How far apart should deck joists be?

Joists are typically 16 inches on-center, or 12 inches for composite decking or diagonal patterns.

How many screws per board?

Plan for two screws at every joist crossing, which usually means about 350 screws per 100 square feet.

Deck Material Calculator

Estimate decking boards, joists and fasteners for a deck.

100% free to useNo sign-up requiredImperial & metric units

How much decking material do I need?

To find decking material, divide the deck area by the coverage of one board, including the gap. A 16 ft by 12 ft deck is 192 square feet and needs about 24 boards of 16 ft, 5.5 inch decking, plus joists at 16 inches on-center and roughly 3.5 screws per square foot.

Inputs

Deck lengthft

Deck widthft

Board width

Board length

Joist spacing

Railing lengthft

Waste allowanceApplied to material quantities

Results

Deck Boards

Add waste for cuts

Joists

Footings

Posts + concrete piers

Deck Screws

672

Deck Area

192 sq ft

Rail Posts

Balusters

120

Estimates update instantly as you type. Confirm against local code before ordering materials.

A deck takeoff has more moving parts than it looks: boards, joists, and fasteners all scale differently with the footprint. This calculator turns deck length and width into board counts based on your chosen board size and length, plus joist counts at your spacing and an estimate of deck screws.

Board orientation, picture-frame borders, and diagonal patterns all change the count, so the result is a strong baseline you refine for your specific layout. Apply the waste allowance to cover cut-offs and defective boards, which add up fast on patterned or angled decks.

How deck materials are estimated

Board count is the deck area divided by the coverage of a single board, including the gap between boards. Joist count is driven by the deck width and the on-center spacing.

Boards = Deck Area ÷ (Board Width + Gap) × Board Length

Decking board coverage

Approximate boards needed per 100 square feet of deck.

Board width	Board length	Coverage each	Boards / 100 sq ft
5.5 in	12 ft	5.75 sq ft	18 boards
5.5 in	16 ft	7.67 sq ft	14 boards
5.5 in	20 ft	9.58 sq ft	11 boards
3.5 in	16 ft	5.00 sq ft	20 boards

Composite and PVC decking usually require 12 inch joist spacing; check the manufacturer span table.

How to use it

1Enter the deck length and width in feet.
2Choose your board width and length.
3Set the joist spacing (16 in on-center is standard).
4Review the board, joist and fastener estimates.

Key terms explained

On-center spacing: The distance between joist centers, commonly 16 inches; composite decking often requires 12 inches.
Coverage width: The actual face width of a board plus the gap between boards, used to count rows.
Picture frame: A border board around the deck perimeter that adds material and cuts.
Fastener: Screws or hidden clips that secure decking. Hidden systems change the count from face-screwing.

Worked examples

Backyard deck

A 16 ft × 12 ft deck with 6 in boards needs about 39 sixteen-foot boards.

Small platform

A 10 ft × 10 ft platform needs roughly 21 boards plus 8 joists at 16 in centers.

Pro tips from the field

1Composite and PVC decking usually require 12 inch joist spacing, check the manufacturer span table.
2Order a few extra boards from the same lot to match color and grain for future repairs.
3Diagonal and herringbone layouts can add 15 percent or more in waste, raise the allowance.
4Hidden fastener systems use clips per square foot, not the face-screw count, plan accordingly.

Common mistakes

Forgetting the gap between boards in coverage math.
Skipping a 10% waste allowance for cuts and defects.
Using interior fasteners outdoors instead of coated deck screws.

Where it gets used

Material orders

Build a clean board, joist, and fastener list before the lumberyard run.

Client estimates

Quote decking material quickly with a defensible quantity.

Composite upgrades

Recheck joist counts when switching to a 12 inch composite layout.

Frequently asked questions