Wood & Trim
Deck Calculator
Estimate deck boards and joist support-line counts from deck size and board layout.
Enter your measurements
Results update as you type.
About this calculator
Enter the deck length and width, actual board width, gap, stock length and joist spacing. The calculator estimates board rows, whole stock pieces, extra allowance and joist support-line count. Fastener quantities require the decking manufacturer’s fastening schedule and the actual support and joint layout.
How to use this calculator
- Measure the deck length and width.
- Confirm board width and the gap you want.
- Choose the stock length you can buy and the joist spacing from the framing plan.
- Add waste for end cuts and defects.
- Check the layout against the joist and beam plan before ordering.
Formula
Rows = ceil((deck width + gap) ÷ (board width + gap)); no trailing gap after the final board.
Boards per row = ceiling(length ÷ stock length)
Boards to buy = ceiling(rows × boards per row × (1 + waste ÷ 100))
Joists = ceiling(length ft × 12 ÷ spacing in) + 1
Assumptions
- Boards run along the deck length. Each row uses whole stock pieces and offcuts are not reused across rows; multiple pieces per row require a separately designed supported-joint layout.
- The deck is a simple rectangle with no stairs, borders or picture framing.
- Joists span the deck width and are counted along the length.
Deck quantity scope
| Project part | Treatment |
|---|---|
| Board rows | No trailing gap |
| Offcuts | Not reused |
| Joist support lines | Counted from the spacing entered |
| Fasteners | Use the manufacturer’s schedule and actual support/joint layout; not calculated here |
| Loads and structural connections | Separate design |
Use the decking manufacturer’s actual support, joint, span and fastening instructions. This table does not specify a fastening pattern or fastener count.
Common mistakes
- Counting a gap after the final board.
- Treating the joist quantity as a span or footing design.
- Assuming all partial stock-board offcuts are reused.
Worked example
Try it: press “Load example” above the form.
A 16 × 12 ft deck with 5.5 in boards and 1/4 in gaps needs 26 rows × 1 stock board; with 10% extra, buy 29 boards. At a specified 16 in joist spacing the model counts 13 support lines. This is not a framing or fastener design.
Limitations
Not a structural design or span table. Beam, footing, ledger, guard and stair material are not included; confirm joist spans and fasteners with local code and an engineer.
Frequently asked questions
Which way do the boards run?
The model runs boards along the entered deck length and counts rows across the deck width. It includes gaps between rows and no extra gap after the final board. Offcuts are not reused.
Does the joist count design the structure?
No. It counts support lines from spacing. It does not choose joist depth, span, bearing, footings or connections. Obtain a design and follow the decking manufacturer’s support requirements.
What does the worked example show?
A 16 × 12 ft deck with 5.5 in boards and 1/4 in gaps needs 26 rows × 1 stock board; with 10% extra, buy 29 boards. At a specified 16 in joist spacing the model counts 13 support lines. This is not a framing or fastener design.
Which assumptions should I check before using the result?
Not a structural design or span table. Beam, footing, ledger, guard and stair material are not included; confirm joist spans and fasteners with local code and an engineer.
Sources
Sources explain the methods and any named product presets. Check your product label where coverage, yield or density differs.
- NIST: PS 20 American Softwood Lumber Standard
DOC PS 20-25, Table 3: nominal and minimum dressed sizes. Confirm moisture, grade, species and actual stock; this is not a fastener schedule, span or structural-design reference.
Calculation documentation · Updated 2026-11-11 · Version 1.6.4. Estimates are planning aids. Confirm quantities, coverage and prices with your supplier before purchasing. Found an error? Report it.
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