Deck Material Calculator

Type the deck's length and width, choose the board, the gap, the board length you will buy and the joist spacing, and read a materials list: how many decking boards, how many joists and rim joists, how many screws or hidden clips and how much fascia, with an allowance for cuts. It works in feet and inches with fractions or in metres and millimetres, handles diagonal decking, and shows the working under the list so you can check it against a quote.

How it works

Deck Material Calculator is a web page that does its work on your own device. Everything you type or choose is worked out by your browser and never sent to a server, so nothing is stored and there is nothing to delete. There is no account, no sign-up and no limit on how often you use it.

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Decking Calculator · Deck Board Calculator

How the count is made

Decking boards lie side by side across the joists, so the number of rows is the deck's width divided by one board's face width plus the gap between boards. Each row is made from whole boards of the length you buy, and the leftover from each row is usually too short to reuse, which is why the count works in rows rather than dividing the area by a board's area. Ten percent is added for cuts, split ends and the odd twisted board. Joists run the other way, across the width, spaced along the length: the length divided by the spacing, plus one so there is a joist under both ends, with two rim joists closing the ends of the joists.

Board sizes and gaps

In North America a deck board is sold as 5/4 × 6 or 2 × 6 but measures 5 1/2 inches across the face; a 2 × 4 measures 3 1/2. Composite boards are made to the same 5 1/2 inch width. Elsewhere decking is sold by its actual width, commonly 90, 120 or 140 mm. Timber boards need a gap of about 1/8 to 1/4 inch (3 to 6 mm) for drainage and movement; kiln-dried and composite boards go down at the maker's stated gap, and wet pressure-treated boards are often butted because they shrink as they dry. The gap changes the count, so choose it before buying.

Joist spacing and fixings

Joists are set at 16 inches (400 or 450 mm) on centre for most timber decking laid square; some composite boards and all diagonal decking need 12 inches (300 mm), and 24 inches is only for thick boards rated for it. Check the board maker's span table and your local code. Face fixing takes two screws at every joist crossing; hidden clip systems take one clip per crossing plus starter clips along the first board. Buy about 5% spare, because screws snap and clips get lost.

What is not counted

• Beams, posts, footings and the ledger against the house, which depend on the span, the height and the soil and come from the design and your code.
• Stairs, railings, blocking between joists and joist hangers.
• Picture-frame borders and inlays, which add boards and cuts beyond the allowance.

What it does

• Decking boards counted by rows and the length you buy, with waste
• 5/4 × 6, 2 × 6, 2 × 4, composite or any width; metric 90, 120, 140 mm
• Joists and rim joists at 12, 16 or 24 in (400, 450, 600 mm) centres
• Screws or hidden clips from the number of board-to-joist crossings
• Fascia around the edge, and diagonal decking with its extra waste
• Optional prices per board, joist and 100 fixings
• No upload, no account, no sign-up

Questions

How many deck boards do I need?

Divide the deck's width by one board's face width plus the gap to get the rows, then multiply by the boards per row for the length you buy, and add about 10% for cuts. A 16 by 12 ft deck in 5 1/2 inch boards with a 3/16 inch gap is 26 rows of one 16 ft board, so 29 boards with waste. The calculator does this for any size.

How many joists for a 16 ft deck?

At 16 inches on centre, 16 ft divided by 16 inches is 12, plus one for the far end: 13 joists, plus two rim joists. At 12 inches it is 17. Joists run the way the deck is wide, so their length is the deck's width.

How many screws per deck board?

Two at every joist the board crosses. A board 16 ft long over joists at 16 inch centres crosses 13 joists and takes 26 screws; a 16 by 12 ft deck with 26 rows takes about 676, so buy 700 to 750. Hidden clip systems take one clip per crossing.

How wide is a 5/4 × 6 deck board?

5 1/2 inches, the same as a 2 × 6. The 6 is the nominal size before planing. Composite boards are made to 5 1/2 inches too. The calculator uses the actual width, and Other width takes any measurement.

What gap should I leave between deck boards?

About 1/8 to 1/4 inch (3 to 6 mm) for dry timber and composite, or as the board maker states. Wet pressure-treated boards are often laid tight because they shrink as they dry. The gap changes the row count, so set it before you buy.

Does diagonal decking need more boards?

Yes. It covers the same area but every board end is a mitre cut and the offcuts are triangles, so allow at least 15% extra, and the joists usually need to be closer, 12 inches on centre. Tick Boards laid diagonally and the calculator uses 15% or your own figure if higher.

Is this calculator free?

Yes, completely free, with no sign-up. The numbers are worked out in your browser and never sent to us.

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