CategoriesDecking

Decking Calculators

Decking calculators for pressure treated, cedar, and composite decks. Calculate boards, joists, footings, railing, and total project costs.

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Deck material comes from board coverage; deck cost comes from the framing under it.

Board count is deck area divided by the coverage of one board, and coverage uses actual width plus the gap. A 5/4 by 6 board is 5.5 inches wide, so with a 3/16 inch gap it covers 5.69 inches, which is 5% fewer boards than the nominal 6 inches suggests. On a large deck that difference is real money.

The framing below is usually the bigger half of the bill and it is set by span tables, not preference. Joist spacing of 16 inches is standard, but composite boards often need 12 inches, and a diagonal board layout needs 12 regardless of material. Deciding the board pattern changes the joist count.

Settle this before you calculate

Board direction and pattern
Diagonal or herringbone layouts need tighter joist spacing and waste 10 to 15% more board than a straight run.
Joist span and spacing
Span tables set joist depth from the distance between beams. Composite decking often requires 12 inch centres where timber allows 16.
Height above grade
Anything over 30 inches needs guardrails, and that adds posts, rails and balusters that dwarf the extra decking cost.

Where the estimate goes wrong

Using nominal board width
Calculate on actual width plus the gap. A 5/4 by 6 covers 5.69 inches, not 6, which is about 5% more boards than you thought.
Framing composite like timber
Composite sags between widely spaced joists. Check the manufacturer's required spacing before ordering the frame.
Costing boards and forgetting fasteners
Hidden fasteners and joist tape add up on a big deck. Budget them with the boards rather than as an afterthought.

Deck Calculator

Calculate deck boards, joists, footings, and costs for pressure treated, cedar, and composite decks

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