Decking Calculator
Calculate deck boards, joists, screws and cost for any deck size, by board width and spacing.
What you need
29
Deck boards to buy — Includes waste. Assumes you can cut boards to length.
- Deck area
- 192.0 sq ft
- Rows of decking
- 25.3
- Joists needed
- 13
- Total joist length to buy
- 156.0 ft
- Deck screws
- 744
How many board widths fit across the deck, including gaps.
Spanning the deck width at your chosen spacing, plus one to close the end.
Two screws per board at every joist crossing.
Estimates for planning. Confirm quantities with your supplier before ordering — see the assumptions below for the figures used.
How to use the decking calculator
A deck is two material lists: the boards you walk on and the frame underneath. This calculator does both — board count from your deck size and board width, joists from your span and spacing — plus fasteners and an optional cost.
What this calculator assumes
Every estimate depends on figures that vary by supplier and conditions. These are the ones used here, so you can sanity-check the result against your own quote.
- Board count assumes you can cut from stock lengths freely. If you want unbroken boards running the full deck length, buy one board per row at that length instead.
- Joists are counted spanning the deck width at your chosen spacing, plus one to close the far end. Rim joists, beams, posts, footings, ledger boards and hardware are not included.
- Screws are estimated at two per board per joist crossing, plus 10%. Hidden fastener systems are priced per square foot instead.
- Joist size and span, beam sizing, post spacing and footing depth are all governed by building code and depend on your loads and species. This calculator does not size the structure — a deck almost always needs a permit.
Frequently asked questions
How many deck boards do I need?
Divide the deck width by the board width plus the gap. A 12 foot wide deck with 5½ inch boards and a ¾6 inch gap fits 144 ÷ 5.6875 ≈ 26 rows. Multiply by the deck length for total linear feet, then divide by the stock length you can buy.
What gap should I leave between deck boards?
About ⅛ to ¼ inch for drainage and airflow. Wet pressure-treated boards shrink as they dry, so butt them tight or gap them only ⅛ inch and they will open up on their own. Kiln-dried and composite boards should be gapped to the manufacturer's spec, which is often ¼ inch for expansion.
How far apart should deck joists be?
Sixteen inches on centre is standard for wood decking laid perpendicular to the joists. Drop to 12 inches for diagonal decking and for most composite boards, which sag between wider spans. Always check the decking manufacturer's requirement — it voids warranties.
Do I need a permit to build a deck?
Almost always, if it is attached to the house or above a certain height — often 30 inches. Permits cover footing depth, ledger attachment, guard height and stair geometry, which are the things that actually make a deck safe. Check with your building department before ordering material.
Related calculators
Lumber Calculator
Work out how many boards you need for a run, plus linear feet, board feet and total cost.
Joist Calculator
Count floor or deck joists for a given span and spacing, plus rim joists, blocking and hangers.
Stair Calculator
Calculate risers, treads, stringer length and total run from your total rise.
Patio Calculator
Get a complete paver patio material list: pavers, base gravel, bedding sand, joint sand and edging.
Beam Calculator
Estimate the load a beam carries, its tributary width, and the built-up beam material you need.
Board Foot Calculator
Convert lumber dimensions into board feet and total cost — the unit hardwood is sold in.
Some links on this page are affiliate links. If you buy through them we may earn a commission at no extra cost to you, which is what keeps these calculators free. How this site is funded.