Deck Board Calculator
Count the deck boards your project needs — enter the deck size, board width, and gap to get the exact board count, a buy-list by stock length, and the spacing.
Free · no sign-up ·
You need · incl. +10% waste
- Deck area
- 192 ft²
- Exact decking
- 416 lin ft
- Joists
- 13
- Joist material
- 156 lin ft
Boards to buy by stock length
Board counts include a 10% waste allowance. Linear feet is what lumberyards price from; the buy-list shows how many boards at each stock length.
The formula & specs
Board rows = Deck width ÷ (board width + gap), rounded up. Boards to buy = (rows × deck length) ÷ stock length, rounded up, plus waste. A “6-inch” board is 5.5 inches actual.
| Board face widths (actual) | 5.5" · 5.25" · 3.5" |
| Board gaps | 1/8" · 3/16" · 1/4" |
| Joist spacing (on-center) | 12" · 16" · 24" |
| Board stock lengths | 8 ft · 10 ft · 12 ft · 16 ft · 20 ft |
Nominal board sizes differ from actual: a “6-inch” board is 5.5" wide. The calculator uses actual widths so the row count matches your real material.
How to estimate deck board materials
The board count is set by how many board widths fit across the deck. Deck boards are sold by nominal size but installed at their actual width: a “6-inch” board (5/4×6 or 2×6) is really 5.5 inches, a “4-inch” board is 3.5 inches, and composites run 5.25–5.5 inches. Add the gap you will leave between boards, divide the deck width by that, and round up — that is your number of rows. Multiply by the deck length and divide by your board stock length to get boards to buy.
Spacing is where board counts go wrong. Pressure-treated boards are installed tight (about 1/8 inch) because they shrink as they dry; kiln-dried and composite boards need a 1/8–1/4 inch gap so they can move with the weather. If you plan a picture-frame border, add the perimeter boards separately — a border adds a few boards and changes how you cut the field. Set the exact board width and gap above so the row count matches the product you are buying.
Buy the length that minimizes butt joints. The calculator lists how many boards you would need at 8, 10, 12, 16, and 20-foot lengths so you can pick the one that spans your deck with the fewest seams and least offcut. Where you must join boards end to end, stagger the joints across rows so they do not line up, and always order about 10% extra for cuts, miters, and the occasional cupped or split board you set aside.
Frequently asked questions
How many deck boards do I need?+
Divide the deck width by the board width plus the gap to get the number of rows, multiply by the deck length for linear feet, then divide by your board stock length and round up. A 16 ft × 12 ft deck in 5.5-inch boards with a 1/8-inch gap is about 26 rows and roughly 29 sixteen-foot boards. Enter your size and board width above for an exact count.
Are deck boards actually 6 inches wide?+
No — a “6-inch” deck board (5/4×6 or 2×6) is actually 5.5 inches wide, and a “4-inch” board is 3.5 inches. Composite boards are usually 5.25–5.5 inches. The calculator uses these actual widths because that is what determines how many boards fit across your deck.
How much gap should I leave between deck boards?+
Install pressure-treated boards tight, about 1/8 inch, because they shrink as they dry. Use a 1/8–1/4 inch gap for kiln-dried wood and composite so the boards can expand and contract. The gap also lets water and debris fall through instead of collecting on the deck.
How do I stagger deck board joints?+
Where boards meet end to end over a joist, offset those seams from row to row so they do not line up in a straight line across the deck — it is stronger and looks far better. Plan joints to land on joists, and a longer board stock length means fewer joints to stagger.
Do composite and wood decks need the same number of boards?+
Yes — the board count depends on the deck size and board width, which are the same for composite and wood at a given nominal size. Only the recommended gap differs slightly. Composite boards sometimes come in fixed lengths, so check the buy-list against the lengths your supplier stocks.
Should I add picture-frame or border boards?+
A picture-frame border (boards mitered around the deck edge) adds the perimeter boards on top of the field count and usually needs extra blocking under the seams. Add the border boards separately, and bump your waste allowance for the mitered corner cuts.
How many screws or fasteners per deck board?+
Face-screwed boards take about 2 screws at every joist they cross. Hidden-fastener systems use one clip per board at each joist instead. The deck calculator estimates total face screws; for clips, multiply your board rows by the number of joists.
Which board length wastes the least?+
Buy the stock length that spans your deck with the fewest joints and least offcut. The calculator shows the board count for 8, 10, 12, 16, and 20-foot lengths side by side — a 16-foot deck is most efficient in 16-foot boards, but odd sizes sometimes waste less split into two shorter lengths.
How many extra deck boards should I order?+
Add about 10% for cuts, miters, and the odd board you reject for cupping or splits. For a 45-degree diagonal layout, add about 15% because the angled end cuts waste more. The deck calculator includes an adjustable waste allowance and a diagonal option.
Is this deck board calculator free?+
Yes — it is completely free, needs no sign-up, and runs entirely in your browser, so nothing you enter is stored or sent anywhere.