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Deck Calculator

Use this deck calculator to find how many deck boards and joists a deck needs from its length, width and board size.

  • Instant answer as you type
  • Nothing you type is sent anywhere
  • Worked example with clear steps

Your measurements

ft
Boards run along this side
ft
ft
Typical value, not a standard
%
Typical value, not a standard

Result

Calculated live
Deck boards 416 linear ft, 16 ft boards, 10% extra
29 boards
Joists 16 in apart, each 12 ft long
13 joists
Deck area
192 ft²
Rows of boards across the width
26
Screws 2 per row of boards at each joist: our own assumption, not from a source
676

Based on NIST PS 20 NIST SP 811 AWC deck guide Full list of sources

Assumptions and constants

You can change

The limits on the other boxes, such as the largest size you can enter, are ours, not from a source.

Length of one board you will buy Typical value

ft
Same as the box in the form above

Typical value, not a standard. Enter the length your supplier sells.

Allowed range: 4 to 24 ft. The limits are ours.

Extra boards for cuts and waste Typical value

%
Same as the box in the form above

Typical value, not a standard. If you lay boards on a diagonal, ask your supplier how much more to order.

Allowed range: 0 to 100 %.

Width of one deck board (actual)

in
Fractions are fine, e.g. 5 1/2

A nominal 6 inch board is dressed to at least 5 1/2 inches when dry, under the American Softwood Lumber Standard. For composite boards, enter the width the maker gives.

Allowed range: 1 to 12 in. The limits are ours.

Gap between boards Typical value

in
Fractions are fine, e.g. 1/8

Typical value, not a standard. Follow the decking maker's instructions.

Allowed range: 0 to 1 in. The limits are ours.

Joist spacing, centre to centre Typical value

in

Typical value, not a standard. Ask your local building authority what spacing applies. The screw count assumes 2 screws where a row of boards crosses a joist: our own assumption, not from a source.

Allowed range: 8 to 48 in. The limits are ours.

Fixed values (exact definitions and references)

These are exact definitions and cannot be edited.

Inches in a foot Reference [2]

12 in per ft

The foot is 0.3048 m and the inch is 0.0254 m, both exact, so 12 inches make a foot.

How to use the deck calculator

  1. Enter the Deck length, the side the boards run along, and the Deck width, both in feet.
  2. Enter the Length of one board you will buy and the Extra for cuts.
  3. Open Assumptions and constants to check the Width of one deck board (actual), Gap between boards and Joist spacing, centre to centre.
  4. Read Deck boards and Joists. The calculator does not size joists, beams, posts or footings.
  5. Use Copy result or Print, and confirm the boards with your supplier and the structure with your local building authority.

How the deck calculator works

Rows of boards = the deck width in inches (12 inches in a foot) [2] / (board width + gap), rounded up. Linear feet = rows x deck length. Boards = linear feet x (1 + extra) / the length of one board, rounded up. Joists = the deck length in inches / the joist spacing, rounded up, plus one so that both ends have a joist (our counting rule), and they run along the width. Screws are two for every row of boards at every joist, which is our own assumption, not from a source, and information only. Use the width a board is dressed to: a nominal 6 inch board is dressed to at least 5 1/2 inches when dry. [1]

Worked example

A 12 ft x 16 ft deck, with the boards running along the 16 ft length. Rows of boards = 12 x 12 / (5.5 + 0.125) = 25.60, rounded up to 26 rows. Linear feet = 26 x 16 = 416. With 10% extra and 16 ft boards, 416 x 1.1 / 16 = 28.60, rounded up to 29 boards. Joists 16 in apart along the 16 ft side: 16 x 12 / 16 + 1 = 13 joists.

Deck from above: length 16 ft, width 12 ft The deck seen from above, a rectangle 4 to 3 as long as it is wide. Its length, 16 ft, is marked along the long side, which is the direction the boards run. Its width, 12 ft, is marked along the short side, which is the direction the joists run. From above Length 16 ft Width 12 ft
Boards run along the 16 ft length. Joists run along the 12 ft width, across the boards.

How many deck boards do I need?

Enter the deck length and width, the length of the board you will buy, and the extra for cuts. The boards are the linear feet of decking divided by one board's length, rounded up. A deck longer than one board needs boards joined end to end. We found no source for where the end joints go, so follow the decking maker's instructions. For composite boards, enter the width and gap the maker gives under Assumptions and constants.

How far apart should deck posts be?

