Free decking calculator
Tell the decking calculator your deck size, board width and gap, and it counts the rows of boards across the deck, the linear feet of decking, how many full-length boards to buy and the hidden fasteners to go with them — updated live, as you type.
On this page14 sections
| Item | Quantity | Est. cost |
|---|---|---|
| Deck boards | 36 × 16 ft | $1,440 |
| Hidden fasteners | 432 clips | $238 |
Prices are regional estimates: composite board ≈ $40/16 ft, hidden clips ≈ $0.55 each. Wood decking and screws cost less; confirm pricing and board dimensions with your supplier.
Board dimensions, gaps and prices vary by product. How accurate is this?
Results are estimates. Consult a professional.
How the decking calculator works
Deck boards are sold one board at a time, but a deck is a surface — so the job is to work out how many boards it takes to cover that surface once you allow for the gap between every board. The calculator runs the boards along the length of the deck, then counts how many rows it takes to march across the width. Each row eats up its own face width plus the gap you leave for drainage and expansion. From the rows it gets the total linear feet of decking, adds a waste margin, and divides by the length of the boards you are buying to tell you how many to put in the cart.
What the result tells you
Two numbers carry the order. The board count is how many full-length boards you buy and carry home. The linear-feet figure is the same deck expressed as a running total — useful when a supplier prices by the foot or when you are mixing board lengths to cut down on seams. The calculator also estimates hidden fasteners, because on a composite deck the clips are a real line on the receipt, not an afterthought.
What goes into your decking estimate
A decking estimate is built from a handful of measurements and one habit of thumb. Get the board width and gap right and the row count is right; get the row count right and everything downstream — linear feet, boards, fasteners — falls into place.
Deck length and width — the surface
Length times width is the floor area, but the two are not interchangeable here. Length is the direction the boards run, so it sets how long each row is. Width is the direction the rows stack, so it sets how many rows you need. Swapping them changes how many board ends you cut and how much you waste, even though the area is identical.
Board width and gap — the effective width
The number that drives the row count is not the board's face width on its own, but the face width plus the gap. A 5.5-inch composite board with a 1/8-inch gap effectively occupies 5.625 inches of deck. Always use the actual face width, not the nominal name: a "2×6" measures 5.5 inches, and a 5/4×6 composite is 5.5 inches too — the names round up, the boards do not.
Board length — how you buy them
Decking comes in fixed lengths, commonly 12, 16 and 20 feet. Picking a length that divides cleanly into your deck's run leaves less off-cut at the end of each row. A 20-foot deck run filled with 16-foot boards leaves a 4-foot gap every row that has to be pieced — which is exactly the off-cut your waste factor is there to absorb.
Waste factor — the safety margin
Boards split, ends get trimmed square, and the leftover at the end of one row is rarely long enough to start the next. A 10% margin covers a straight, parallel layout. The calculator adds 10% by default and lets you raise it for diagonal or picture-frame patterns, which cut far more.
A worked example using the decking calculator
Maria is building a 20 ft × 12 ft deck with 5.5-inch composite boards, a 1/8-inch gap, and 16-foot boards running the 20-foot length. She wants the board count, the linear feet, and a 10% waste margin.
Step 1 — Count the rows across the width
Effective board width = 5.5 + 0.125 = 5.625 in. The deck is 12 ft = 144 in wide, so 144 ÷ 5.625 = 25.6, rounded up to 26 rows.
Step 2 — Find the linear feet of decking
Each row spans the 20-foot length, so 26 × 20 = 520 linear feet of decking before any waste.
Step 3 — Add the waste margin
With 10% added, 520 × 1.10 = 572 linear feet to buy.
Step 4 — Convert to whole boards
At 16 feet per board, 572 ÷ 16 = 35.75, rounded up to 36 boards. The area cross-check (240 ft² ÷ 7.5 ft² per board, plus waste) lands on the same 36, and the deck needs about 432 hidden clips at 1.8 per ft².
How many deck boards do I need?
