Footings are the part of a deck nobody admires and everybody depends on. They're the concrete plugs in the ground that take the whole weight of the deck, the furniture, and a dozen people at a barbecue, then hand that load off to soil that won't budge. Get them too small, too shallow, or too few, and the deck either settles into a slope or lifts a corner every winter. Most failed decks I've crawled under didn't fail in the boards — they failed in the dirt.
This guide covers the three questions that actually decide whether your footings hold: how big around, how deep, and how many. We'll work through the concrete math too, because once you know the diameter and depth, the bag count is just a cylinder volume and some rounding.
Why footings matter more than the lumber
A deck has one job at the foundation: move load straight down into ground that can carry it. The posts collect the weight from the beams, the footings spread that point load across enough soil that the ground doesn't compress, and the whole thing stays put. A 4x4 post sitting on bare earth concentrates a couple thousand pounds onto a few square inches — the soil gives, the post sinks, and the deck tilts. A footing is just a wide concrete foot that turns that knife-edge load into a gentle press the soil can shrug off.
Then there's frost heave, which is the quiet killer in any climate that freezes. Water in the soil expands when it freezes, and if the bottom of your footing sits inside that freezing zone, the ground shoves it upward — sometimes an inch or two a season. It doesn't come back down evenly, so the deck racks, doors stop closing, and the ledger pulls at the house. The fix is simple and non-negotiable: the bottom of every footing has to sit below the local frost line, on undisturbed soil.
How deep: the frost-line rule
Frost depth is the single biggest reason footing depth varies across the country. Your local building department publishes the official figure, and that number wins over any chart, including this one. As a rough geography:
| Region | Approx. frost line | Common required footing depth |
|---|---|---|
| Deep South (FL, TX Gulf, AZ low) | 0–12″ | 12″ |
| Mid-South / Southeast | 12–18″ | 18–24″ |
| Mid-Atlantic / lower Midwest | 24–30″ | 30–36″ |
| Upper Midwest / Northeast | 36–48″ | 42–48″ |
| Far North (MN, ND, ME interior) | 48–60″+ | 48–60″+ |
So a deck in Orlando might need a footing only a foot deep, while the same deck in Minneapolis needs one four feet down. That difference is enormous for the amount of concrete you'll mix — a 12-inch-diameter footing at 12 inches deep is about one 80 lb bag; the same diameter at 48 inches deep is roughly four. Plan your back and your wheelbarrow accordingly.
Footing depth rules of thumb
- Bottom of footing goes below the frost line, always — this is non-negotiable in freeze climates.
- Bear on undisturbed soil, not loose backfill. If you over-dig, fill with compacted gravel, not loose dirt.
- Add a few inches of gravel subbase at the bottom of the hole for drainage so water doesn't pool under the concrete.
- When in doubt, call your building department before you dig — the official frost depth is a free phone call.
How big around: footing diameter by load
Diameter is about spreading load, and the load on any one footing depends on how much deck it carries — what builders call tributary area. A post in the middle of a beam carries half the deck depth on each side and half the post spacing on each side, so a wider post spacing or a deeper deck means more weight per footing and a bigger foot. Soil matters too: soft clay needs a wider footing than dense gravel to carry the same load.
For a typical residential deck on average soil (assume around 1,500 psf bearing, which is the conservative default many codes use), these diameters are a sensible starting point. They're starting points, not gospel — your inspector and your soil have the final say.
| Deck depth (joist length) | Post spacing 6 ft | Post spacing 8 ft |
|---|---|---|
| Up to 10 ft | 12″ dia. | 14″ dia. |
| 10–14 ft | 14″ dia. | 16″ dia. |
| 14–16 ft | 16″ dia. | 18″ dia. |
| Soft / clay soil | Go up one size or use a bell (flared) base. | |
This is where a lot of DIY decks under-build. People grab the cheapest 8-inch tube because it's light to carry and easy to dig, then wonder why a corner settles. An 8-inch footing has less than half the bearing area of a 12-inch one — area scales with the square of the radius, so going from 8 to 12 inches nearly triples the foot. When in doubt, the bigger tube is cheap insurance.
The concrete math, one footing at a time
Every footing is a cylinder, and the volume of a cylinder is π × r² × h. Work in feet and you get cubic feet straight out. Say you've got a 12-inch-diameter footing (radius 0.5 ft) dug 36 inches deep (3 ft):
Volume = 3.1416 × (0.5)² × 3 = 3.1416 × 0.25 × 3 = 2.36 cubic feet.
Now convert to bags. A standard 80 lb bag of concrete mix yields about 0.60 cubic feet; a 60 lb bag yields about 0.45 cubic feet. So that footing needs 2.36 ÷ 0.60 = 3.9, call it four 80 lb bags, or 2.36 ÷ 0.45 = 5.2, round to six 60 lb bags. Always round up — a footing short of concrete is not a thing you can fix later.
| Diameter | 24″ deep | 36″ deep | 48″ deep |
|---|---|---|---|
| 10″ | 2 bags | 3 bags | 4 bags |
| 12″ | 3 bags | 4 bags | 6 bags |
| 14″ | 4 bags | 6 bags | 8 bags |
| 16″ | 5 bags | 8 bags | 10 bags |
Notice how fast it climbs. Bag yield is the lever people forget when they're standing in the store aisle doing mental math — the difference between a 60 and an 80 is a third more concrete per bag, which changes your pallet count on a deck with a dozen footings. Once you've got more than eight or ten footings, or any are 16 inches and deep, it's usually cheaper and faster to order ready-mix by the yard than to mix bags one at a time.
