Deck Calculator
Estimate deck boards by stock length, joists, rim and ledger, beam plies, posts and screws or hidden clips for a rectangular deck from board width and gap.
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About this toolWhat is the Deck Calculator?
Deck materials come from two simple grids: boards laid across joists, and joists laid across a ledger and a beam. The counts depend on the actual board width plus the gap, the joist spacing, and the lengths your yard sells, so a square-footage rule of thumb is easy to get wrong by a dozen boards. This calculator counts the rows of decking, the joists, the fasteners at every crossing, and the framing members you choose.
Enter the deck length along the house and the projection out from it, the board width and gap, the board length you will buy, and whether the boards run parallel to the house or on a 45 degree diagonal. Pick the joist spacing and stock length, enter the beams, plies and posts from your design, and choose face screws or hidden clips.
Results give boards to buy with waste, joists and their cut length, rim and ledger, beam plies, posts, joist hangers, the fastener count with boxes, the joist span from ledger to beam, a cut list and the decking cost.
- Rows and boards by stock length, with nesting of short rows
- Straight or diagonal decking and adjustable waste
- Joists, rim, ledger, beam plies, posts and hangers
- Face screws or hidden clips per crossing, with boxes and rate per area
- Cut list, joist span readout, decking cost, metric and imperial
How to use the Deck Calculator
- Choose imperial or metric and enter the deck length along the house and the projection out from it.
- Enter the actual board width, the gap between boards and the board length you will buy, then choose straight or diagonal boards.
- Set the joist spacing, joist size label and stock length, the rim thickness, and whether a ledger attaches the deck to the house.
- Enter the beams, plies, posts and the joist cantilever past the beam from your design or permit drawing.
- Choose face screws or hidden clips, the count per box and the waste, then read the materials and cut list.
Worked examples
16 x 12 ft attached deck
Input
16 ft along the house, 12 ft out, 5-1/2 in boards, 3/16 in gap, 16 ft boards, 16 in joists, one 2-ply beam with 3 posts, 18 in cantilever, 10% waste
Output
29 boards, 13 joists at 11' 10-1/2", 676 screws (2 boxes of 350), joist span 10' 6"
144 / 5.6875 gives 25.3, so 26 rows, each one 16 ft board. Every row crosses 13 joists with two screws: 26 x 13 x 2 = 676, about 3.5 per square foot.
20 x 14 ft diagonal deck with hidden clips
Input
20 x 14 ft, 45° boards, 12 in joists, 16 ft boards, 15% waste, hidden clips
Output
43 boards, 21 joists, 418 clips
Diagonal decking covers the same area with the same linear footage, but offcuts at both ends push waste to about 15%, and 12 in joist spacing keeps the boards stiff.
Metric 4.8 x 3.6 m deck
Input
4.8 x 3.6 m, 140 mm boards, 5 mm gap, 4.8 m lengths, 450 mm joists
Output
28 boards, 12 joists, 600 screws, about 34.7 screws per m²
Each row covers 145 mm, so 3.6 m takes 25 rows. Twelve joists at 450 mm cover 4.8 m with the end joist.
Background and insight
Joists are counted like studs: ceil(length / spacing) + 1 including both end joists. Rows of decking are ceil(projection / (board width + gap)), using the actual width. A 5/4x6 board is 5-1/2 in wide, so with a 3/16 in gap each row covers 5-11/16 in. When the deck is shorter than the boards, rows nest two or more to a board; when it is longer, butt joints have to land on joists and each row takes more than one board. Two face screws per board at every joist is the standard pattern, a little over 3 screws per square foot for 5-1/2 in boards at 16 in, and hidden clips use one per crossing.
Joist sizes and spans are a code question. In the US, IRC Section R507 holds the deck joist and deck beam span tables, which depend on species, grade, spacing and cantilever, and the American Wood Council publishes the same rules as DCA 6. Composite decking makers often require 12 in joist spacing for diagonal boards. The calculator shows your joist span so you can compare it with the table, but it does not size members.
Common use cases
Pricing a deck before a permit
Get board, joist and fastener counts to compare composite and timber quotes.
Planning a resurface
Count new boards and screws for existing framing by entering the deck size and joist spacing.
Choosing board direction
Compare straight and diagonal layouts for board count, waste and fasteners.
Ordering stock lengths
Try 12, 16 and 20 ft boards to find the length that nests best with your deck size.
Tips and limitations
- Use the actual board width. A 5/4x6 is 5-1/2 in, a 2x6 is 5-1/2 in and composite boards are often 5-1/2 in too, but check the product sheet.
- Lay boards with the bark side up or follow the maker's gap for composite; gaps vary from 1/8 to 1/4 in with temperature and moisture.
- Stagger butt joints and land each one on a doubled joist or a blocking piece.
- Ledger fasteners, lateral load connectors, footings, stairs and railings are not included. Plan them from the code or DCA 6.
- Picture-frame borders and breaker boards need extra boards beyond this count.
Frequently asked questions
How many deck boards do I need?
Divide the deck width across the boards by the board width plus the gap and round up to get rows, then multiply by the boards per row. A 12 ft deck with 5-1/2 in boards and 3/16 in gaps has 26 rows; with 16 ft boards on a 16 ft deck that is 26 boards plus about 10% waste.
How many screws do I need per square foot of decking?
About 3.2 for 5-1/2 in boards with 3/16 in gaps on joists at 16 in, from two screws at every joist crossing; a small deck comes out nearer 3.5 because of the end joists. At 12 in joists it is about 4.2, and at 24 in about 2.1. The calculator counts the crossings for your layout.
How many joists does a deck need?
Divide the length of the ledger by the joist spacing, round up and add one. A 16 ft deck at 16 in centres needs 13 joists, counting the two end joists.
How far can deck joists span?
It depends on species, grade, size and spacing. Look it up in the deck joist span table in IRC Section R507 or the AWC DCA 6 guide; for example, the span for a 2x8 is much shorter than a 2x10 at the same spacing.
What gap should deck boards have?
About 1/8 to 1/4 in (3 to 6 mm) between boards for drainage, or the spacing printed on the composite maker's installation guide. Wet pressure-treated boards shrink, so they are often butted tight and left to open up.
Do diagonal deck boards need closer joists?
Usually. A board on a 45 degree angle spans further between joists, so many manufacturers ask for 12 in joist spacing for diagonal decking where 16 in is fine for straight runs.
Is there an in-depth guide to the Deck Calculator?
Yes. “Estimating a Deck: Boards, Joists, Screws and Span Tables” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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