Boards and framing
Decking calculator
Enter the deck's dimensions and the real width of your board. The calculator gives you board count, lineal feet, joists at your spacing, and the fasteners to put it all together.
OrderExact · https://orderexact.com/decking/
Board width plus the gap is what actually covers
A 2x6 is 5.5 inches wide, not 6. With a 3/16 inch gap each row covers 5.6875 inches, so a 12 foot width takes 26 rows, not 24. That difference is two full rows of decking across an average deck, and it is the most common reason a delivery comes up short.
The calculator works from the real width and the gap you choose, and reports lineal feet as well as a board count — lineal feet is how a lumber yard will quote composite and hardwood, board count is how you buy pressure-treated.
Actual widths of common boards
| Sold as | Actual width | With a 3/16 in gap, each row covers |
|---|---|---|
| 2x4 | 3.5 in | 3.6875 in |
| 5/4x6 | 5.5 in | 5.6875 in |
| 2x6 | 5.5 in | 5.6875 in |
| 2x8 | 7.25 in | 7.4375 in |
Buy the length that spans cleanly
A 16 foot board across a 15 foot deck is one cut and no joint. Two 8 foot boards across the same deck leave a butt joint that has to land on a joist, needs blocking to support both ends, and will hold water for the life of the deck. Where the price per lineal foot is similar, the longer board is always the better buy.
Where a deck is wider than any board you can get, plan the joint line deliberately — stagger the joints across the deck rather than running them all down one seam, and double the joist underneath.
Joists, spans and what this does not tell you
Joists are spaced along the deck's length and span its width, so the count comes from the sides rather than the area: a 16 foot length at 16 inches on center takes 13 joists, each one 12 feet long on a 12 foot deck.
What spacing you are allowed to use is a different question. Decking carries a span rating — a lot of 5/4 and composite board is rated for 16 inches and will feel springy at 24, and laying boards diagonally increases the effective span by about 40 percent, which often forces a tighter spacing.
Joist size, beam size, post spacing and footing depth are structural and governed by code. This calculator counts materials; it does not size them. Post holes and footings are usually the next thing you will need — the concrete calculator covers those.
Common 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, then divide the deck length by your board length for boards per row. This calculator does both and adds your waste allowance on top.
What gap should I leave between deck boards?
Between 1/8 and 1/4 inch for kiln-dried and composite boards. Wet pressure-treated lumber can be laid tight because it shrinks as it dries, and will open up to roughly the right gap on its own.
How many screws do I need for a deck?
Two per board at every joist it crosses. The calculator multiplies the board rows by the joist count for you — it adds up fast, and running out on a Sunday is the classic way to lose a weekend.
Is a 2x6 really six inches wide?
No. It measures 5.5 inches. Nominal lumber names refer to the rough-sawn size before drying and planing, and every calculation here uses the actual width.
How far apart should deck joists be?
Sixteen inches on center is the common default, but the governing figure is the span rating of your decking and the load your framing has to carry. Check the decking manufacturer's span table and your local code before ordering.
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