Before you call the lumberyard, take two measurements on site. Walk out the deck and write down the length and width of the flat area you plan to cover. Then bend down and look at the board you want to use. Most treated lumber sold as a 2x6 actually measures 5.5 in across, and that real width is what rows are built from.
Step one, measure the box
Measure along the longest edge and the shortest edge of the deck in feet. Ignore the railing for now, and skip the stair treads because they are quoted separately. If the deck jogs around a corner, break it into two rectangles and add the areas later.
Step two, pick the board
A 5.5 in wide board is the default. Standard lengths run 12, 16 and 20 ft. Choose 16 ft when your deck depth is 12 ft, because one 16 ft board crosses the depth and leaves only a small tail to cut off. Longer boards mean fewer seams, which look better and leak less.
Step three, set the gap
Boards move as humidity changes, so leave a gap. For wood, 1/4 in is the usual opening. For composite, follow the wrapper, which often says 1/8 in. The gap also keeps water from pooling between boards.
Step four, add waste
Order about 10% extra. That buffer covers cuts at the house wall, around vents, and any board that splits on the fastener. It also saves you a trip back to the store when the third board has a twisted edge.
Run the boards the right way
Most decks run the long boards parallel to the house or out from it. Running them perpendicular to the house means each board spans the full deck depth, so you waste fewer offcuts and the pattern looks longer. If you plan a picture frame border, order one extra box of matching boards, because the border pieces are cut from full lengths and do not count toward the field rows.
Wood versus composite in practice
Pressure-treated pine is cheap and goes down fast, but it needs stain every couple of years. Composite costs two to three times more per board, never splinters, and washes with a hose. The row count is identical because both come in 5.5 in widths, so the material choice does not change the order, only the budget.
Worked example
A 16 ft by 12 ft deck, 5.5 in boards, 1/4 in gap, 16 ft lengths: roughly 33 rows, and about 42 boards once the 10% waste is folded in. Round up at the counter.
Decking Calculator
Start by measuring the deck footprint and picking a 5.5 in board, and the count lands on the rows and 16 ft boards you need to buy.
How to use
- Measure the deck length and width in feet
- Enter the actual board width, 5.5 in for a common 2x6
- Set the board length, 16 ft is a popular pick
- Choose the gap, 1/4 in is standard for wood
- Add the 10% waste factor and read the board count
Supported formats and limits
- Boards only; joists, railing and stairs are not counted
- Actual count shifts with corners, stair openings and seam layout
- US units only, feet and inches
Examples
A 16 ft by 12 ft deck with 5.5 in boards, 1/4 in gap, 16 ft lengths lands near 42 boards with 10% waste.Common errors
- Deck length and width must be positive
- Gap should sit between 0 and 0.5 in
- Board width must be greater than zero
Privacy and browser support
All calculations run locally in your browser. No measurements are uploaded.
FAQ
How wide is a typical deck board?
A surfaced 2x6 measures about 5.5 in, not the nominal 6 in. That half inch matters once you count rows, so always use the real width.
How big should the gap between boards be?
Leave 1/4 in for cedar or redwood, and 1/8 in for pressure-treated or composite. The gap lets wet boards swell without cupping.
How many 16 ft boards do I need?
A 16 ft by 12 ft deck needs about 33 rows across the 12 ft width. With 10% waste for cuts, order around 42 boards.
Wood or composite?
Wood costs less up front and cuts with a saw. Composite carries a higher price but never needs staining and lasts decades. Both boards are the same 5.5 in width, so the count works either way.
Which board length minimizes seams?
Buy the longest board that spans your deck depth in one piece. If the deck is 12 ft deep, a 16 ft board gives one seam per row on the far side.
Do I need to double joists?
Yes, under stairs and hot tubs. The calculator counts decking only, not the framing.