Deck Calculator
Estimate the decking boards, joists, and screws for a rectangular deck from its length and width. Factors in your board width, joist spacing, and a waste allowance so you buy the right lumber count in one trip.
How to Use
- Enter the deck length and width in feet to set the surface area you are building.
- Pick the decking board width — 5/4×6 (5.5"), 5/4×4 (3.5"), or 5/4×8 (7.25") — which sets how many rows it takes to cover the deck.
- Choose the joist spacing on center: 16" is standard, 12" is stiffer for diagonal boards, 24" is the wider limit.
- Set a waste percentage (10% is typical) to cover cut-offs, defects, and the gaps between boards.
- Read the board count, joist count, and fastener estimate, and check the step-by-step Show Work below.
Show Work
Notes
About the Deck Calculator
Need a hand with building, materials estimation and site work? The Deck Calculator does the work for you — free, and right here in your browser. Estimate the decking boards, joists, and screws for a rectangular deck from its length and width. Factors in your board width, joist spacing, and a waste allowance so you buy the right lumber count in one trip.
How it works
Type your numbers into the boxes. The answer shows up right away — you do not have to press a button. If you change a number, the answer changes too. So you can try different numbers and watch what happens, or check an answer you worked out yourself. Just make sure each box has the right kind of number in it.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
How many deck boards will I need?
It depends on the board width and the gap between boards. This tool divides the deck width by the board face plus a small drainage gap to get the number of rows, then accounts for length. Narrower 5/4×4 boards take more rows than 5/4×8 to cover the same deck, so the board count rises as the boards get skinnier.
Why leave a gap between deck boards?
A small gap, usually about 1/8 inch, lets water drain and air circulate so the boards dry out and do not trap debris or rot. Pressure-treated lumber is often wet when you buy it and will shrink as it dries, so some installers butt those boards tight and let shrinkage open the gap. The tool builds a gap into the board count.
What joist spacing should I use?
Sixteen inches on center is the standard for most decking laid straight across. Drop to 12 inches if you are running boards diagonally or using thinner composite that needs more support, and 24 inches is the practical maximum and only for certain rated materials. Closer spacing means more joists but a stiffer, less bouncy deck.
How many screws does a deck take?
Plan on roughly two screws per board at every joist crossing, which works out to about 7 fasteners per square foot, or 350 screws for a 12 by 16 deck before waste. The tool estimates from your deck area. Buy hidden clips separately if you are using a grooved-edge composite system, since those use a different count.
Does this size the framing and footings too?
No — this estimates the decking boards, the field joists, and fasteners. Beams, posts, footings, ledger boards, and railing are separate and depend on your span and local code. Use it to budget the deck surface and joists, then size the structural framing and footings with the appropriate span tables or a pro.
How do I use the Deck Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
Ordering decking lumber
Convert deck dimensions into an exact board count plus waste so you buy the right number of boards without a second trip.
Budgeting a new deck
Get boards, joists, and screws up front to price the surface materials before you start the build.
Comparing board widths
See how 5/4×4 versus 5/4×6 versus 5/4×8 changes the board count and the look before you choose a profile.
Planning joist layout
Check how 12, 16, or 24 inch spacing changes the joist count so you balance stiffness against material cost.
Stocking fasteners
Estimate the screw count so you buy enough boxes and avoid stopping the job to run for more.
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