Plan a rectangular deck from its length and depth: this calculator counts decking boards (mixing 8, 10, 12, 16, and 20-foot lengths per row to waste the least), joists, rim and ledger lumber, doubled beams, posts and footings, joist hangers, and deck screws, and adds up a materials estimate when you enter prices. It counts material only — always size joists and beams from the IRC R507 span tables or the AWC DCA 6 guide.
Add steps with the stair stringer calculator, mix footings with the concrete calculator, and price hardwood or dimension lumber with the board foot calculator. Fencing the yard around it? The fence calculator counts posts, rails, and pickets. For an accessible entrance instead of steps, the ramp slope calculator checks length and slope against ADA limits. Quoting the job for a client? Turn these numbers into a line-item PDF with the construction estimate generator. For footing concrete by the bag or by the yard, see our guide to cubic yards, tons, and density.
Frequently asked questions
How many deck boards do I need?
Divide the deck depth (the direction the boards stack) by the board width plus the gap, and round up — that is the number of rows. For 5½" boards with a 3⁄16" gap on a 12-foot-deep deck, 144 ÷ 5.6875 ≈ 25.3, so 26 rows. Each row is covered with the stock lengths you choose; a 16-foot deck uses one 16-foot board per row. Add 10% for waste on a straight layout and 15% for diagonal decking, where every board end is cut at an angle.
How many joists does a deck need?
Divide the deck length along the house by the joist spacing and add one: a 16-foot deck at 16" on center has 192 ÷ 16 = 12, plus 1 = 13 joists. Use 12" spacing for most composite decking and for diagonal boards, which span farther between joists. You also need a rim joist across the front and a ledger board on the house (or a second rim for a freestanding deck). Joist size is a span-table question — see IRC Table R507.6. Beams that support the joists must be sized separately.
How many screws per square foot of decking?
Two screws per board at every joist it crosses. With 6-inch boards on joists 16" on center that works out to roughly 350 screws per 100 square feet, or about 3.5 per square foot; 12" spacing or narrower boards push it higher. The calculator counts the actual crossings for your layout. Hidden-clip fastening systems replace face screws with one clip per crossing, so check the manufacturer's count if you use them. Buy screws rated for your lumber: stainless or coated for pressure-treated wood, and color-matched ones for composite boards. A box or two extra covers dropped and stripped screws.
How far apart should deck posts and footings be?
Post spacing is set by the beam: a doubled 2×10 or 2×12 beam typically spans about 6 to 8 feet between posts, but the allowed span depends on beam size, species, and how far the joists span onto it (IRC Table R507.5). Footing size and depth depend on soil and frost depth. Enter the post spacing from your plans and the calculator counts posts and footings per beam; it does not check that the spacing is safe. Minimum post size by height is in IRC Table R507.4; 6×6 posts are the usual choice.
Do I need a permit for a deck?
In most US jurisdictions, yes — especially for a deck attached to the house, one more than 30 inches above grade, or one over a certain area (often 200 square feet). The ledger connection is a common failure point and inspectors look closely at flashing, fasteners, and lateral-load hardware. Take this materials list to your building department along with a framing plan based on the span tables. Getting the permit first is far cheaper than rebuilding after a failed inspection, and an unpermitted deck can complicate selling the house later.