Deck Calculator: How Many Deck Boards, Joists and Screws Do You Need?

A deck is a set of rectangles, so the arithmetic should be straightforward. Even so, a careless count costs money if you buy the wrong amount, so this page is written to do the arithmetic for you.

Our deck calculator is designed for the general builder, and covers the most common deck build styles — a standard flat deck with 5.5-inch-wide decking boards on 16-inch spaced joists. Enter the length, width, board length and board size, and the calculator returns deck board, joist, beam, post and screw counts that include a sensible waste allowance.

Table of Contents

• How to Use the Deck Calculator
• How Many Deck Boards Do I Need?
• Worked Example: A 16 × 12 Foot Deck
• Joists, Beams and Framing
• How Many Screws Do I Need?
• Wood or Composite Decking?
• FAQs

How to Use the Deck Calculator

The deck calculator needs the deck length and the deck width, in feet, and the length of the boards you are buying. Most boards are sold in 8, 10, 12, 14 or 16 foot lengths — the calculator assumes the board length you choose and works out how many boards cover the deck, allowing the usual 10% for cutting and waste.

Two further choices affect the answer. Joist spacing determines how many joists support your decking boards, so the calculator assumes the standard 16 inches between joists. It also assumes your decking boards are laid with a 1/8 inch gap to allow them to drain and to swell. If your layout uses double joists or blocking, swap out the spacing for what you actually have and rerun the numbers.

If your deck is an L shape rather than a rectangle, split it into two rectangles and calculate each separately, then add them together. Trying to use one calculator for a complicated shape gives you the wrong answer. You can also use the Fence Calculator for the railings and posts for a deck with a balustrade.

How Many Deck Boards Do I Need?

Deck boards cover the deck lengthways, so the coverage of one board is its width multiplied by the length of each piece. Each board covers its own length, so the count is deck length divided by the coverage of one board. The worked example below shows how it works in practice.

The table below shows the usual numbers based on 5.5 inch wide decking boards — the standard size for pressure-treated pine and cedar decking.

One standard trick: buy whole boards in the longest lengths your supplier stocks, even if your deck is short, because fewer joints at the ends mean fewer seams and less visible wear.

The deck calculator estimates the number of boards you need by dividing the total deck area by the coverage of a single board. For a 16-foot board, fewer boards are needed. To cost them, divide the price of the boards by the number of boards you need, and our Board Foot Calculator converts board sizes into board feet for you.

Worked Example: A 16 × 12 Foot Deck

Here’s a full estimate for a 16-by-12-foot deck using 12-foot treated pine decking boards at the standard spacing of 16 inches:

• Deck area: 16 × 12 = 192 square feet.
• Deck boards: 16 ft ÷ (5.5 in wide board) = 34.9, so 34 boards (you will have offcuts for the last board).
• Boards with waste: 34 × 1.1 = 37.4, so 38 boards to order.
• Joists: 16 ft ÷ 16 in spacing = 12, plus one joist for the edge = 13 joists.
• Screws: 2 screws per board per joist × 34 boards × 13 joists = 884 screws.
• Beams: 2 beams of 2 × 10 or 2 × 12 timber, each 16 feet long, plus 6 concrete pier blocks.

The screws disappear quickly because each board crosses every joist, so each crossing point gets its own two screws on most decks. A pilot hole is wasteful, but if you plan to do the decking work yourself, a carload of screws will be needed.

Joists, Beams and Framing

Joists do the actual work of supporting the decking boards, and they sit on beams that run the length of the deck. Standard spacing in many guides is 16 inches on center, which with 2 × 8 inch timber gives a comfortable deck for the usual domestic load of a person and furniture.

Joists also have a maximum span that depends on the timber, the joist spacing and the size of the joists. The industry decking guide with IRC span tables has a useful summary, and deck estimating guides provide helpful spacing and sizing data for deck builders. The two links open separate calculators and are good checks of your own numbers.

How Many Screws Do I Need?

Screws are the hidden cost of a decking project, and like screws in general a little goes a long way — you will need far more than you think. Boards are fixed at every crossing, so each board that meets a joist needs two screws, and most decking boards cross as many joists as they have across the deck.

The deck calculator’s screw count is derived from the number of crosses in the grid. A 16 × 12 deck with 34 boards and 13 joists needs 884 screws, which is close to two boxes of 500. Buying individually is more expensive and you will run out mid-project; buying a big box and sharing it with neighbours costs less.

Screws and fixings may look like a minor item but they add up: you will need the fixings for the joists and the beams too, usually dozens of ties and hundreds of screws. A concealed fixing system is neater and costs more, but isn’t recommended for a board that is already on a fixed deck.

Wood or Composite Decking?

Pressure-treated pine is half the price of composite, and it lasts 10–15 years if you keep up with staining and sealing. Composite costs more up front but does not need staining and never rots. If you can afford the upfront cost, it saves labour over the life of the deck.

Cost is not the whole story: if you know how to look after it, a beautiful hardwood deck will outlast a humble softwood deck and the composite deck. Exotic hardwoods like ipe and cedar price out at a higher rate, but look at the alternatives before you choose because the deck you build will set the pattern for the rest of the garden.

For now, the deck calculator works the same way for both, because it calculates the board dimensions, the screw pattern and the joists. If you are pouring concrete footings for the posts, the Concrete Calculator gives the concrete by volume and depth, down to the pier blocks.

FAQs

How many deck boards do I need for a 12 × 12 deck?

A 12 × 12 foot deck (144 square feet) needs roughly 22 deck boards, and we suggest adding 10% extra, bringing the total to 25 boards. Buying 82 and 83 board lengths will improve the result considerably, and skill availability may affect your choice.

How far apart should deck joists be?

Deck joists are usually spaced 16 inches apart — this is the standard for most residential decks. If you’re using composite decking, a 12-inch spacing is recommended.

How many screws do I need per deck board?

Two screws per board per joist is the standard used by builders. For a 16-by-12-foot deck with decking boards 5.5 inches wide, you need around 884 screws.

What size deck boards should I use?

The most common decking boards are 5.5 inches wide and 12 feet long, laid with a 1/8-inch gap between boards for drainage.

Do I need a permit to build a deck?

Many regions require a permit for decks over 30 inches high or attached to the house. Check with your local council before building.

How much does it cost to build a deck?

A 16 × 12 foot deck in pressure-treated pine typically costs £2,000 to £3,500 in materials, depending on the timber quality and fixings. Composite would roughly double that.

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