The frame is not the deck boards
Two calculators cover a deck and they answer different questions. The deck board calculator works out the surface: how many boards, in what stock lengths, with what gap, and how much you rip off the last row. This one works out what is holding that surface up. Joists, the rim around them, the beam, the posts, the footings and the hardware at every connection. On a typical residential deck the frame is more lumber by board foot than the decking is, and it is nearly all of the labour.
The joist count works the same way as any framing layout: a joist at each end and one per space in between, so floor(length / spacing) + 1. A 16 foot ledger at 16 inch centers carries 13 joists. Everything else follows from the two dimensions and the choices about how the outer edge is supported.
Ledger or freestanding
A ledger bolts to the house and lets the house carry half the deck. It is cheaper, it uses half as many posts and footings, and it is the single most consequential connection on the whole structure. Deck collapses are overwhelmingly ledger failures, and they are usually water failures first: the flashing was missing or lapped the wrong way, the rim behind the ledger rotted, and then the fasteners had nothing to hold. Flashing that sheds water over the ledger rather than behind it is not a detail you improvise.
Freestanding means a second beam and a second row of posts along the house side, with the frame standing on its own footings and only touching the house incidentally. It costs more in concrete and posts. It is the right answer when the house wall cannot take a ledger — brick veneer, stucco over foam sheathing, a cantilevered floor above, or any wall where you cannot get to solid rim framing behind the sheathing. Switching the house-side field to freestanding doubles the beam and post counts here so you can see the price of that decision before you make it.
Posts, beams and where the numbers come from
| Piece | Count | Sized by |
|---|---|---|
| Joists | floor(length / spacing) + 1 | Span table for the projection |
| Beam plies | 2 or 3, full deck length | Beam span table for the tributary width |
| Posts | ceil(length / post spacing) + 1 | Beam span, then post height and load |
| Footings | One per post | Soil bearing and frost depth |
| Hangers | One per joist at the ledger | Joist depth, and the manufacturer table |
Notice how few of those are decided here. The count of posts follows from a maximum post spacing you supply, and that spacing is the output of a beam span table matched to your beam size, species and the tributary width the beam carries. This page will not invent it for you. What it will do is show you immediately that moving from 8 foot post spacing to 6 foot on a 16 foot deck goes from three posts to four, which is one more hole to dig and one more bag of concrete, and that is often the whole decision.
The footing itself is a soil question. Depth is driven by the frost line where you are, diameter by how much load the post brings down and what the soil under it can carry. Once you know the diameter and depth, the concrete slab calculator in circle mode gives the bags. To turn the deck area into the load each post carries, put the numbers through the header and beam load calculator.
Hardware is not optional and it is not generic
Connectors have published capacities, and those capacities assume a specific fastener in every specified hole. A joist hanger installed with roofing nails, or with half its holes empty because they were awkward to reach, is not the connector on the label. The same goes for post caps: a beam that sits on top of a post and is toenailed is relying on the nails to resist uplift and lateral movement, which is what a cap is for. Post bases with a standoff keep the end grain of the post out of standing water on the footing, which is where posts rot from.
Fastener metallurgy matters with treated lumber. Modern treatments are corrosive to plain steel, so hardware and fasteners in contact with treated wood need to be rated for it. Mixing metals in a wet connection is a slow way to lose a deck.
Permits and inspection
Decks need a permit in most jurisdictions, and the inspection usually happens twice: once with the footing holes open, once with the frame up and before the decking covers the connections. The open-hole inspection is the one people skip and regret, because a footing that is judged too shallow after it is poured is a jackhammer job. Book it, and take photographs of the ledger flashing before the boards go down, because that is the connection nobody can see afterwards and the one a future buyer will want evidence about.
Questions people ask
How many footings does a 16 by 12 deck need?
With a ledger on the house and 8 foot maximum post spacing, three: posts at each end of the beam and one in the middle. Go freestanding and it becomes six, because there is a second beam and post line along the house. Tighten the post spacing to 6 feet and the ledgered deck goes to four. The count is ceil(deck length divided by the maximum post spacing) plus one, and the maximum post spacing is not a number this page invents — it comes from the beam span table for the beam size, species and tributary width you are working with.
What is the difference between the deck board calculator and this one?
The board calculator sizes the surface you walk on: board count, rows, gaps, lineal feet and offcut. This one sizes the structure under it: joists, rim, beam, posts, footings, blocking and hardware. Most people need both, and the two share only the joist spacing. If you are pricing a deck, run this first, because the frame is usually the larger board-foot number and it determines how many holes you have to dig.
Do I need blocking between deck joists?
It depends on the joist depth and on what the decking is doing. Deeper joists on longer spans want blocking or bridging to keep them from rolling over under load, and it stiffens the whole frame noticeably. Blocking also gives you something to land a picture-frame border on, and something to fasten stair stringers and rail posts to. It is cheap: one piece per bay per row, cut to the spacing less 1-1/2 inches, so 14-1/2 inches at 16 inch centers. Whether it is required for your joist depth and span is a question for your adopted code and your inspector.
Can I bolt a ledger to any house wall?
No. The ledger has to reach solid framing — normally the floor rim joist of the house — and the fasteners have to pass through the sheathing into it with the pattern from a published fastener table. Brick veneer will not carry it. Stucco or siding over thick foam sheathing puts a compressible layer in the load path and needs a specific detail. A cantilevered floor above is generally not a ledger substrate. And regardless of substrate, the ledger has to be flashed so water runs out over it rather than down behind it. If any of that is uncertain on your house, the freestanding option removes the question entirely at the cost of one more beam and one more row of footings.
What size joists should I use for a 12 foot deck projection?
That is a span table question and it depends on the species and grade of the treated lumber you can actually buy locally, the joist spacing, and the loads your jurisdiction assumes. The table your building department works from is the one that counts. What this calculator does is take whatever size you select and count the pieces, the rim, the blocking and the hangers, so you can price two options side by side. If you cannot get a straight answer from a table, that is exactly the point at which an engineer is cheap relative to the alternative.