For a ballpark before you measure, this table gives the linear feet and 16-foot board count for common deck sizes, using 5.5-inch boards with a 1/8-inch gap and a 10% waste margin. Boards run along the longer (length) dimension.
| Deck size (L × W) | Area (ft²) | Rows | Linear ft (+10%) | 16 ft boards |
|---|---|---|---|---|
| 10 ft × 10 ft | 100 | 22 | 242 | 16 |
| 12 ft × 12 ft | 144 | 26 | 343 | 22 |
| 16 ft × 12 ft | 192 | 26 | 458 | 29 |
| 20 ft × 12 ft | 240 | 26 | 572 | 36 |
| 20 ft × 16 ft | 320 | 35 | 770 | 49 |
| 24 ft × 20 ft | 480 | 43 | 1,135 | 71 |
Assumes 5.5 in boards, 1/8 in gap, boards run the length, 10% waste. Rows = ceil(width in inches ÷ 5.625). Diagonal or picture-frame layouts need more — raise the waste factor.
When to use this decking calculator
Reach for it whenever a deck surface turns on how many boards to buy — which is every build, because decking is one of the priciest line items and both over- and under-ordering hurt.
- Building a new deck — to size the decking order from your framed footprint.
- Resurfacing an old deck — reusing the joists but replacing the boards on top.
- Comparing board lengths — to see whether 12, 16 or 20 ft boards leave the least off-cut for your run.
- Pricing composite vs. wood — same board count, sharply different per-board cost, plus the hidden-clip line for composite.
Common deck board sizes and gaps
The row math is the same for every board, but the face width and recommended gap change with the material. Use the actual face width — the nominal name always rounds up.
| Board | Actual face width | Typical gap | Note |
|---|---|---|---|
| Composite (Trex etc.) | 5.5 in | 1/8–1/4 in | Hidden clips set a consistent gap; 1/8 in above 40°F |
| 5/4×6 wood (radius-edge) | 5.5 in | 1/8 in | The classic pressure-treated decking board |
| 2×6 lumber | 5.5 in | 1/8 in | Thicker; used for docks and heavier spans |
| 5/4×4 / 2×4 | 3.5 in | 1/8 in | Narrow boards — more rows, more fasteners per ft² |
Face widths follow standard lumber dressing and Trex published dimensions. Composite gap and end-gap recommendations follow Trex installation guidance (1/8 in above 40°F, wider below).
Composite is dimensionally stable and installs with hidden fasteners that hold the gap for you; wood needs you to set the gap by eye or with a spacer, and saturated treated lumber is laid tight because it shrinks as it dries.
Waste factor: how many extra boards to order
Order exactly the calculated linear feet and you will come up short. The leftover at the end of one row rarely starts the next, ends get trimmed square, and the occasional board is cracked or bowed enough to set aside. A modest overage covers all of it.
10% for straight, more for angles
For boards laid straight and parallel to the deck edge, add 10%. A 45-degree diagonal layout wastes far more off-cut at the corners — figure 15%. Herringbone and picture-frame borders cut the most: 15–20%. The calculator defaults to 10% and adjusts up to 25%.
Why running short is its own problem
Leftover boards are wasted money, but running short means a second trip — and a real risk the store's next batch of composite is a slightly different color lot or shade, leaving a visible mismatch on a finished deck. The overage exists so the whole surface comes from one order.
Decking definitions
How accurate is this decking calculator?
The board geometry is exact. Deck width divided by the effective board width (face plus gap), rounded up, is the precise number of rows, and rows times length is the exact linear feet your boards must cover. If your measurements and your board's actual face width are right, the count is right.
The fastener estimate and the board count past the linear-feet stage are planning figures. Hidden-clip coverage shifts a little with joist spacing and board width — 1.8 per ft² assumes 16-inch on-center joists and 5.5-inch boards — so confirm the coverage on your specific fastener box. How many whole boards you buy also depends on how cleverly you nest the off-cuts; a careful layout can save a board or two, a careless one can cost extra. Treat the board and clip counts as a confident estimate. Confirm board dimensions against the manufacturer's spec sheet, and order to the high side: a color-lot mismatch from a second trip costs far more than a couple of spare boards.
Frequently asked questions about the free decking calculator
About this decking calculator
This decking calculator runs entirely in your browser — nothing you type is sent anywhere or stored. It turns your deck's length and width, your board's actual face width and gap, and a waste factor into a board count, linear feet of decking and a hidden-fastener estimate, recomputing instantly as you adjust any input.
It is one of the material estimators on our construction calculators shelf. For the concrete footings and piers underneath the deck try the concrete calculator, or browse every tool in the full calculator directory.