How many footings does a deck need?
Footing count comes out of your beam and post layout, not the square footage directly. Posts under a beam typically sit 6 to 8 feet apart, and you need a footing under each post plus one at each beam end. A common setup: figure the beam length, divide by your maximum post spacing, and round up, then add one. A 16-foot beam at 8-foot spacing wants posts at each end and one in the middle — three footings per beam line.
Most rectangular decks run on one beam at the outer edge with the house-side ledger carrying the other end, or two beams on a freestanding deck. So a freestanding 12x16 deck might use two beam lines of three footings each — six footings total. Tighter post spacing means more footings but smaller beams; wider spacing means fewer, heavier footings and a chunkier beam. The deck joist span calculator helps you balance that trade because beam size and joist span move together.
Sonotubes, bell footings, and gravel
For most decks you'll pour into a cardboard form tube — Sonotube is the brand name everyone uses — set in the hole and backfilled. The tube keeps the concrete round and clean above grade and gives you a tidy pier to set a post base on. Cut it so it stands an inch or two above the soil to keep the post hardware off wet ground.
In frost country, a bell footing (also called a flared or belled base) earns its keep. You widen the bottom of the hole into a flared foot — either by hand or with a special form — so the footing has a big bearing base down low and a narrower pier above. The wide base resists frost from pushing it up and spreads load on soft soil, while the narrower pier saves concrete. A few inches of compacted gravel at the very bottom of the hole gives water somewhere to drain so it isn't sitting against the concrete when the freeze comes.
A worked example: freestanding 12 x 16 deck
Let's size the footings for a freestanding 12 ft deep × 16 ft wide deck in a region with a 36-inch frost line. Joists run the 12-foot direction onto two beams (one at the house side, one at the outer edge), posts at 8-foot spacing.
- Depth: frost line is 36″, so dig to 42″ and bear on undisturbed soil with a few inches of gravel below.
- Diameter: 12-foot deck depth at 8-foot spacing → 16-inch footings from Table 2.
- Count: two 16-foot beam lines, posts at each end plus the middle → 3 footings per beam × 2 = 6 footings.
- Concrete per footing: 16″ dia at 42″ deep ≈ 7 bags (interpolating Table 3) → round to 7.
- Total: 6 footings × 7 bags = 42 bags of 80 lb mix, or about 2 cubic yards if you order ready-mix.
Forty-plus bags is the moment most people switch to ordering a small ready-mix delivery, because hand-mixing two yards is a brutal day. Run your own dimensions through the deck footing calculator for the exact volume, and use the concrete calculator to convert that volume into bags or cubic yards whichever way you're buying.
Code, permits, and inspections
Footings are almost always inspected, and usually twice — once as an open hole before the pour so the inspector can confirm depth and bearing, and again as part of the framing inspection. Don't pour before the hole inspection if your jurisdiction requires it; a footing you have to dig back out is a miserable do-over. The International Residential Code and the American Wood Council's deck guide (DCA 6) spell out the prescriptive requirements most inspectors work from, but local amendments are common, especially on frost depth and required bearing.
The honest summary: confirm your frost depth and required footing size with the local building department before you buy a single bag of concrete or rent an auger. The cost of a phone call is nothing next to the cost of re-digging six holes. Once you know the depth, diameter, and count, the deck cost calculator folds the footing concrete into your overall build budget so nothing sneaks up on the final invoice.
Common questions about deck footings
How deep should deck footings be?
Deep enough that the bottom sits below your local frost line and bears on undisturbed soil. That's roughly 12 inches in the Deep South and 42 to 48 inches or more across the northern states. Your building department publishes the official frost depth for your area, and that number overrides any general chart. In freeze-thaw climates, a footing that stops short of the frost line will heave every winter and rack the deck.
What size footing do I need for a deck?
For a typical residential deck on average soil, 12 to 16 inches in diameter covers most cases, with the bigger diameter for longer joist spans, wider post spacing, or soft soil. Diameter spreads the load, and because bearing area grows with the square of the radius, jumping up one tube size adds a lot of capacity for little extra concrete. Soft clay or a heavily loaded deck may call for a wider footing or a belled base.
How much concrete is in one deck footing?
Treat the footing as a cylinder and use volume = π × radius² × depth. A 12-inch footing dug 36 inches deep holds about 2.4 cubic feet, which is roughly four 80 lb bags or six 60 lb bags. An 80 lb bag yields about 0.6 cubic feet and a 60 lb bag about 0.45, so always round your bag count up — you can't top off a footing after it sets.
Can I use a Sonotube for deck footings?
Yes, cardboard form tubes are the standard way to pour deck piers. Set the tube in the hole, brace it plumb, and pour. Leave it standing an inch or two above grade so the post base and hardware stay off wet soil. In frost-prone areas, pair the tube with a flared or belled base at the bottom of the hole so the footing resists being pushed up by freezing ground.
The bottom line
Footings live or die on three numbers: deep enough to clear the frost line, wide enough to spread the load, and enough of them to match your post layout. Nail those, pour onto undisturbed soil over a little gravel, and the concrete you bury this weekend will outlast the boards on top. Size it with the deck footing calculator, confirm the depth with your building department, and you won't be the deck I'm crawling under in five years wondering why a corner sank.