How to calculate deck materials
A deck takeoff has two halves: the walking surface (boards and fasteners) and the structure underneath (joists, beams, posts and footings). This deck calculator works both out from three numbers: the deck length along the house, the depth out from the house and the joist spacing.
Boards = rows x boards per row x (1 + waste %)
Joists = deck length (in) / joist spacing, rounded up, + 1
Posts = (deck length / max post spacing, rounded up, + 1) x number of beams
Worked example: a 16 x 12 ft deck
With 5.5" boards and a 1/8" gap, each row covers 5.625". The deck is 144" deep, so 144 / 5.625 = 25.6, which rounds up to 26 rows. Using 16 ft boards there is one board per row, and 10% waste brings it to 29 boards. At 16" on center, 192" / 16 = 12 spaces, so 13 joists. One beam with posts no more than 8 ft apart needs 3 posts and 3 footings.
Assumptions
- Boards run parallel to the house and joists run out from it. Picture-frame borders, angled boards and stairs are not included, so add those separately.
- Attached decks have a ledger on the house and one dropped beam at the outer edge. Freestanding decks have two beams.
- Screws are counted at two per board at every joist. Butt joints on long runs need a few more.
- Beam size, post spacing, footing size and ledger fastening come from your local code and span tables. The calculator counts pieces; it does not size them.
Building these for a living? See Bolster for deck builders, or how digital takeoff and AutoCost turn measurements into priced estimates.
FAQ
How many deck boards do I need?
Divide the deck depth in inches by the board width plus the gap to get the number of rows, round up, then multiply by the number of boards each row needs and add 10% for waste. A 16 x 12 ft deck with 5.5" boards and 1/8" gaps needs 26 rows, or 29 sixteen-foot boards with waste.
How many joists do I need for a deck?
Divide the length of the deck in inches by the joist spacing, round up and add one for the end joist. A 16 ft deck at 16" on center needs 13 joists. Composite and PVC decking usually calls for 16" or 12" spacing, so check the board manufacturer before you frame.
How far apart should deck posts be?
It depends on the beam size, the joist span and local code, but 6 to 8 ft is common for residential decks. Set the max post spacing field to the number your beam span table allows and the calculator counts posts and footings for every beam.
How much waste should I add for decking?
About 10% for straight boards on a simple rectangle. Use 15% or more for diagonal boards, picture-frame borders or decks with angles, because every cut leaves an offcut.
Can I turn this into a deck estimate?
Yes. Add your unit prices to see a material total here. In Bolster, AutoCost prices the same items with live regional material and labor costs and builds the estimate and proposal for you.
