screws should I use?
2½ inch exterior deck screws for 5/4 boards and 3 inch screws for 2× decking.