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MaterialCalc — Deck Board & Joist Calculator

Estimate prepared 09/01/2026. Estimates only; verify against your plans before ordering.

Deck Board & Joist Calculator

The dimension the joists span.

Over 30 in and the code wants guardrails.

%

Result

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Two numbers decide a deck material order: how many rows of decking cross the width, and how many joists run under them. Both come from spacing, not from area, so a deck one foot wider can need a whole extra row of boards. This counts the boards, joists, beam plies, posts and hangers precisely.

Assumptions used

These are the numbers behind the result. Where one of them is soft, the cost range widens rather than pretending to a precision the data does not have.

Lumber waste allowance
0.1

Cut-offs, crooked stock and the boards you set aside.

Standard industry overage — Industry standard, varies by supplier, checked 09/01/2026

Concrete waste allowance
0.1

Covers spillage, over-excavated holes and forms that bulge. Footings dug by hand run higher than forms.

Standard industry overage — Industry standard, varies by supplier, checked 09/01/2026

Deck board coverage width
5.5 in

A nominal 5/4x6 or 2x6 board covers 5.5 in. Gaps come out of the waste allowance.

Home Depot / Lowe's published product data — Published product data, checked 09/01/2026

Gap between deck boards
0.1875 in

Drainage and expansion. Composite wants a wider gap than wood — check the maker.

Trex and TimberTech installation guides — Industry standard, varies by supplier, checked 09/01/2026

Deck joist spacing
16 in o.c.

Composite decking laid diagonally usually requires 12 in instead.

International Residential Code (2021) — Published product data, checked 09/01/2026

Deck beam post spacing
8 ft

IRC deck provisions (DCA 6) — Industry standard, varies by supplier, checked 09/01/2026

Deck screws per square foot
3.5 screws/ft²

Fastener manufacturer coverage tables — Industry standard, varies by supplier, checked 09/01/2026

Prices verified 09/01/2026 (US national average, big-box retail shelf price and regional ready-mix quotes). Regional adjustment: Blended from RSMeans city cost index bands and BLS construction wage differentials.

The formula

Board rows = width ÷ (5.5 in coverage + 3/16 in gap), rounded up. Boards = rows × the number of 16 ft lengths needed to cover the deck length. Joists = (length ÷ 16 in) + 1. Beam plies = two boards per beam run. Posts sit at 8 ft centers.

Worked examples

16x12 deck, boards running the 16 ft length

Deck length: 16 Deck width: 12 Height above grade: 36 Decking material: pt Waste: 10

25 rows across the 12 ft width, one 16 ft board per row. 13 joists on 16 in centers, 6 hangers per beam bay.

20x16 deck needing joist splices

Deck length: 20 Deck width: 16 Height above grade: 36 Decking material: pt Waste: 10

At 16 ft of width the joists still make it in one length, but the 20 ft run means two board lengths per row and a butt joint that must land on a joist.

12x10 small deck

Deck length: 12 Deck width: 10 Height above grade: 18 Decking material: pt Waste: 10

120 square feet, no railing required below 30 in, and everything fits in single lengths. This is the cheapest deck geometry per square foot.

Choosing materials

  • A nominal 6 in deck board actually covers 5.5 in, and with a 3/16 in gap each row takes 5.69 in. Working from the nominal size gives you a row short on anything over about 10 ft wide.
  • Order boards that span the full deck length if you can. Butt joints between boards must land on a joist, need doubled blocking, and are where water sits.
  • Joists span the width, so the width is what sets the joist size. 2x8 gets you about 12 ft in southern pine at 16 in on center; beyond that you need 2x10 or a mid-span beam.
  • Two hangers per joist — one at each end. Buy the hanger sized for the joist, use the structural screws or nails the manufacturer specifies, and fill every hole.
  • Run a beam under the joists rather than hanging everything off ledgers where you can. A dropped beam with joists over the top is stronger and easier to build level.

Common questions

How many deck boards do I need?

Divide the deck width by 5.6875 in — the 5.5 in a board actually covers plus a 3/16 in gap — and round up for the row count. Then multiply by how many board lengths it takes to cover the deck length. A 12 ft wide deck needs 26 rows.

How far apart should deck joists be?

Sixteen inches on center for wood decking. Twelve inches for diagonal layouts and for many composite products, which sag between joists at 16 in. The manufacturer instruction overrides the general rule.

What gap should I leave between deck boards?

About 3/16 in for kiln-dried or composite boards. Wet pressure-treated lumber shrinks as it dries, so those can be installed tight and will open up on their own to roughly the right gap.

How many joist hangers do I need?

Two per joist, one at each end, wherever joists hang from a ledger or beam face. Joists sitting on top of a dropped beam only need hangers at the ledger end, plus hurricane ties at the beam.

Should deck boards run the length or the width?

Boards run perpendicular to the joists, so the direction you want the boards decides which way the joists run. Running boards the long direction of the deck usually means fewer butt joints, which looks better and lasts longer.

How do I handle butt joints in decking?

Land every joint on a joist and double that joist or add blocking so each board end has its own bearing and fasteners. Stagger the joints across rows so they do not line up in a seam across the deck.

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