What to measure
How to use the deck calculator
Enter the deck’s length along the house and its width out from the house. The calculator assumes the boards run along the length, across joists that span the width. Enter the actual board width and the gap, pick the board length you’ll buy, and set the joist spacing. Add a price per board for the decking cost.
Pick a board length that divides the deck length evenly, or is just longer: a 16 ft deck takes one 16 ft board per row with no joints, but two 12 ft boards and a lot of offcut.
The formula
- Rows = (width + gap) ÷ (board width + gap), rounded up
- Boards = rows × boards per row × (1 + waste), rounded up
- Joists = length ÷ spacing + 1, rounded up
- Screws = rows × joists × 2
Worked example: 16 × 12 ft deck, 5.5 in boards, 3/16 in gaps
- (144 + 0.1875) ÷ (5.5 + 0.1875) = 25.4 → 26 rows
- 16 ft boards: 1 per row → 26 × 1.10 = 28.6 → 29 boards
- 26 × 16 = 416 linear ft of decking
- Joists at 16 in: 192 ÷ 16 + 1 = 13 joists, 12 ft long
- 26 rows × 13 joists × 2 = 676 screws
Deck boards by deck size
| Deck | Board length | Boards (+10%) | Joists at 16 in |
|---|---|---|---|
| 10 × 10 ft | 10 ft | 25 | 9 |
| 12 × 12 ft | 12 ft | 29 | 10 |
| 16 × 12 ft | 16 ft | 29 | 13 |
| 16 × 16 ft | 16 ft | 38 | 13 |
| 20 × 12 ft | 20 ft | 29 | 16 |
| 20 × 16 ft | 20 ft | 38 | 16 |
| 24 × 16 ft | 12 ft | 75 | 19 |
5.5 in boards with 3/16 in gaps; the width is out from the house.
Common mistakes
- Using the nominal width. A “6 inch” deck board is 5.5 inches wide. Using 6 inches undercounts by about 8%.
- Board lengths that don’t fit the deck. Joints must land on a joist, and leftover pieces rarely fit elsewhere. Match the board length to the deck length, or plan the joints first.
- Forgetting the gap in the count. Gaps add up: 26 gaps of 3/16 inch are almost 5 inches, close to a whole board.
- Treating this as a framing plan. The joist count assumes one span from the house to a beam. Beams, posts, footings, ledger fastening and joist sizes come from the span tables, not from the decking count.
Frequently asked questions
How many deck boards do I need for a 16 × 12 deck?
With 5.5 in boards and 3/16 in gaps running along the 16 ft side, 26 rows of 16 ft boards. With 10% extra, buy 29 boards.
How do I calculate deck boards?
Divide the deck width (in inches) plus one gap by the board width plus the gap and round up; that's the number of rows. Multiply by the boards needed to span each row's length, then add waste.
How many linear feet of decking per square foot?
About 2.11 linear feet per sq ft with 5.5 in boards and 3/16 in gaps, or 211 linear feet per 100 sq ft.
How wide is a 5/4 × 6 deck board?
5.5 inches wide and about 1 inch thick. A 2 × 6 is also 5.5 inches wide but 1.5 inches thick. Composite boards are usually 5.5 inches too; check the label.
What gap should I leave between deck boards?
Trex spaces its composite boards 3/16 to 3/8 inch side to side, and wider under trees so debris falls through. Pressure-treated boards shrink as they dry, so their gaps open up over time, as Trex also notes. Follow the maker's guide for your boards.
How many screws per deck board?
Two at every joist. IRC R507.7 requires wood decking to be fastened to each supporting member with at least two 8d threaded nails or two No. 8 wood screws. A 16 × 12 deck on joists 16 in apart takes about 676 screws.
What joist spacing do I need?
16 inches on center is the usual starting point, but the maximum depends on the decking. IRC Table R507.7 sets it for wood decking and the manufacturer for composite. Diagonal decking needs closer joists.
Does the calculator size joists, beams and footings?
No. Joist and beam sizes depend on species, grade, span and load. Use the span tables in the IRC (section R507) or the American Wood Council's DCA 6 deck guide, and check what your building department requires.
Sources
Formulas and defaults last reviewed October 30, 2026. How we check our math.