Deck Calculator
Deck boards, joists, and screws for that new deck?
How this is calculated
- Deck area = length × width; order amount adds the waste factor
- Boards = ⌈order amount ÷ (board width × board length)⌉
- Joists = ⌊width × 12 ÷ spacing⌋ + 1 (spaced along the width, plus one at each end)
- Screws ≈ 3.5 per sq ft (2 per joist crossing per board)
Common questions
How many deck boards do I need for a 12×20 deck?
A 12 × 20 deck is 240 sq ft. With standard 5.5 in boards in 12 ft lengths and a 10% waste allowance, that is about 48 boards. Joists at 16 in on center across the 20 ft span: 16 of them.
How far apart should deck joists be?
16 in on center is the residential standard for wood decking laid perpendicular. Drop to 12 in for diagonal installations (30-45° cuts span more joists), most composite boards (check the manufacturer — voided warranties are real), and 5/4 boards at the max. 24 in only works for wood boards laid perpendicular on a robust frame.
How many screws per deck board?
Two screws at every joist crossing is the norm, which works out to roughly 3.5 screws per sq ft of deck. A 5 lb box of deck screws (~320 screws) covers about 90-100 sq ft. Stainless or coated screws matter — the wrong fastener streaks composite and stains cedar.
Wood or composite decking?
Pressure-treated wood runs $3-8/sq ft and needs periodic cleaning/sealing; composite runs $8-15/sq ft and needs near-zero maintenance but heavier framing (12 in joists usually required, and it sags more between joists). Many homeowners go wood for the price and feel, composite for the "never touch it again".
Do I need a permit for a deck?
In most jurisdictions, yes for anything attached to the house or above ~18 in off the ground — footings, frost depth, and railings all get inspected. A freestanding ground-level platform deck often escapes, but check before building: undoing an unpermitted deck costs multiples of the permit fee.