Deck & Fence
Deck Calculator
Estimate the decking for a rectangular deck. Enter its size, board width, gap and board length, and this deck calculator works out the deck boards, linear footage, joists and screws, with a material cost. It is a planning estimate, not a structural design.
Result
Enter your measurements and press Calculate. What to buy, the working and the cost will appear here.
How to use the deck calculator
- Enter the deck length (the direction the boards run) and width (out from the house, the direction the joists run).
- Enter the board face width and gap. 5/4 × 6 and 2 × 6 boards are 5 1/2 in wide.
- Choose the board length and joist spacing.
- Add prices and press Calculate.
How deck boards are calculated
rows of boards = deck width ÷ (board width + gap), rounded up
Each row runs the full deck length; rows longer than one board are joined over a joist. The calculator plans the cuts from your board length, then adds the allowance.
joists = deck length ÷ joist spacing, rounded down, + 1
Worked example: a 16 × 12 ft deck
A 16 × 12 ft deck uses 16 ft boards, 5.5 in wide with 3/16 in gaps, on joists 16 in on center. Boards cost $24 and joists $22:
- Rows: 12 ft ÷ 5.6875 in per row, rounded up = 26
- Boards: 26 rows of 16 ft cut from 16 ft boards = 26 × 1.1 → 29
- Joists: 16 ft ÷ 16 in, rounded down, + 1 = 13
Deck boards: 29 × 16 ft; Deck area: 192 ft²; Joists: 13; Deck screws: 676; Material cost: $982.00. Open this example in the calculator.
Choosing boards
- Match board length to the deck where you can: a 16 ft deck with 16 ft boards has no joints. Otherwise stagger joints and land them on joists.
- Gaps: about 1/8–1/4 in for drainage; wet pressure-treated boards shrink, so they are often laid tight.
- Composite decking often needs joists at 12 in on center and its own gap and fastener system; follow the maker.
- Diagonal decking needs closer joists and 15% or more extra boards.
What this calculator does not cover
The frame of a deck (joist size and span, beams, posts, footings, ledger attachment, stairs and railings) is structural and is governed by your local building code, and most decks need a permit. This calculator counts decking and joists for planning only. For footings, see the concrete footing calculator; for steps, the stair calculator.
Assumptions and limitations
- The deck is a rectangle with boards running along its length.
- Fasteners are estimated at two per board at every joist.
- Rim joists, blocking, beams, posts and hardware are not counted.
Deck questions
How many deck boards do I need for a 12 × 12 deck?
With 5.5 in boards and 3/16 in gaps, 26 rows across 12 ft. Using 12 ft boards that is 26 boards, or 29 with 10% extra.
How far apart should deck joists be?
16 in on center is the most common spacing for wood decking; many composite boards require 12 in. Joist size and span are set by code tables.
How many screws do I need for a deck?
About two per board at each joist: roughly 350 per 100 ft² of deck at 16 in joist spacing.
Formulas, unit conversions and default values are explained on our methodology page. Results are estimates; check manufacturer data and local codes before you buy or build.