Deck Calculator
Estimate the decking boards and joists you need for a deck from its size, and get a material cost.
Free · No sign-up · Runs in your browser · Last reviewed November 2026
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About the Deck Calculator
This deck calculator estimates the decking boards and joists for a rectangular deck. It assumes 5/4 by 6 inch boards, which cover about 5.625 inches each with a small gap, runs them across the joists and adds 10% for waste. Joists are spaced 16 inches on center by default. Posts, beams, footings, fasteners and railings are not included.
How to use it
- Enter the deck length and width in feet.
- Choose the board length and joist spacing and enter prices.
- Press Calculate deck for boards, joists and a material cost.
How the estimate works
- Rows of boards = deck width × 12 ÷ 5.625.
- Boards per row = deck length ÷ board length, rounded up.
- Joists = deck length × 12 ÷ spacing + 1.
Example: 16 × 12 ft deck
The deck is 192 sq ft. With 16 ft boards there are 26 rows of 1 board, so 26 boards, or 29 with 10% waste, and 13 joists at 16 inches on center.
Wood vs composite
| Factor | Pressure-treated wood | Composite |
|---|---|---|
| Upfront cost | Lower | Higher |
| Maintenance | Stain or seal regularly | Minimal |
| Lifespan | 15 to 20 years | 25 years or more |
Deck planning: a second example and what the estimate leaves out
Take a 12 × 10 ft deck using 12 ft boards at 16 inches on center. The width of 10 ft is 120 inches, which needs 22 rows (21.3 rounded up). With one board per row you need 22 boards, or about 25 with 10% waste. Joists come to 10 (12 ft × 12 ÷ 16 + 1).
What the quick estimate does not cover
- Structure. Posts, beams, footings, ledger board, flashing and hardware are separate items that often cost more than the surface boards.
- Permits. Many jurisdictions require a permit and inspection for decks, especially those attached to the house or raised above a certain height.
- Fasteners and finish. Use screws or hidden clips rated for treated lumber or composite, and budget for stain or sealer on wood.
- Joist span. Joist size and span limits depend on the lumber species and code; check span tables before settling on 2x6 or 2x8.
Estimating lumber volume for a custom frame is easier with the lumber calculator.
Frequently asked questions
How many deck boards do I need?
Divide the deck width in inches by 5.625 for the number of rows, then multiply by boards per row and add 10%.
How far apart should deck joists be?
Sixteen inches on center is standard. Composite and diagonal patterns may need 12 inches.
How much does a deck cost per square foot?
Materials and labor often range from about $25 to $65 per square foot depending on material and region.
Do I need a permit to build a deck?
Most jurisdictions require one for decks attached to a house or above a certain height. Check local rules.
How much waste should I plan for?
About 10%, or 15% for diagonal layouts.
Do I need a permit to build a deck?
In many areas, yes, particularly for attached or elevated decks. Rules vary, so call your local building department before you buy materials.
Should I run decking boards diagonally?
You can, but diagonal patterns require more waste (often 15%) and may need closer joist spacing, such as 12 inches on center, depending on the product.
Last reviewed: November 2026.