How to use the deck calculator
- Enter the deck length and width in feet and choose whether the boards run along the length or the width.
- Check the board width and gap against your decking product, and enter the board length you plan to buy.
- Set the joist spacing on center (16 inches is the starting value) and keep or change the 10% waste allowance.
- Read the rows, linear feet, boards to buy and joist count. Add prices for a cost estimate of the boards and joists.
Formula
Rows of boards = (distance across the boards x 12) / (board width + gap), rounded up
Linear feet = rows x length of each row
Boards to buy = linear feet x (1 + waste) / board length, rounded up
Joists = (length the joists are spaced along x 12) / spacing, rounded up, plus 1
The extra joist is the one at the end of the run, because n spaces between joists need n + 1 joists. Joists run perpendicular to the boards, so they span the other direction.
Worked example: a 16 ft by 12 ft deck with 5.5 inch boards
Boards run along the 16 ft length and are laid across the 12 ft width. Each board plus a 1/8 inch gap takes 5.625 inches, so the rows are 144 / 5.625 = 25.6, rounded up to 26 rows.
Linear feet are 26 x 16 = 416 ft, or 457.6 ft with a 10% allowance. In 16 ft boards that is 28.6, so 29 boards.
Joists run across the boards, each 12 ft long, spaced 16 inches on center along the 16 ft length. That is 192 / 16 = 12 spaces, so 13 joists and 156 linear feet.
If the boards ran along the 12 ft width instead, you would need 35 rows of 12 ft, still 29 boards of 16 ft, and 10 joists of 16 ft.
Board direction and joists
Decking boards sit across the joists, so the direction you choose sets the direction of the framing. Running boards along the longer side often means shorter joists, while running them across the shorter side means longer joists with fewer boards per row. The structure under the deck must be designed for its span, and this tool counts joists but does not check whether a given joist size can span a given distance.
Gaps, widths and waste
Most decking is laid with a small gap so water can drain and boards can move with the weather. The right gap depends on the material and the installation instructions, so enter the figure from your manufacturer. Diagonal and picture-frame patterns waste more board than a straight lay, so raise the allowance if you plan one.
The tool also shows a second figure for the case where offcuts cannot be reused. If your boards are shorter than the deck, each row needs more than one piece, and joining pieces over joists lets you reuse offcuts and buy fewer boards.
What is not included
Beams, posts, footings, ledger boards, flashing, hangers, fasteners, stairs and railings are separate and depend on your design and local code. You can size footings with the concrete calculator, and compare boards by volume with the board foot calculator.
Assumptions and limits
- The deck is a rectangle with straight boards. Angled corners, curves and multiple levels are not modeled.
- Board width (5.5 in) and joist spacing (16 in on center) are editable assumptions. Use your product and your plan.
- Joists are counted along the length perpendicular to the boards, with one extra joist for the end of the run. Doubled joists, rim joists and blocking are not counted.
- Boards are assumed to be joined over joists so that offcuts can be reused. A second figure shows the case where they cannot.
- This is a planning estimate. Always follow your approved plans, manufacturer instructions and local building code.
Frequently asked questions
How many deck boards do I need for a 12 by 12 deck?
With 5.5 inch boards and a 1/8 inch gap, boards along a 12 ft side need about 26 rows of 12 ft, or about 312 linear feet. A 10 percent allowance brings it near 343 feet.
What joist spacing should I use?
Sixteen inches on center is a common starting point, but the correct spacing depends on the decking product, joist size and span, and local code. Use what your plan requires.
Should boards run lengthwise or crosswise?
It is a design and appearance choice that also determines which way the joists run. Try both in the tool to compare the number of rows and joists for your deck.
How much gap should I leave between deck boards?
Follow your decking manufacturer, because the right gap depends on material and conditions. The tool uses 1/8 inch as a starting value and lets you change it.
Does the calculator include railings and stairs?
No. It counts boards and joists only. Railings, stairs, beams, posts and footings need to be added separately from your plans.