Deck board calculator

Rows, not square feet

Decking is sold as boards of a fixed length, so the useful answer is "how many 16 ft boards", not "how many square feet". The row count is what drives it:

rows = deck width in inches ÷ (board width + gap)

A "5/4x6" deck board is 5-1/2 in wide, not 6. That is the minimum dressed dry width for a nominal 6 in board in the American Softwood Lumber Standard (PS 20-20, Table 3); the 5/4 refers to the nominal 1-1/4 in thickness, which dresses to a full 1 in. A 2x6 is the same 5-1/2 in wide.

For a 12 ft × 16 ft deck with boards running the 16 ft way and a 3/16 in gap: 144 ÷ (5.5 + 0.1875) = 25.3, so 26 rows of 16 ft board. Run the same calculation on a nominal 6 in and you get 24 rows — two boards short, leaving about 11 in of open joist along the last edge.

The gap depends on what you bought

Trex publishes 3/16 in as the width-to-width gap for composite decking, and most composite makers are in the same range; the gap is a drainage and expansion requirement, not a style choice, so it is worth honouring.

Wet pressure-treated southern pine is the opposite case. It leaves the treating cylinder at roughly 25% moisture content, and it shrinks across the width as it dries. Install those boards butted tight and they open to something close to 1/8 in on their own. Gap them at 3/16 as well and you end up with 3/8 in slots that drop tools and phone cases. Kiln-dried-after-treatment (KDAT) boards have already shrunk, so they get the gap.

Pick the stock length before you buy

Stock lengths are 8, 10, 12 and 16 ft, and 20 ft is usually a special order. Two rules decide the cheapest buy:

• Any butt joint has to land on a joist, and the two boards each need their own fasteners into it. That is why a 16 ft board across a 16 ft deck — no joints at all — is worth paying for.
• Price per foot is not flat. Ten-foot boards are often dearer per foot than twelve, so a 10 ft deck can be cheaper to build from 12 ft stock cut down. The offcut figure above tells you how much you are throwing away per row.

Screws come from joist crossings

Two screws per board at every joist, so rows × joists × 2. That 12×16 deck has 13 joists at 16 in on centre, giving 26 × 13 × 2 = 676 screws. Rules of thumb that quote screws per square foot miss the fact that tightening the joist spacing to 12 in raises the count by a third without changing the deck area.

Buy them by piece count, not by weight. Deck screws are shelved in pound boxes, but the count in a 5 lb box swings with gauge and length, so the only reliable number is the one printed on the box. Enter that above and the box count follows.

What this counts, and what it leaves on the ground

This page counts the decking and the screws that hold it down. The frame underneath — joists, beams, posts, ledger, hangers, flashing, blocking — is not here, and neither are stairs, railing, fascia or skirting. A picture-frame border or a mitred corner changes the row arithmetic as well, and there is no field for either.

The deck is taken to be a plain rectangle laid straight. A diagonal layout is handled only by raising the waste percentage, which is an allowance for the loss rather than a calculation of it — nothing here works out where a 45° cut lands. Cut-outs for a tree, a hot tub or an angled bay are outside it altogether.

Offcuts are not pooled between rows. Each row is cut from its own boards, so a 20 ft deck built out of 16 ft stock is charged two boards per row and the 12 ft tail of the second one is written off. Some of those tails would in fact do for the short piece in the next row, so the answer comes out high rather than low. That is the safe direction, and there is a reason for it beyond caution: a tail is only worth something if the joint it makes lands over a joist.

Two things about the screw count. It is two face screws per board at every joist crossing — hidden fastener clips for grooved composite are a different system, sold by coverage, and are not counted here. And the joists are assumed evenly spaced with nothing doubled, so the extra joist under a butt joint, the blocking, and anything screwed into a rim or fascia are all on top.

The joist spacings offered are the ones decking is normally laid on. Whether your joists can span their run at that spacing is a framing question, and this page does not ask it.