Deck Calculator
Deck boards, joists, and screws for a rectangular or L-shaped deck — counted the way the lumber yard sells them, with waste built in.
Split the L into two rectangles; boards are counted per section.
You need
Enter dimensions
| Detail | Amount |
|---|---|
| Deck area | — |
| Linear feet (with waste) | — |
| Joists (16″ OC default) | — |
| Deck screws (2½″) | — |
Treated 5/4×6 commonly runs $0.80–$1.50/lin ft; composite $2.50–$5+.
How to Calculate Decking
Deck boards are counted in rows across the width, then bought as linear feet (or whole boards at your deck's length):
- Rows: deck width in inches ÷ (board width + gap), rounded up.
- Linear feet: rows × deck length, plus 5–15% waste depending on the layout.
- Joists: deck length in inches ÷ spacing, rounded up, plus one.
- Screws: two per joist crossing — about 350 per 100 sq ft at 16″ spacing.
Decking formulas
Rows = ceil(width ÷ (board width + gap))
Linear ft = rows × length × (1 + waste)
Joists = ceil(length ÷ spacing) + 1
Worked example — 12 × 16 ft deck, 5.5″ boards, ¼″ gap: 144″ ÷ 5.75″ = 26 rows × 16 ft = 416 lin ft × 1.10 = 458 linear feet — 29 sixteen-foot boards, on 13 joists, with ~672 screws.
Boards Needed for Common Deck Sizes
5.5″ boards, ¼″ gap, 10% waste, boards run the long dimension:
| Deck size | Rows | Linear feet | Boards (at deck length) |
|---|---|---|---|
| 10 × 10 ft | 21 | 231 | 24 |
| 12 × 12 ft | 26 | 344 | 29 |
| 12 × 16 ft | 26 | 458 | 29 |
| 16 × 20 ft | 34 | 748 | 38 |
Tips Before You Buy
- Buy boards the length of your deck when possible — butt joints mid-span look patchy and need doubled joists.
- Crown side up, bark side down on wood boards; cup direction sheds water.
- Check your framing first. Composite decking on 24″ joists feels like a trampoline — most brands require 16″ or less.
- Hidden fasteners vs. screws: clips give a clean face but cost 2–3× and are slower; coated screws are the workhorse choice.
- Permits matter for decks. Most jurisdictions require them above 30″ height — the AWC DCA 6 guide is the standard reference inspectors use.
Frequently Asked Questions
How many boards for a 12×16 deck?
26 rows — 29 sixteen-foot boards with 10% waste (about 458 linear feet).
What gap between boards?
⅛–¼″; wet treated lumber can be butted tight since it shrinks. Composites: follow the manufacturer's spec.
How far apart are deck joists?
16″ on center standard; 12″ for diagonal decking and many composites; span limits per AWC DCA 6.
How many screws per board?
Two per joist crossing — roughly 350 per 100 sq ft, about 4.5 lb.
Which way should boards run?
Perpendicular to the joists — typically parallel to the house. Long-dimension boards make the deck look bigger.
Actual size of a 5/4×6?
About 1″ × 5.5″. A 2×6 is 1.5″ × 5.5″ — same coverage, stiffer feel.
References
- American Wood Council — DCA 6: Prescriptive Residential Wood Deck Construction Guide: awc.org
- Simpson Strong-Tie — deck framing and fastening guides: strongtie.com
- Trex — composite decking installation requirements: trex.com