About the Deck Calculator
Deck Calculator estimates the materials for a simple rectangular deck. Enter the length and width, choose the board width and board length, set the gap between boards, the joist spacing and an allowance for waste, and the page works out the number of rows of boards, the number of boards to buy, the number of joists and how many deck screws you will use. The boards run along the length, and joists run across it. Use it to plan a shopping list and compare board lengths, then check the sizes, spans and footings against your local building code.
Updated October 2026
How to use the Deck Calculator
- 1Enter the length and width of the deck in feet.
- 2Choose the board width and the board length you will buy.
- 3Set the gap between boards and the joist spacing.
- 4Add an allowance for cuts and mistakes, 10% is common.
- 5Read the number of boards, joists and screws.
How the estimate is made
The deck surface is a grid of rows. The number of rows is the deck width divided by the board width plus the gap. Each row is made of one or more boards laid end to end along the length. The calculator rounds up so that you do not run out, and then adds the waste allowance.
Choosing board length
Buying boards as long as the deck avoids joints and cuts, but long boards cost more and are harder to carry. If the deck is longer than the boards, plan joints that land on a joist and stagger them so that they do not line up.
- Match the length to the deck to cut waste
- Stagger end joints on different joists
- Keep end joints over the middle of a joist
- Round up and keep a few spare boards
Before you build
Check whether your deck needs a permit, how deep footings must go and what railings are required. Use pressure-treated or rot-resistant lumber for the frame, and flashing where the deck meets the house. Work out gravel and concrete for the footings with the Concrete Calculator, and boards for floors with the Flooring Calculator.
Related tools
Plan the base with the Concrete Calculator and the Gravel Calculator, compare with the Flooring Calculator, work out fences with the Fence Calculator and stairs with the Stair Calculator.
Features
- Boards, joists and screws
- Common board widths and lengths
- Gap and joist spacing options
- Waste allowance
- Deck area
- Private: nothing is uploaded
Frequently asked questions
How many deck boards do I need?
Divide the deck's width by the board width plus the gap to find the number of rows, multiply by the boards needed per row and add about 10% for waste. The calculator does this for you.
What gap should I leave between boards?
Wood decking is usually laid with a gap of about 1/8 inch when dry and 1/4 inch if wet. Composite makers give their own gap, so follow the manual.
What joist spacing should I use?
16 inches on centre is the most common. 12 inches suits diagonal decking or composites, and 24 inches only suits some thick boards. Check your board maker's rules and your local code.
Does it include the frame and posts?
It counts joists and the surface boards. Posts, beams, ledger boards, hangers, footings, railings and stairs are not included.
How many screws do I need?
About two per board at each joist crossing. The estimate includes 5% extra. Hidden fasteners and clips use different counts.
What waste factor should I use?
10% for a plain rectangle and 15% for diagonal boards, picture frames or a lot of cuts.
How do I find the board length to buy?
Pick lengths that match your deck length with the least cutting, such as 16 ft boards for a 16 ft deck. Use the shorter boards if joints are staggered.
Is this a replacement for plans?
No. It is a planning estimate. Always follow local building rules and the manufacturer's instructions.