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Deck Calculator

Enter deck size and board width for deck board pieces, joists, posts, concrete bags and all the hardware.

Deck dimensions

Deck board size
Gap between boards
Railing linear feet
Waste allowance15%

192 sq ft · 40 boards (12 ft each) · 13 joists · 6 posts

You'll need
40 boards
  • pieces at 12 ft length
  • 12 bags of 60 lb bags for footings

Materials list

Joist hangers26 hangers
2 hangers per joist · match hanger size to joist lumber size
Structural screws100 screws
Structural screws for ledger, headers and post connections — use code-approved connectors
Concrete bags (60 lb)12 bags
2 bags per post hole · mix in the hole dry or pre-mix · allow 24 h before framing
Post bases6 bases
Standoff post bases keep wood off concrete and are required by most codes
Deck screws2 lb
3 in coarse-thread deck screws · ~1 lb per 100 sq ft · use stainless for cedar/redwood
Railing posts10 posts
1 post per 4 lin ft · typically 4×4 at 36 in height
Rail kit (top + bottom rail)5 sections
8 ft sections · includes top rail, bottom rail and balusters
Deck stain / sealer1 galoptional
~250 sq ft per gallon · apply first season after install

Tools you'll want

Buy-once items — skip any you already own.

Circular saw
For cutting boards to length and ripping if needed.
Post hole diggeroptional
Manual clamshell for small jobs; rent a gas auger for many posts.
Impact driver
A corded or brushless impact is far faster than a standard drill for deck screws.

Where to buy (optional)

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Estimates only — verify quantities before buying.

What actually affects how much deck material you need

Board spacing and layout direction change material counts more than deck size alone. A 1/8 to 1/4 inch gap between boards is standard for drainage and seasonal expansion — composite decking needs less gap than wood, since it moves less with temperature. Running boards perpendicular to the joists (the usual layout) is the most efficient use of material; a diagonal pattern looks striking but increases waste by roughly a third because of the angled end cuts. Joist spacing is governed by building code, not preference: 16 inches on-center is standard for most decking, but some composite products require 12-inch spacing to prevent sagging, so always check the manufacturer's span rating before finalizing joist count. Footing depth follows the local frost line, not a fixed number — footings that don't go below frost depth can heave and shift over winter, so check with your municipality before digging.

How to calculate deck materials

Deck board count is driven by the number of rows across the deck width:

rows = ceil(deck width ÷ (board width + gap))
linear feet = rows × deck length × (1 + waste%)
board pieces = ceil(linear feet ÷ 12)
joists = floor(deck length ÷ 1.333) + 1  (16 in o.c.)
posts = max(4, ceil(deck length ÷ 8) × 2)

Post spacing of 8 ft is standard for most beam/post systems. Concrete: 2 bags of 60 lb per post (10 in diameter hole, 18–24 in depth depending on frost line).

Common mistakes

  • Skipping permits — most decks over 30 in from grade require a permit and inspection.
  • Not accounting for crown direction — install boards crown-up so water runs off, not pools.
  • Using indoor-rated lumber — always use pressure-treated, cedar or composite for exterior decking.
  • Undersizing post footings — check your frost depth; footings must go below frost to prevent heaving.

Frequently asked questions

How many deck boards do I need for a 16x12 foot deck?+

A 16 × 12 ft deck using 2×6 boards (5.5 in actual) with 1/4 in gaps needs about 25 rows across the 12 ft width. At 12 ft board lengths, that is 25 pieces before waste. Adding 15% waste brings the total to about 29 pieces.

How far apart should deck joists be?+

Deck joists are typically spaced 16 in on-center for 5/4 or 2× decking. For 1× or thinner composite boards, 12 in spacing is recommended. A 16 ft deck needs about 13 joists at 16 in o.c. (including both rim joists).

How many concrete bags do I need per deck post?+

Use 2 bags of 60 lb concrete per post hole for typical 4×4 posts in 10 in diameter holes. For larger 6×6 posts or in frost-heave areas, use 2 bags of 80 lb or 3 bags of 60 lb per post. Allow concrete to cure 24 hours before framing.

What size deck posts do I need?+

For a low-to-ground deck under 6 ft, 4×4 posts on 8-ft centers are typical. For elevated decks over 6 ft, or for any beam span over 12 ft, use 6×6 posts. Always check local building codes, which often require specific post sizes based on deck load and height.

How much does it cost to build a deck per square foot?+

Pressure-treated lumber decks typically cost $15–25 per sq ft for DIY material costs. Cedar decks run $25–40/sq ft. Composite decking like Trex is $35–60/sq ft. These are material costs only — contractor labor adds $20–40/sq ft depending on complexity and region.

Do I need a permit to build a deck?+

In most jurisdictions, a permit is required for decks over 30 in above grade or attached to the house. Permits ensure the deck is engineered correctly for load, frost depth, and connection to the house rim joist. Always check with your local building department before starting.

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Reviewed by the RenoSheets team for calculation accuracy. Method: row count × deck length with adjustable waste; joists at 16 in o.c.; posts at 8 ft centers. Last updated 2026-05-26. Estimates only — verify with your local building code before purchasing.