How to measure a deck for boards
Enter the length of the deck in the direction the boards will run, and the width across the boards. On most decks the boards run parallel to the house, so the length is the side along the house. Use the board width the lumber yard lists as the actual size: a 5/4 × 6 or 2 × 6 board is really about 5.5 inches wide, and most composite boards are similar.
How the calculation works
Rows of boards = deck width in inches ÷ (board width + gap), rounded up
Linear feet = rows × deck length × (1 + waste %)
Boards = linear feet ÷ board length, rounded up
Screws = rows × joists × 2, with joists = deck length in inches ÷ joist spacing + 1
In the example above, a deck 16 feet long and 12 feet wide needs 26 rows of 5.5 inch boards with 1/8 inch gaps. That is 416 linear feet, or about 458 with 10% extra, which is 29 boards 16 feet long. With joists every 16 inches there are 13 joists, and two screws per board at each joist comes to about 680 screws.
Choosing board length
The fewer joints, the better a deck looks. If you can buy boards as long as the deck, each row is a single board and waste is low. For longer decks, stagger the joints between rows and make sure each joint lands on a joist. Picking a board length that divides evenly into your deck length, such as 12 ft boards for a 24 ft deck, keeps offcuts to a minimum.
How big should the gap be?
- Pressure-treated lumber is often sold wet and shrinks as it dries, so many builders install it tight and let the gaps open up on their own.
- Kiln-dried wood is usually gapped about 1/8 inch so water can drain and the boards can expand.
- Composite and PVC boards have gap requirements set by the manufacturer, often around 3/16 inch. Follow the installation guide, or the warranty may not apply.
Joists, framing and fasteners
Joists are usually 16 inches apart for boards laid straight. Composite decking and diagonal patterns often call for 12 inch spacing, so check the board manufacturer's requirements before you build the frame. This calculator covers the deck surface only. Framing, footings, railings and stairs depend on your design and local code, and many areas require a permit for a deck.
The screw count assumes two face screws per board at every joist. Hidden fastener systems use clips instead; their packaging lists how many square feet each box covers.
Frequently asked questions
How many deck boards do I need for a 12 × 12 deck?
With 5.5 inch boards and 1/8 inch gaps, a 12 ft wide deck has 26 rows. Using 12 ft boards, that is 26 boards, or 29 with 10% extra.
How many linear feet of decking per square foot?
About 2.1 linear feet per square foot for 5.5 inch boards with a 1/8 inch gap, since each foot of deck width holds a little over two boards. Add your waste allowance on top.
How many screws do I need per deck board?
Two screws at every joist the board crosses. A 16 ft board over joists 16 inches apart crosses 13 joists and takes 26 screws.
Should deck boards run parallel or perpendicular to the house?
Most decks have boards running parallel to the house, across joists that run away from it. Diagonal boards look striking but need closer joist spacing and more waste.
This calculator estimates decking materials only. Deck framing and footings must follow your local building code.