How Many Deck Boards Do I Need?

Start with the direction of the boards

Most decks have boards running parallel to the house, across joists that run out from the house. In this guide the deck length is the side the boards run along and the deck depth is the side the boards are laid across. If your boards run the other way, swap the two measurements.

Step 1: count the rows

Each row of decking covers one board width plus one gap. Use the actual board width. Most nominal 6 inch boards measure 5.5 inches, and a 3/16 inch gap is common for wood. That makes a module of 5.6875 inches.

A 12 foot (144 inch) deep deck needs 144 ÷ 5.6875 = 25.3 rows, rounded up to 26. The last row is usually ripped to width.

Step 2: convert rows to linear feet and boards

Multiply rows by the deck length to get the total decking length. On a 16 foot long deck, 26 rows is 416 linear feet. Add a waste allowance, then divide by the board length:

• 416 × 1.10 = 457.6 feet.
• 457.6 ÷ 16 = 28.6, so buy 29 boards of 16 feet.

Quick reference for common deck sizes

These counts use 5.5 inch boards, a 3/16 inch gap, boards as long as the deck and a 10 percent allowance:

When the deck is longer than the boards you can buy, the board count goes up because each row needs more than one board. The deck calculator handles that from the board length you enter.

Step 3: count the joists

Joists run under the boards at a fixed spacing, usually 16 inches on center, sometimes 12. Divide the deck length by the spacing and add one for the starting joist. A 16 foot deck at 16 inches is 192 ÷ 16 = 12 spaces, so 13 joists. A 20 foot deck at 12 inches needs 21 joists. Rim joists, doubled joists at splices, blocking and beams are extra.

Step 4: estimate screws

Face-screwed decking usually gets two screws at every joist. Rows × joists × 2 gives the count: 26 rows × 13 joists × 2 = 676 screws for the 16 by 12 deck. Deck screws are sold by the pound or by box count, so check the count per box. Hidden fastener systems use clips instead, at a rate set by the maker.

Layout tips that save boards

• Match board length to the deck. A 16 foot deck with 16 foot boards wastes almost nothing in length. A 17 foot deck with 16 foot boards can waste a lot.
• Stagger butt joints and land every joint on a joist, often a doubled joist.
• Use a breaker board on long decks to avoid scattered joints.
• Picture-frame borders and diagonal patterns need more waste, often 15 percent or more.

Worked example: a deck longer than your boards

A 24 by 12 foot deck needs 26 rows of 5.5 inch boards with a 3/16 inch gap. If the longest boards you can get are 16 feet, each row needs 24 feet of board:

• Total decking: 26 rows × 24 feet = 624 linear feet.
• With 10 percent waste: 686.4 feet ÷ 16 = 42.9, so 43 boards.
• A common layout is one 16 foot and one 8 foot piece per row, alternating the side the joint falls on so joints are staggered.

Another option is to buy 12 foot boards and run two per row with every joint on the same doubled joist, often with a contrasting breaker board. That gives a clean look and makes the board count simple: 26 rows × 2 = 52 boards plus spares.

Fascia, borders and stair treads

Several parts of a deck use decking or matching boards but are not part of the surface count:

• Fascia boards cover the rim joists. Measure the exposed perimeter and divide by board length.
• Picture-frame borders run around the edge of the deck surface. Add the border length and expect more waste at the mitered corners.
• Stair treads are often two boards per tread. A stair with 3 treads 4 feet wide needs 6 pieces of 4 feet, which can come out of two 12 foot boards. The stair calculator gives the tread count.

Fasteners in more detail

Two screws per board at every joist is the common pattern for face-screwing. Deck screws are packed by count or by pound, and the box usually states roughly how many square feet it covers. Use corrosion-resistant fasteners rated for the lumber you are using, since treated wood can corrode some metals. Hidden fastener systems use one clip per board at each joist, with special clips at the start and end rows; the maker lists coverage per box.

Before you buy: a checklist

• Confirm the actual board width and the gap recommended by the maker.
• Check the joist spacing the decking requires, especially for composite or diagonal layouts.
• Decide on board lengths and joint layout before ordering.
• Add fascia, borders and stair treads as separate lines.
• Buy fasteners suited to treated lumber or to the composite product.

Turning boards into a budget

Decking prices differ greatly between pressure-treated wood, cedar and composite, and between brands. The deck calculator accepts your price per board and multiplies it by the board count. Framing lumber, hardware, footings, railings and labor are separate lines to add after that.

Common mistakes

• Using the nominal board width (6 inches) instead of the actual width (5.5 inches), which undercounts rows.
• Forgetting the gap, or using a gap that does not suit the decking material.
• Counting square feet of boards instead of linear feet.
• Assuming 16 inch joist spacing for composite boards that require 12 inches.

Frequently asked questions

How many linear feet of decking per square foot?

With 5.5 inch boards and a 3/16 inch gap, each square foot of deck takes about 12 ÷ 5.6875 = 2.11 linear feet of board.

Do I need a permit to build a deck?

Many jurisdictions require a permit for decks attached to a house or above a certain height. Footings, ledger attachment and railings are commonly inspected. Check with your local building department.

Wood or composite?

The math is the same. Composite usually costs more per foot, has stricter gap and joist spacing rules and needs less maintenance. Enter the width and length of the product you choose.