Contractor & construction
Deck Calculator
Works deck quantities, where the structural members follow span requirements.
Result
Covers decking and joist count only. Beams, posts, footings, ledger, hardware and railing are additional, and structural spans must follow local code.
Get a Free Business AuditHow the calculation works
Joists at the required spacing across the span, beams and posts to carry them, footings sized for the load, and decking boards for the area including the gap between boards. The structural member sizes and spacings follow the adopted code's span requirements rather than the deck's dimensions.
What moves the number
- Joist span and spacing, which the code governs.
- Beam and post sizing for the load path.
- Footing size and depth, including frost depth.
- Decking board width and the gap between boards.
Where this one goes wrong
Joist spans, beam sizes, post spacing and footing dimensions are code requirements determined by the load and the species, and a deck built to a quantity calculation rather than to the span requirements is a structural problem rather than a material shortfall. Use the code's tables and get it inspected. The ledger connection to the house is the other critical detail, since deck failures concentrate there and the fastening requirements are specific.
Before you quote
- Take spans and member sizes from the adopted code's tables.
- Set footing depth below the local frost line.
- Follow the specific requirements for the ledger connection.
Where the rules come from
Questions
About this calculator
What determines joist size and spacing?
The span, the load and the species, under the adopted code's tables rather than the deck's overall dimensions. A deck built to a material calculation rather than to the span requirements is a structural problem, not a shortfall.
Why do decks fail at the house connection?
Because the ledger connection carries a large share of the load and the fastening requirements are specific. Deck collapses concentrate there, which is why that detail is prescribed in the code and inspected closely.
How deep should footings be?
Below the local frost line, which varies by region and is set locally. A footing above it will heave with freezing ground, which lifts and racks the structure over successive winters whatever its size.
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How to use this calculator
- Deck length (ft) (ft). Enter your own figure; the page starts at 16 ft.
- Deck width (ft) (ft). Enter your own figure; the page starts at 12 ft.
- Board width (inches) (in). Enter your own figure; the page starts at 5.5 in.
- Gap between boards (inches) (in). Enter your own figure; the page starts at 0.25 in.
- Joist spacing (inches) (in). Enter your own figure; the page starts at 16 in.
- Waste allowance (%). Enter your own figure; the page starts at 10%.
The result updates as you type, so you can try a figure, see what it does to decking needed (linear ft), and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Deck length (ft) | 16 ft |
| Deck width (ft) | 12 ft |
| Board width (inches) | 5.5 in |
| Gap between boards (inches) | 0.25 in |
| Joist spacing (inches) | 16 in |
| Waste allowance | 10% |
| Cost per linear foot of decking | $3.8 |
What comes out
| Result | Value |
|---|---|
| Decking needed (linear ft) main result | 458 |
| Deck area (sq ft) | 192.0 |
| Joists needed | 13 |
| Decking cost | $1,739 |
A 16 × 12 deck is 192.0 sq ft, needing 458 linear feet of decking across 26 rows and 13 joists, about $1,739 in boards.
How sensitive the answer is
Holding everything else steady and changing only cost per linear foot of decking, this is what happens to decking needed (linear ft). It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per linear foot of decking ($) | Decking needed (linear ft) |
|---|---|
| $1.9 | 458 |
| $2.85 | 458 |
| $3.8 example above | 458 |
| $5.7 | 458 |
| $7.6 | 458 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Deck length (ft) (ft)
- Measured in ft, starting at 16 ft.
- Deck width (ft) (ft)
- Measured in ft, starting at 12 ft.
- Board width (inches) (in)
- Measured in in, between 0 and 24, starting at 5.5 in.
- Gap between boards (inches) (in)
- Measured in in, between 0 and 2, starting at 0.25 in.
- Joist spacing (inches) (in)
- Measured in in, between 0 and 36, starting at 16 in.
- Waste allowance (%)
- A percentage, between 0 and 40, starting at 10%.
- Cost per linear foot of decking ($)
- A money amount, starting at $3.8.
What the results tell you
| Result | What it is |
|---|---|
| Decking needed (linear ft) | The headline result. A number. |
| Deck area (sq ft) | A decimal figure. |
| Joists needed | A number. |
| Decking cost | A money figure. |
Overhead is where these go wrong
Pricing and margin figures are only as good as the cost you put in. If your cost covers parts and wages but not the van, the insurance, the phone and the unbilled hours, the markup is paying for those before it becomes profit, and the margin you think you are making is not the one you are making.
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