Decking Calculator
How many deck boards and screws you need by deck size.
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Decking is the surface you walk on — the satisfying part that goes on fast once the frame is right. Straight starter boards, consistent gaps, and seams that land on joists are what separate a pro-looking deck from a wavy one.
DifficultyIntermediate
TimeA weekend for the boards on a typical deck, once the frame is done
SkillsComfortable with a drill and a circular saw; patience for keeping lines straight
PermitsDecking itself usually rides on the framing permit — but the deck as a whole almost always needs one.
Tools you'll need
- Drill/driver and coated deck screws (or a hidden-fastener kit)
- Circular saw or miter saw
- Spacing tool or a couple of 16d nails for gaps
- Chalk line and tape measure
- Pry bar (for straightening bowed boards)
- Orbital sander for any rough spots
Step by step
- Acclimate and sort the boards Let boards sit a few days near the site, then sort out the worst bows for short runs and cuts.
- Start straight off the house Snap a chalk line and fasten the first board dead straight — every board after it follows this one.
- Gap the boards consistently Leave about a 1/8" gap between boards for drainage and expansion (pressure-treated boards can be butted tighter since they shrink as they dry).
- Stagger seams over joists Any end-to-end seam must land on a joist, and stagger seams between rows so they don't line up.
- Fasten and trim Two screws per joist (or hidden clips), then run a chalk line and trim the overhang flush in one pass for a clean edge.
- Sand and seal Knock down any splinters, then seal or stain — see the deck stain calculator for how much you'll need.
Common mistakes
No gap between boards
Boards need a drainage and expansion gap or they trap water and cup. Use a spacer for consistency (PT lumber is the exception — it shrinks).
Seams that miss a joist
Every butt seam must be supported by a joist. Plan board lengths so ends land on framing, or add blocking.
Uncoated or wrong-length screws
Use exterior-rated coated or stainless screws long enough to bite ~1" into the joist — interior screws rust and stain.
Forgetting to seal cut ends
Field cuts expose untreated wood. Brush end-cut preservative on cuts for pressure-treated boards.
Safety
- Wear eye protection when cutting — composite and PT dust is irritating.
- Keep fasteners and offcuts off the walking surface as you go.
Frequently asked questions
How big a gap between deck boards?
About 1/8" for dry boards. Wet pressure-treated boards can be installed nearly tight because they shrink as they dry.
How many deck boards do I need?
Enter your deck length, width, and board size above — the calculator returns boards (with 10% waste) plus screws.
Composite or pressure-treated?
Composite costs more up front but barely needs maintenance; pressure-treated is cheaper but wants cleaning and re-sealing every couple of years.
Should I seal a new deck?
Seal composite never; seal pressure-treated after it dries out (often a few weeks to months), then re-coat as needed.
Next steps for this project
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