How Much Concrete Do I Need? Slabs, Footings and Bags Explained

Buy too little concrete and your pour seizes up halfway through with no way to add more — a cold joint and a weak slab. Buy too much and you’ve wasted money and have a wheelbarrow of setting concrete to dump somewhere. Getting the number right matters more here than almost any other DIY estimate, because concrete doesn’t wait for a second trip to the store. The good news: the math is just volume, and a couple of unit tricks are the only place people go wrong.

The core idea: it’s all volume

Concrete is sold by volume, not weight or area. In the US that’s cubic yards (yd³); most of the rest of the world uses cubic metres (m³). Every estimate is the same two steps: work out the volume of the space you’re filling, then convert it to the unit your supplier sells in.

For a rectangular slab or footing, volume is simply:

volume = length × width × depth

The catch is units. You measure a slab’s length and width in feet, but its depth in inches — and you can’t multiply feet by inches. Convert the depth to feet first by dividing by 12:

4 inch depth = 4 / 12 = 0.333 ft

That single mistake — forgetting to convert the depth — is the number one reason home estimates come out 12× too large or too small.

From cubic feet to what you actually order

Once everything is in feet, you have a volume in cubic feet (ft³). Suppliers don’t talk in cubic feet, so convert:

cubic yards = cubic feet ÷ 27      (because 1 yd = 3 ft, so 1 yd³ = 3³ = 27 ft³)
cubic metres = cubic feet × 0.0283

That ÷ 27 is the conversion people forget exists. A yard is three feet, so a cubic yard is 27 cubic feet, not three.

A worked example: a small patio slab

Say you’re pouring a patio 10 ft × 12 ft, 4 inches thick:

1. Depth in feet: 4 / 12 = 0.333 ft
2. Volume: 10 × 12 × 0.333 = 40 ft³
3. Cubic yards: 40 / 27 = 1.48 yd³
4. Add 10% waste (more on that below): 1.48 × 1.10 ≈ 1.63 yd³

So you’d order about 1.6 cubic yards of ready-mix. The Concrete Volume & Bags Calculator does this for slabs, round columns/post holes (π × r² × height) and stairs, and prints the answer in yd³, m³ and the number of bags at once — so you don’t have to juggle the unit conversions by hand.

Bags of mix — and the yields that matter

For smaller jobs you buy pre-mixed bags and add water. Each bag yields a fixed volume once mixed:

Bag size	Yields (ft³)	Yields (yd³)
40 lb	0.30	0.011
60 lb	0.45	0.017
80 lb	0.60	0.022
25 kg	0.44 (0.0125 m³)	—
30 kg	0.53 (0.015 m³)	—

To get the number of bags, divide your total volume by the bag’s yield. For four post holes, each 10″ across and 36″ deep:

• Radius: 5″ = 0.417 ft, height: 36″ = 3 ft
• Each hole: π × 0.417² × 3 ≈ 1.64 ft³
• Four holes: ≈ 6.5 ft³, plus 10% waste ≈ 7.2 ft³
• In 80 lb bags: 7.2 / 0.60 = 12 bags

Twelve bags is a very manageable bagged job. The patio above, by contrast, needs 40 ft³ × 1.10 ÷ 0.60 ≈ 74 bags — which is where bagging stops making sense.

When to stop bagging and call a truck

There’s a rough crossover point: around one cubic yard, or roughly 40+ bags, a ready-mix truck becomes cheaper, faster and far less back-breaking than mixing bag after bag by hand. Below that, bags win because trucks carry a minimum-load fee. Knowing your volume up front tells you which side of that line you’re on before you commit.

Why the 10% waste factor isn’t optional

Always order 5–10% more than the bare calculation. Sub-grade is never perfectly level, so some areas are deeper than planned; forms bow slightly under the weight; and some concrete is always lost to spillage and what clings to the mixer and wheelbarrow. Running short by even a few percent on a single-pour slab means a weak cold joint — vastly more expensive to live with than a few dollars of extra mix. The calculator builds in 10% by default, and you can toggle it off to see the raw geometric volume.

Frequently asked questions

How many 80 lb bags are in a cubic yard? A cubic yard is 27 ft³, and an 80 lb bag yields 0.60 ft³, so 27 / 0.60 = 45 bags. That alone shows why anything approaching a yard is truck territory.

What’s the difference between cement and concrete? Cement is the grey powder that acts as the glue; concrete is the finished mix of cement, sand, aggregate (gravel) and water. You pour concrete, not cement.

How thick should a slab be? Four inches is standard for patios, walkways and shed bases. Driveways and anything carrying vehicle loads usually want 5–6 inches, often with reinforcing mesh or rebar. Always defer to local code.

Does the bag’s printed weight tell me its volume? No — weight and yield are different things. An 80 lb bag doesn’t yield twice a 40 lb bag’s volume by coincidence; use the published yield (0.30 / 0.45 / 0.60 ft³) rather than guessing from the weight.

Measure your forms, drop the numbers into the Concrete Volume & Bags Calculator, and order with confidence. Planning the rest of the project too? The Paint Coverage Calculator and Tile & Flooring Coverage Calculator do the same job-sizing math for walls and floors.