Deck boards are counted in rows: each row runs the length of the deck, and the number of rows depends on the board width plus the gap between boards. This deck board calculator works out the rows, the total linear feet and the number of boards to buy at the length you choose, plus a rough count of face screws. It works for pressure-treated pine, cedar, hardwood and composite or PVC boards; just enter the actual width and the gap the manufacturer recommends.
Joist spacing affects both the fastener count and what decking you can use. The American Wood Council's Prescriptive Residential Wood Deck Construction Guide (DCA 6) limits joist spacing by decking type, and composite manufacturers publish their own maximum spans. Building steps down to the yard? Use the stair stringer calculator. For posts and footings, the concrete slab calculator handles concrete volume.
How to use the deck board calculator
- Enter the deck length in the direction the boards run, and the width across the boards.
- Enter the actual board width: 5.5 in for nominal 6-inch boards (5/4×6 and 2×6), 3.5 in for 2×4, or the exact width of your composite profile.
- Enter the gap. Many installers leave about 1/8–1/4 inch between wood boards; composite manufacturers specify their own gaps, often with hidden fastener clips that set it automatically.
- Choose the board length you will buy. Picking a length that matches the deck (or divides into it evenly) cuts waste.
- Enter the joist spacing for the screw estimate and a waste allowance (10% is typical; 15% for diagonal decking or picture-frame borders).
Formula
The screw estimate assumes two face screws per board at every joist. Hidden-fastener systems use one clip per board per joist; check the manufacturer's coverage per box.
Worked example
A 16 × 12 ft deck with boards running the 16 ft direction, 5.5 inch boards, 3/16 inch gaps, 16 ft boards, joists at 16 inches on center and 10% waste.
- Rows: 12 × 12 = 144 in ÷ (5.5 + 0.1875) = 144 ÷ 5.6875 = 25.3, so 26 rows (rip the last board).
- Linear feet: 26 × 16 = 416 ft.
- Boards: 416 × 1.10 = 457.6 ÷ 16 = 28.6, so 29 boards at 16 ft.
- Joists: 16 × 12 = 192 in ÷ 16 + 1 = 13 joists. Screws: 26 × 13 × 2 = 676 screws.
Because the deck is exactly 16 ft long, each row is one board with no butt joints; the 10% waste covers end trimming and bad boards.
Choosing board length to reduce waste
When a deck is longer than one board, rows need butt joints, and each joint must land on a joist (often a doubled joist or blocking). Board lengths that divide evenly into the deck length, such as two 12 ft boards for a 24 ft deck, produce the least waste. Staggering joints from row to row looks better and spreads load, but can mean buying a mix of lengths. If your deck is 14 ft, buying 16 ft boards wastes 2 ft per row; buying 14 ft boards wastes almost nothing.
Joist spacing and decking
DCA 6 lists allowable joist spacing by decking material: thinner 5/4 (1 inch nominal) wood boards are commonly limited to 16 inches on center, while 2× lumber decking can span farther. Diagonal decking reduces the allowable span. Composite and PVC decking spans are set by each manufacturer and are often 16 inches for perpendicular installation and 12 inches for diagonal. Check your product's installation guide before ordering joists.
Caveats and assumptions
- Gaps change as wood dries; wet pressure-treated boards are often installed tight and shrink to form gaps. Follow the supplier's guidance.
- Picture-frame borders, breaker boards and diagonal patterns add waste and cuts. Raise the waste allowance to 15–20%.
- Fastener counts are estimates. Stair treads, fascia and rim boards need additional fasteners.
- This calculator does not size framing. Joists, beams, posts and ledger attachment should follow DCA 6 or your local code.
Frequently asked questions
How many deck boards do I need for a 12 × 16 deck?
With 5.5 inch boards and 3/16 inch gaps running the 16 ft direction, you need 26 rows of 16 ft boards, or 29 boards with 10% waste.
How many linear feet of decking per square foot?
With 5.5 inch boards and a 3/16 inch gap, each foot of deck width takes 12 ÷ 5.6875 = 2.11 rows, so about 2.11 linear feet of board per square foot of deck.
How much gap should be between deck boards?
It depends on the material and its moisture content. Many installers leave about 1/8 to 1/4 inch between dry wood boards; composite manufacturers specify exact gaps in their installation guides.
How many screws per deck board?
Typically two face screws per board at each joist. A 16 ft board on 16 inch joist spacing crosses 13 joists, so about 26 screws.
Which way should deck boards run?
Usually perpendicular to the joists, and often parallel to the house. Diagonal layouts are possible but require closer joist spacing and more waste.
Sources
- American Wood Council — DCA 6 Prescriptive Residential Wood Deck Construction Guide — joist spacing by decking type and framing rules
- NIST — PS 20 American Softwood Lumber Standard — actual vs nominal board sizes
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.