Concrete Footing Calculator: Size, Depth & Cost

I Almost Poured a Footing That Was Way Too Small

So there I was, about to pour footings for a deck addition — nothing fancy, just a 12x16 attached deck — and I was doing the math on the back of a lumber receipt. I had the post loads figured out (or so I thought), I had my sonotube sizes picked, and I was ready to call the concrete truck. But something felt off. The numbers I was getting for the footing width seemed too narrow for the soil type in my yard, which is basically clay with some gravel mixed in. Turns out I was using a bearing capacity number for well-compacted gravel when my soil was nowhere near that. If I'd poured those footings, they might've settled unevenly within a year or two, and honestly, I've seen that happen on other people's projects and it's not pretty.

That experience is basically why I built the concrete footing calculator on this site. Because getting footings right isn't just about "dig a hole, pour some concrete" — there's actual math involved, and the variables matter more than most people realize.

How Footing Sizing Actually Works

The whole point of a footing is to spread the load from your structure across enough soil area that the ground can handle it. That's it. A footing is just a load-spreading device made of concrete.

But here's where people get tripped up: the size of your footing depends on two things that are pulling in opposite directions. You've got the load coming down (the weight of whatever you're building), and you've got the bearing capacity of the soil pushing back up. The footing has to be wide enough that the pressure on the soil stays below what the soil can handle.

💡 THE FORMULA

Footing Area = Total Load ÷ Soil Bearing Capacity

Total Load = dead load + live load + snow load (etc.), in pounds
Soil Bearing Capacity = how much weight the soil can support per square foot (in psf)
Footing Area = minimum square footage of the footing base

Let me walk through a real example. Say you've got a deck post carrying about 4,800 lbs of total load (that includes the deck weight, furniture, people, snow — all of it). And your soil bearing capacity is 1,500 psf, which is pretty typical for average residential clay soil.

4,800 ÷ 1,500 = 3.2 square feet of footing area needed.

For a round footing (like a sonotube), you'd need a diameter of about 24 inches to get roughly 3.14 square feet. For a square footing, you're looking at about 21.5 inches on each side. Most people would round up to 24 inches square just to keep things clean and add a little safety margin, which I always recommend.

Now, depth is a whole different animal. Your local building code will specify a minimum footing depth based on your frost line — that's the depth below grade where the ground freezes in winter. In places like Minnesota, that's 42 inches or deeper. In Georgia, maybe 12 inches. You absolutely need to check your local code on this one because there's no universal answer, and getting it wrong means your footing can heave up out of the ground when it freezes. I've seen frost heave crack a garage slab right in half.

If you don't know your soil type, a safe assumption for most residential work is somewhere around 1,500 psf. But if you're building anything structural — like a house foundation — you really should get a soil test. They run maybe 300 to 800 depending on your area, and it's cheap insurance against a foundation failure that could cost you tens of thousands.

Figuring Out the Cost

Once you know the size and depth of your footings, the cost part is honestly pretty straightforward. You need to know the volume of concrete, and then multiply by your local price per cubic yard.

For a single round footing — say 24 inches in diameter and 42 inches deep — you're looking at about 0.116 cubic yards of concrete. That doesn't sound like much, and it isn't. But if you've got 8 footings for a deck, that's roughly 0.93 cubic yards total, which is just under a full yard. A bag of premix (80 lb) gives you about 0.6 cubic feet, so you'd need around 53 bags for that project. At maybe 5 to 7 per bag, you're in the range of 265 to 370 just for the concrete.

Ready-mix delivery is a different calculation entirely. Most plants have a minimum order (usually 1 yard) and a short-load fee if you order less than their minimum full truck (often 7-10 yards). For small residential footings, bags are usually more practical unless you're pouring a continuous strip footing for a wall or something bigger.

Don't forget sonotubes, rebar, form lumber, and gravel for the base — those add up. A 24-inch sonotube that's 4 feet long runs about 25 to 40 each. I usually budget an extra 20-30% on top of the raw concrete cost for all the extras and a little waste factor.

Our concrete volume calculator can help you figure out exactly how many cubic yards you need, and the footing calculator will size everything for you based on your loads and soil type.

A Few Things People Always Forget

Rebar. Seriously, don't skip it. A footing without reinforcement is just a block of concrete sitting in the ground, and concrete is terrible in tension. Even a couple of #4 bars running through the footing makes a huge difference in crack resistance. For pier footings, a single piece of rebar running vertically through the center and bent into an L at the bottom is standard practice.

Also — and I cannot stress this enough — call 811 before you dig. Every time. I don't care if you "know" there's nothing down there. I once hit a gas line that wasn't on any map, about 18 inches below grade in what was supposed to be a clear area. That's a phone call you do not want to make to the gas company.

Drainage matters too. If water pools around your footings, the soil bearing capacity drops. Some builders put a few inches of compacted gravel at the bottom of the hole before pouring, which helps with drainage and gives you a more stable base. It's a small step that takes maybe 10 extra minutes per footing.

If you're working on a bigger project and need to figure out material quantities for walls or slabs too, the concrete block calculator and concrete slab calculator are worth checking out. And for anyone trying to estimate total project costs, our construction cost estimator pulls a lot of these pieces together.

For unit conversions while you're on site — because nothing is ever in the units you need it to be — the percentage calculator and unit converter are both handy to have bookmarked.

How deep should a concrete footing be?

It depends almost entirely on your local frost line. In cold climates, footings need to go 36 to 48 inches below grade (sometimes deeper). In warmer areas, 12 to 18 inches might be enough. Your local building department will tell you the exact minimum for your area — and you should always meet or exceed that number, never go shallower.

Can I pour footings with bags of concrete instead of ordering a truck?

Absolutely, and for most residential footing projects — decks, small additions, fence posts — bags are the way to go. You'll need a wheelbarrow or a mixer, and it's definitely more labor, but you avoid minimum order fees and short-load charges from the concrete plant. I'd say anything under about 1.5 cubic yards is bag territory. Above that, start pricing a truck.

What happens if my footings are too small?

Settlement. The footing sinks into the soil unevenly, which causes cracks in walls, doors that won't close, sloping floors — basically all the stuff that makes a building feel "off." In extreme cases, structural failure. Oversizing footings slightly is always better than undersizing them. The extra concrete cost is negligible compared to a foundation repair.