What Is the Decking Calculator?
The Decking Calculator estimates the surface boards and joists for a rectangular deck. You enter the deck dimensions, the width of the boards, the gap between them and the joist spacing. It returns the number of boards to buy, the number of joists, and the number of screws to fasten the surface.
Decking is priced by the board or by the linear foot, and composite boards in particular are expensive. Knowing the exact count lets you compare products fairly and avoid over-ordering. The joist count helps you plan the frame, which is where a deck’s strength comes from.
How the Calculator Works
Deck boards run in one direction, and joists run perpendicular to them underneath. The calculator treats the deck length as the direction the boards run and the deck width as the direction the joists run.
To find the number of rows of boards, it converts the deck width to inches and divides by the board width plus the gap. Each row is as long as the deck. Multiplying rows by deck length gives the total linear feet of decking.
It adds the waste percentage, then divides by the board length you intend to buy and rounds up. Joists are counted by dividing the deck length by the joist spacing and adding one for the end joist. Screws are estimated at two per board at every joist.
The Decking Formula
Rows = Deck width (in) ÷ (Board width + Gap), rounded up
Linear feet = Rows × Deck length × (1 + Waste % ÷ 100)
Boards = Linear feet ÷ Board length, rounded up
Joists = Deck length (in) ÷ Joist spacing + 1, rounded up
Screws = Rows × Joists × 2
Actual board widths
Lumber is sold by nominal size. A 2×6 or 5/4×6 deck board is 5.5 inches wide. A 2×4 is 3.5 inches. Most composite boards are 5.5 inches as well, but check the product sheet.
Step-by-Step Explanation
- Measure the deck length along the direction the boards will run, and the width across them.
- Convert the width to inches.
- Divide by the board width plus the gap and round up for the number of rows.
- Multiply rows by the deck length for linear feet.
- Add 10 percent for waste.
- Divide by the board length and round up for the number of boards.
- Divide the deck length in inches by the joist spacing, round up, and add one for joists.
Worked Example
A deck is 16 feet long and 12 feet wide. The boards are 5.5 inches wide with a 1/8 inch gap, bought in 16 foot lengths. Joists are at 16 inches on centre.
- Width in inches: 12 × 12 = 144.
- Rows: 144 ÷ 5.625 = 25.6, so 26 rows.
- Linear feet: 26 × 16 = 416.
- With 10 percent waste: 458 linear feet.
- Boards: 458 ÷ 16 = 28.6, so 29 boards.
- Joists: 192 ÷ 16 = 12, plus 1 = 13 joists, each 12 feet long.
- Screws: 26 × 13 × 2 = 676.
Because the deck is exactly 16 feet long and the boards are 16 feet, each row is a single board with no butt joints. In that case 26 boards cover the deck and the three extra are your margin for defects.
How to Use the Calculator
- Enter the deck length in the direction the boards will run.
- Enter the deck width in the direction the joists will run.
- Enter the actual board width and the gap.
- Choose the board length you plan to buy.
- Choose the joist spacing.
- Set the waste allowance and press Calculate.
The frame is bought as dimensional lumber, and the Lumber Calculator converts joists, beams and posts into board feet for pricing. Footings are sized with the Concrete Calculator. Steps down to the yard are laid out with the Stair Calculator. If you are adding a privacy screen or enclosing the yard, the Fence Calculator uses similar board arithmetic, and the Square Footage Calculator gives the area for stain or sealer coverage.
Practical Tips
- Choose board lengths that match the deck where possible. Full-length boards look better and avoid butt joints.
- If butt joints are needed, stagger them and land each one on a doubled joist or blocking.
- Use 12 inch joist spacing for diagonal decking and for most composite boards. Check the manufacturer’s span table.
- Leave a gap of 1/8 to 3/16 inch between boards for drainage. Wet treated lumber shrinks as it dries, so it can be laid tighter.
- Snap a chalk line every few rows and measure to the end to keep rows parallel.
- Let boards overhang the frame and trim them all at once with a circular saw for a straight edge.
- Seal cut ends of treated lumber with preservative.
- Use hidden fasteners for grooved composite boards, and buy them by the square foot coverage listed on the box.
Common Mistakes
Using nominal board width. Counting a 6 inch board as 6 inches undercounts rows by about 8 percent.
Forgetting the gap. Over 12 feet, an eighth of an inch per board adds up to more than 3 inches.
Joists too far apart. Boards flex and feel bouncy when spacing exceeds what the product allows.
No allowance for picture framing or fascia. A border board around the deck and fascia on the rim are extra.
Ignoring the frame. Decking is the visible part, but beams, posts, ledger, hangers and footings make up much of the cost.
Skipping the permit. Many decks require one, especially when attached to the house or raised above a certain height.
Wood or Composite?
Pressure-treated pine is the least expensive and lasts well when sealed regularly. Cedar and redwood are more stable and attractive but cost more. Tropical hardwoods are extremely durable and very dense. Composite and PVC boards cost the most up front, need little maintenance and do not splinter. They expand and contract with temperature, so follow the gapping instructions closely.
Joist Sizing Basics
Joist size depends on span and spacing. As a rough guide for common species at 16 inches on centre, a 2×8 spans about 12 feet and a 2×10 about 15 feet. Span tables vary by species, grade and local code, so use the table your building department accepts. Joist hangers and structural screws or nails rated for treated lumber are essential.
Railings and Stairs
Decks more than 30 inches above the ground generally need a guardrail at least 36 inches high, with balusters spaced so a 4 inch sphere cannot pass through. Railing posts are typically no more than 6 feet apart. Stair requirements are similar to those indoors, with consistent risers and a graspable handrail.
Diagonal and Patterned Decks
Boards laid at 45 degrees give a deck a custom look and stiffen the frame against racking. They also change the numbers. Each board spans a longer distance between joists, so joists must be 12 inches on centre for most products. Waste rises to about 15 percent because every board is mitred at both ends. Herringbone and picture-frame borders need extra blocking between joists to support the board ends, and that blocking should be counted with the framing lumber.
Fasteners
Coated deck screws at 2.5 to 3 inches long are standard for wood decking. One pound holds roughly 75 to 100 screws depending on size, so a 676 screw deck needs about 8 pounds. Stainless steel is worth the premium near salt water. For hidden clip systems, the manufacturer states how many clips cover 50 or 100 square feet.
Checking the Numbers Against Area
A quick way to sanity check a decking estimate is to work from area. Boards 5.5 inches wide with a small gap cover about 0.47 square feet per linear foot, so each square foot of deck needs roughly 2.1 linear feet of board. The 192 square foot deck in the example needs 192 × 2.1, or about 405 linear feet, which agrees with the row-by-row figure of 416. If your two methods differ widely, check that you used the actual board width.