Deck Calculator
Estimate decking boards, joists, screws and cost for a rectangular deck from its size, board width, gap and joist spacing.
Results
| Item | Value |
|---|
Quick Answer
Rows of boards = deck width in inches ÷ (board width + gap). A 16 ft by 12 ft deck with 5.5 inch boards and 1/8 inch gaps needs 26 rows. With 16 ft boards and 10% waste, that is 29 boards, plus 13 joists at 16 inches on center and about 676 screws.
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Table of Contents
- Deck Calculator
- Quick Answer
- Related Calculators
- How This Deck Calculator Works
- How to Use the Deck Calculator
- Core Formulas
- Key Terms
- Worked Example
- Real-Life Examples
- Wood vs Composite Decking
- Maximum Joist Spacing for Wood Decking
- Common Mistakes to Avoid
- Frequently Asked Questions
- References & Sources
How This Deck Calculator Works
Enter the deck’s length along the boards and its width across them. Choose the board width, gap, board length and joist spacing. The calculator works out how many rows of boards fit across the deck, how many boards you need with waste, how many joists sit under them, and how many screws hold them down.
Quick explanation: deck boards run across the joists, with a small gap between each board for drainage.
- Board width: a 2×6 or 5/4×6 is about 5.5 inches wide.
- Joist spacing: 16 inches is common for 5/4 wood decking.
- Fasteners: two screws per board at each joist.
How to Use the Deck Calculator
- Enter the deck length along the boards and the width across them.
- Choose the board width and enter the gap your decking maker recommends.
- Choose the board length and joist spacing.
- Enter a waste allowance, usually 5% to 15%.
- Press Calculate and read boards, joists and screws.
Core Formulas
- Rows: width × 12 ÷ (board width + gap), rounded up
- Boards: rows × boards per row × (1 + waste %)
- Joists: length × 12 ÷ spacing + 1, rounded up
- Screws: rows × joists × 2
Key Terms
- Joist: a horizontal framing member that supports the deck boards.
- On center (o.c.): the distance from the center of one joist to the next.
- Nominal size: the name size of lumber, such as 2×6, which is larger than its real size.
- Single span: a board supported by only two joists.
- Ledger: a board attached to the house that supports one end of the joists.
Worked Example
A 20 ft by 10 ft deck uses 5.5 inch composite boards with a 3/16 inch gap, 20 ft boards and joists 12 inches apart.
- Rows = 120 ÷ (5.5 + 0.1875) = 21.10, so 22 rows
- Boards with 10% waste = 25
- Joists = 240 ÷ 12 + 1 = 21, and screws = 22 × 21 × 2 = 924
Real-Life Examples
- A small patio deck: 12 ft by 10 ft with 12 ft boards needs 25 boards and 10 joists.
- A larger family deck: 20 ft by 16 ft with 20 ft boards needs 39 boards.
- Diagonal decking: add more waste, often 15% or more, for the angled cuts.
- Screw boxes: a 16 ft by 12 ft deck needs about 676 screws.
Wood vs Composite Decking
- Pressure-treated wood: lower cost, but needs sealing and can split or warp.
- Composite: costs more upfront, needs little upkeep, and often needs closer joist spacing.
- Gaps: composite boards usually need gaps set by the maker for heat expansion.
Maximum Joist Spacing for Wood Decking
Composite and plastic decking follow the maker’s instructions. Your local code may differ.
| Decking | Perpendicular, single span | Perpendicular, multiple span | Diagonal, multiple span |
|---|---|---|---|
| 1 1/4 in thick (5/4) | 12 in | 16 in | 12 in |
| 2 in thick | 24 in | 24 in | 24 in |
Common Mistakes to Avoid
- Using nominal board width. A 2×6 is about 5.5 inches wide, not 6.
- Joists too far apart. Check the decking maker and code for the maximum spacing.
- Forgetting waste. Ends get trimmed and some boards have defects.
- Ignoring board length. Plan seams to land on joists, and stagger them.
- Skipping permits. Most decks need a permit and inspection.
Frequently Asked Questions
Divide the deck width in inches by the board width plus the gap to get rows, then multiply by boards per row and add waste.
For 5/4 wood decking laid across joists, the IRC allows up to 16 inches on center for multiple spans. 2-inch decking allows 24 inches.
The IRC calls for at least two screws or nails at each joist.
Follow the decking maker. Many wood decks use about 1/8 inch, and composite boards often have a set gap.
Usually about 5.5 inches wide and 1 inch thick.
About 5% to 10% for a straight layout, and 15% or more for diagonal or picture-frame layouts.
Divide the length in inches by the joist spacing and add one.
No. Joist size depends on span and species, and must follow code span tables.
Choose lengths that reduce seams, with joints landing on joists and staggered between rows.
Yes, but joists usually need to be closer together, and you need more waste.
It depends on screw size. Check the box label, and compare it with the screw count here.
Most areas require one, especially for decks attached to the house or above a certain height.
References & Sources
| Source | Used For | Link |
|---|---|---|
| ICC, 2021 International Residential Code, Section R507 Exterior Decks | Joist spacing for decking and fastening rules. | https://codes.iccsafe.org/content/IRC2021P2/chapter-5-floors |
| Wikipedia, Deck (building) | How decks are built. | https://en.wikipedia.org/wiki/Deck_(building) |