On center means the distance from the center of one joist to the center of the next. A 16 inch spacing is not 16 inches of air. It is 16 inches minus the thickness of a joist, for the clear gap, and the layout mark is still 16 inches. When you count pieces, use the center-to-center number. The board calculator does that, and it includes a joist at both ends.
Both ends are joists
Divide the run by the spacing, round up, and add one. The round-up is the number of spaces. The added one is the last joist. For 16 feet at 16 inches on center, the run is 192 inches. 192 ÷ 16 = 12 spaces exactly, and 12 spaces need 13 joists. That is the opening deck: 16 feet along the house, joists running out from the house, boards along the house.
If the length does not divide evenly, you do not stretch the last bay past the spacing. You add a joist and the last bay comes out shorter. Sixteen feet and one inch at 16 inches on center is no longer 12 spaces. It is 13 spaces and 14 joists. People miss this when the deck is “about 16 feet” and the tape says 16 feet 2 inches. Measure the framing length, not the rounded story you told a neighbor.
What 12, 16, and 24 do to the pile
Closer joists mean more lumber and more screws, and a stiffer feel underfoot if the joist itself was already adequate. On the 16 foot run, 12 inch spacing is 192 ÷ 12 = 16 spaces, so 17 joists. Sixteen inch spacing is 13 joists. Twenty-four inch spacing is 192 ÷ 24 = 8 spaces, so 9 joists. The calculator will switch among those three because they are the common layout marks. Choosing 24 because it is fewer sticks is how decks get spongy or fail a manufacturer’s fastening schedule.
Screws follow the joists. Two fasteners each time a board crosses a joist. On the opening deck that is 26 installed rows × 13 joists × 2 = 676 screws, then 10 percent waste, which buys 744. Drop to 9 joists and the screw line falls with it. Raise to 17 joists and it climbs. The fastener guide uses this crossing count. Do not pick spacing only to shrink the screw budget.
Spacing is not span
Spacing is how often a joist occurs along the house. Span is how far that joist runs before a beam or a ledger holds it up. On the opening deck the joist span is the 12 foot trip out from the house, and only if there is no beam in the middle. Whether a 2×8, a 2×10, or a 2×12 can make that trip depends on the species, the grade, the spacing, the live load, and the code your town adopted. This site does not print a span table and does not want you to treat $1.60 a foot, the placeholder joist price, as proof that a 2×8 is the right depth.
Check local code before you buy joists. Ask what load the deck must carry, what snow applies if you have snow, and which span table they accept. An inspector who wants 2×10s will not be talked out of it by a shopping page. If you are unsure, stop. A joist that is too shallow fails in the middle, not at the calculator.
What the decking allows
The board between joists has a span of its own: the spacing you just chose. Five-quarter decking is about an inch thick. A lot of it is sold for 16 inches on center, not 24. Many composite and PVC boards say 16 inches on center when the boards are perpendicular to the joists, and 12 inches when the boards run diagonal. Some two-inch-thick wood decking is rated for 24. The rating is on the manufacturer’s sheet or the grade stamp, not in a rule of thumb from a blog.
The calculator will still count joists at 24 inches, and it will warn when the face is 5 1/2 inches or narrower, because that combination is a common way to overspan 5/4 boards. The warning is not a code ruling. It is a prompt to read the board you are buying. If the sheet says 16 inches maximum, do not use the 24 inch button to save three joists.
Rim, ledger, and the joists you already counted
The end joists are inside the count. You do not add two more “band” joists on the ends. You do still need a rim or band along the outer edge, parallel to the house on the opening layout, and the calculator lists one. A ledger against the house is a separate board, usually with flashing and bolts the shopping list does not design. Doubling the rim is a choice for a picture frame or a cantilever. The page mentions it. It does not add a second rim unless you account for it yourself.
Joist hangers, post caps, and structural screws for the frame are not in the decking screw total. Those fasteners are sized to the hanger and the load. Buy them from the hanger maker’s schedule. The footing under the beam is a different calculator: bags for a round tube, after someone who knows the local frost depth has told you the hole.
Layout without a fancy story pole
Hook the tape on the outside of the first joist and mark centers at the spacing, or mark the edge you actually nail to and stay consistent. The last mark should land on the end joist or just shy of it. If the last space is longer than the spacing, you skipped a joist. Add it. The decking will telegraph a wide bay as a soft stripe, and a hidden-clip system will not forgive a bay the clip schedule did not allow.
Crown the joists. Sight down each board and set the crown up, so the load straightens it rather than sagging it further. A crowned joist installed crown-down is a belly you will feel under the board gap you were careful about. The calculator cannot see crown. The yard can, if you look before the bundle is on the truck. The order guide includes that look on purpose.