Pouring Concrete: How to Calculate What You Need for Slabs, Footings, and Posts

Running short mid-pour is not an option

I want to be straight with you on this one. Concrete is unforgiving. It doesn’t care about your schedule, your budget, or the fact that the truck is already on its way. Once you start a pour, you finish it — or you tear it out and start over. There is no “I’ll just top it off tomorrow.” A cold joint (that’s the seam you get when fresh concrete meets yesterday’s hardened slab) is structurally weak and looks terrible.

I learned this the hard way on my second year as an apprentice. We shorted a garage slab by about three-quarters of a yard. By the time we got more mix on site, the edge had already kicked — started setting up — and the patch never bonded properly. We jackhammered the whole thing the next morning. That was a $2,000 lesson in 1998 money, and I’ve never underordered since.

The fix is dead simple: measure properly, calculate accurately, and order a little extra. That’s what this guide is about.

Start with the area you’re pouring

Before you can figure out how much concrete you need, you need to know the exact square footage of the area you’re filling. This sounds obvious, but it trips people up more than you’d think — especially when the slab isn’t a clean rectangle.

A standard patio or garage floor is straightforward: length times width. But driveways with flared aprons, L-shaped paths, sidewalks with curves — those need to be broken into simpler shapes and added together. Miss a section and your volume calculation is off from the start.

Here’s how I do it on site. I break irregular areas into rectangles, triangles, and partial circles. I measure each one separately, calculate the area, and add them all up. For a driveway that’s 40 feet long and 12 feet wide at the garage but flares to 18 feet at the street, I’d split it into a rectangle and a trapezoid rather than trying to eyeball an average width.

The Square Footage Calculator handles all of these shapes cleanly. Punch in your measurements and let it do the geometry:

Write that total down. You’ll need it for the concrete calculation next.

Calculating your concrete volume

Here’s where people get into trouble. Concrete is sold by the cubic yard (or cubic metre if you’re working in metric). One cubic yard is 27 cubic feet. A ready-mix truck typically carries 8 to 10 yards per load, and most plants have a short-load fee if you order less than a certain minimum — usually around 3 or 4 yards. So getting your number right isn’t just about having enough material, it’s about not paying penalties on a second small delivery.

The formula is simple: take your square footage, multiply by your slab thickness in feet, and divide by 27 to convert cubic feet to cubic yards. A 20x20-foot patio slab at 4 inches thick works out to 400 square feet times 0.333 feet (that’s 4 inches expressed as feet), which equals about 133 cubic feet, or 4.9 cubic yards.

But here’s the thing nobody tells you. The ground underneath your slab is never perfectly flat. Even after you’ve graded and compacted, there are dips and soft spots. Your forms might bow slightly under the weight of wet concrete. The actual thickness will vary — four inches in some places, four and a half or five in others. That all adds up.

My standard rule: order 10% extra for slabs, 15% extra for footings and post holes. Footings have irregular shapes at the bottom where the soil crumbled during digging, and sonotubes (those cardboard cylinder forms you use for deck posts and fence posts) sometimes sit in holes that are wider than the tube itself, so concrete seeps underneath.

Use the Concrete Calculator to get your yardage dialled in:

A note on bagged mix versus ready-mix. For anything under about one cubic yard — say a few post holes or a small set of steps — bagged concrete from the hardware store is fine. An 80-pound bag yields roughly 0.6 cubic feet of mixed concrete. That means you need 45 bags to make a single cubic yard. Trust me, once you’ve mixed 45 bags by hand in a wheelbarrow, you’ll understand why everyone orders the truck for anything bigger.

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Don’t forget the base — gravel matters more than you think

A concrete slab is only as good as what’s underneath it. Pour directly onto clay soil and you’re asking for cracks within two years. The soil heaves when it freezes, shrinks when it dries, and shifts when it gets saturated. Your slab doesn’t flex — it breaks.

The standard approach is a compacted gravel sub-base. For most residential slabs, you want 4 to 6 inches of compacted granular fill — typically 3/4-inch crushed stone or “crusher run,” which is a blend of crushed stone and stone dust that locks together when you compact it. The dust fills the voids between the larger pieces and creates a solid, well-draining platform.

For footings, the gravel base is usually thinner — 2 to 4 inches — but it’s still critical for drainage. Water pooling under a footing accelerates frost heave in cold climates and can undermine the concrete over time.

Calculating gravel volume follows the same logic as concrete: area times depth. But gravel is sold by the ton, not by the cubic yard (at least at most landscape supply yards). And the conversion isn’t one-to-one because of density. A cubic yard of 3/4-inch crushed stone weighs roughly 1.4 tons, though it varies depending on the stone type and moisture content.

The Gravel Calculator accounts for all of this. Enter your area and desired depth and it’ll give you both the volume and the tonnage:

Order your gravel first and compact it before the concrete truck shows up. You want that base plate-compacted in lifts — that means you don’t dump all six inches at once and compact it. You put down three inches, compact it, then add another three inches and compact again. Each pass should be done with a plate compactor (rent one for about $80 a day) until you’re not seeing any movement under the plate. If you can see footprints in the gravel, it’s not compacted enough.

Footings and post holes — the details that matter

For deck footings, fence posts, and structural piers, the dimensions are usually dictated by your local building code. A typical deck footing might be a 12-inch-diameter sonotube sunk 48 inches below grade (below the frost line), sitting on a 20-inch-wide by 8-inch-deep bell footing at the bottom. That bell adds stability and distributes the load.

Each one of those footings uses more concrete than people expect. A 12-inch sonotube at 4 feet deep holds about 3.1 cubic feet of concrete — that’s roughly five 80-pound bags per hole. A deck with eight footings needs about 25 cubic feet total, or just under one cubic yard. Add the bell footings and you’re closer to 1.3 yards.

Always check your local frost line depth before digging. Footings that don’t reach below the frost line will heave and crack, taking whatever they’re supporting with them. In my area, the frost line is 42 inches. Yours might be 12 inches or 60 inches — look it up before you dig.

The ordering checklist

Before you pick up the phone to order concrete, run through this list:

• Square footage confirmed — measured on site, not estimated from drawings
• Slab thickness decided — 4 inches minimum for patios and walkways, 6 inches for driveways and garage floors
• Gravel base planned and ordered — 4 to 6 inches of compacted crusher run
• Volume calculated with overage — 10% extra for slabs, 15% for footings
• Forms built and inspected — check for bowing, loose stakes, and correct elevations
• Reinforcement in place — rebar grid or welded wire mesh, set on chairs to keep it in the middle of the slab, not resting on the gravel

Get those numbers right and you’ll pour with confidence instead of panic. The truck shows up, you screed it off, bull-float it smooth, and walk away knowing the math was solid from the start.

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