This calculator does not size posts. The American Wood Council's deck guide (the 2018 edition, based on the 2015 IRC) gives no single spacing for posts. It limits the beam span, which it measures from post centerline to post centerline when there are overhangs, by the size and species of the beam and the span of the joists. For Southern Pine (its Table 3A), a beam of two 2x6 under joists spanning up to 6 ft may span 6 ft 8 in, and a beam of three 2x12 may span 15 ft 0 in. The table assumes a 40 psf live load, a 10 psf dead load, No. 2 grade and wet service conditions, and the guide applies to single level decks attached to the house. Its posts are at least 6x6 nominal. [3]

That is one guide, and we found no second source for these limits. Your local code applies: ask your local building authority, and follow the plan or deck guide it accepts.

Spans and loads: this is not a structural design tool

The calculator counts boards and joists from the deck size. It does not choose joist sizes, beams, posts or footings, and it does not say how far a deck can span or how much weight it can hold. Ask your local building authority about those. The joist spacing under Assumptions and constants only sets how many joists are counted.

Common deck sizes

Example deck sizes, with the boards and joists the calculator gives. The sizes are examples to show the sum.

Deck boards for common sizes with 5.5 in boards, 16 ft lengths and 10% extra
Deck Area (ft²) Rows Boards Joists
Small deck, 8 x 8 ft 6418107
Deck, 10 x 10 ft 10022169
Deck, 12 x 12 ft 144262210
Deck, 12 x 16 ft 192262913
Large deck, 14 x 20 ft 280304216

Nominal and dressed lumber sizes

The nominal size is the name on the label and is larger than the dressed size. A dry 2 by 4 is dressed to at least 1 1/2 by 3 1/2 inches. [1]

Nominal and minimum dry dressed sizes in inches
Nominal size Minimum dry dressed size (in)
2 x 4 1 1/2 x 3 1/2
2 x 6 1 1/2 x 5 1/2
2 x 8 1 1/2 x 7 1/4
2 x 10 1 1/2 x 9 1/4
5/4 x 6 (board) 1 x 5 1/2

Source: the American Softwood Lumber Standard. [1]

Boards laid on a diagonal

We found no source for how much extra a diagonal layout needs, so ask your supplier before you change the Extra for cuts.

How much deck stain do I need?

The area to coat is the deck length times its width, for the top. Add the railings, steps and sides if you will coat them. Divide by the coverage printed on the product label. We state no coverage figure.

How much does a deck cost?

We state no price. Ask local suppliers for a price per board, and multiply it by the boards above.

Common mistakes and limits

  • Using the nominal board width. A nominal 6 inch board is dressed to at least 5 1/2 inches when dry. [1] Use the dressed width.
  • Forgetting the gap. Gaps between boards add rows.
  • Ignoring joints. Boards shorter than the deck need end joints, so ask the decking maker where they go.
  • Treating the joist count as a design. It only counts joists at the spacing you enter.
  • What this tool does not cover: posts, beams, footings, railings, stairs, ledger attachment and permits.

Frequently asked questions

How many deck boards do I need?

Divide the deck width in inches by the board width plus the gap to get the rows, multiply by the deck length, add the extra, and divide by the length of one board. A 12 ft x 16 ft deck with 5 1/2 inch boards and 1/8 inch gaps has 26 rows and 416 linear feet. With 10 percent extra and 16 ft boards, order 29 boards. [1]

How many deck screws do I need?

The calculator shows two screws for each row of boards at every joist, which is 676 for the example deck (26 rows x 13 joists x 2). That is our own assumption, not from a source, and information only. Follow the decking maker's instructions for the fastener type and spacing.

How many deck posts do I need?

This calculator does not size posts. We found no source we can cite for a post count, so ask your local building authority.

How many deck blocks do I need?

This calculator does not size footings or deck blocks, so ask your local building authority.

Sources and references

  1. National Institute of Standards and Technology (NIST). Voluntary Product Standard PS 20-25, American Softwood Lumber Standard (PS 20-25, accessed November 2026). Used for: Nominal and minimum dry dressed lumber sizes, for example a dry 2 by 4 is dressed to at least 1 1/2 by 3 1/2 inches.
  2. National Institute of Standards and Technology (NIST). NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically (SP 811, accessed November 2026). Used for: the inch and foot relationship (12 inches in a foot).
  3. American Wood Council (AWC). Prescriptive Residential Wood Deck Construction Guide (DCA 6, copyright 2018, based on the 2015 International Residential Code, accessed November 2026). Used for: how the distance between posts is limited by the beam span, and the smallest post size.

How we use sources: methodology.

Estimates only. Confirm quantities with your supplier. See the disclaimer. Found a wrong result? Report an error.