How Much Concrete Do I Need?

Start with volume

Whatever you are building, the amount of concrete depends on its volume. Find the volume in cubic feet by multiplying the dimensions in feet, then divide by 27 to get cubic yards. From there you can convert to bags if the job is small. The yardage calculator handles the conversion for any length, width, and depth.

Cubic yards = (Length ft × Width ft × Thickness ft) ÷ 27

How much concrete for a slab

Slabs are the most common pour. The volume is just area times thickness. Thickness matters as much as the footprint: going from 4 inches to 6 inches adds 50% more concrete. This table shows how much a slab needs at typical thicknesses:

For a quick estimate on any size, use the slab calculator or the square footage calculator if you already know the area.

How much concrete for footings and posts

Footings and post holes are smaller but there are usually many of them, so the count matters. A footing is length × width × depth; multiply by the number of identical footings. A round post hole uses π × radius² × depth.

For example, a single fence-post hole 10 inches across and 2 feet deep holds about 1.1 cubic feet of concrete, roughly two 80 lb bags before you subtract the post. Across a 20-post fence that is more than 20 cubic feet, or about 0.8 cubic yards. The footing calculator totals every footing at once, and the Sonotube calculator covers round forms.

Bags or ready-mix?

Once you know the volume, the practical question is how to buy it. Bagged concrete is convenient for small jobs; ready-mix is delivered by the cubic yard and is better value once you need a lot. As a rule of thumb:

• Under about ½ cubic yard: use bags. That is roughly 23 × 80 lb bags or fewer.
• ½ to 1 cubic yard: bags are possible but heavy; compare prices.
• Over 1 cubic yard: order ready-mix. Mixing 45+ bags by hand is slow and inconsistent.

The bag calculator shows the count for 40, 60, and 80 lb bags so you can see when bagging stops making sense.

Worked example: a shed base

Suppose you are pouring a shed base that is 10 feet by 12 feet at 4 inches thick.

1. Area: 10 × 12 = 120 square feet.
2. Thickness in feet: 4 ÷ 12 = 0.333 ft.
3. Volume: 120 × 0.333 = 40 cubic feet, or 40 ÷ 27 = 1.48 cubic yards.
4. Add 10% waste: about 1.63 cubic yards.

At nearly 1.5 yards this is firmly ready-mix territory. If you bagged it anyway, you would need about 67 × 80 lb bags, a clear sign that delivery is the easier choice.

Why yardage is measured in cubic yards

Concrete fills a three-dimensional space, so it is measured by volume rather than by area or by weight. The unit the trade settled on is the cubic yard, which is a cube three feet on every side. Three times three times three is 27, so one cubic yard holds 27 cubic feet of concrete. That 27 is the number doing all the work in the formula above, and it is the single step most people forget. Everything else is just measuring.

Suppliers load trucks by the yard, price by the yard, and talk in yards, so learning to turn a slab into a yard number lets you order concrete, compare two quotes, and sanity-check an estimate without taking anyone's word for it.

Figuring yardage from square footage

If you already know the area of a slab, there is a shortcut the ready-mix industry has used for years. At a standard 4-inch thickness, one cubic yard covers about 81 square feet, so you can divide your square footage by 81 to get the yardage. ConcreteNetwork.com calls 81 the "magic number" for exactly this reason (Concrete Network).

For the patio above, 168 square feet ÷ 81 = 2.07 yards, which matches the long-form answer. The shortcut only holds at 4 inches, though. Change the thickness and the coverage per yard changes with it, which is the next thing to get right.

Thickness changes everything

Two slabs with the same footprint can need very different amounts of concrete if one is thicker. A 6-inch slab needs half again as much concrete as a 4-inch slab of the same size, because you are pouring 50 percent more depth. This table shows how much a single cubic yard covers as the pour gets thicker, which is the fastest way to feel the effect.

The practical takeaway: always confirm the thickness before you figure yardage, and never assume 4 inches. Driveways that carry vehicles are often poured at 5 or 6 inches, and that extra inch or two moves the yard count more than people expect.

Always add a waste factor

Never order the exact calculated number. The figure to go by comes from the industry body rather than from a contractor forum: NRMCA CIP 8 says to estimate waste, additional thickness, form deflection and other contingencies, and to "order more than the original volume estimate by at least 4 to 10 percent." Those two words matter: 10 percent is the top of a floor, not a ceiling. Concrete Network's guidance is blunter still, "never order the exact amount of needed concrete," though its own quantified advice is to add a quarter inch to the slab thickness, which at 4 inches is only 6.25 percent. Inch Calculator recommends 10 to 15 percent (Inch Calculator). Most of that allowance is not spillage. It is the subgrade, and the yardage calculator has the ACI tolerance figures that explain why.

The reason is that the real world eats concrete. Subgrade is never perfectly flat, so low spots swallow more than the math predicts. Forms bow out under the weight of a full load. Some concrete sticks in the chute, some spills off the wheelbarrow, and the last bit clings to the drum. Add 10 percent for a normal slab on decent ground, and closer to 15 percent if the subgrade is rough or the forms are hand-built.

Contractors in the trade back this up. On the Garage Journal forum, the rule members repeat is to "order 10 percent more than you need or than what you calculate the form will hold," because coming up short creates low spots and cold joints that are far worse than having a little extra (Garage Journal). The same thread is worth reading for what to do with leftover concrete, since a wheelbarrow of extra can become a stepping stone or a trash-can pad instead of going to waste.

For the 2.05-yard patio, adding 10 percent brings you to 2.26 yards, and you would ask the supplier for 2.25 or 2.5 depending on how they sell partial yards.

Why coming up short is the expensive mistake

There is an asymmetry worth understanding. A little extra concrete costs you a few dollars a yard. Coming up short costs you a lot more. If the first truck empties before the pour is finished and you wait on a second, the concrete already down starts to set, and the seam where old meets new becomes a cold joint. A cold joint is a plane of weakness that can crack and let water in. On top of that, a small emergency delivery carries a short-load fee that can rival the cost of the concrete itself. A bit of waste is cheap insurance against both.

Short loads and the cost of small pours

Yardage does not just tell you how much concrete to order, it tells you how you will be charged for it. Ready-mix suppliers price a full truck at around 10 yards, and anything well under that picks up a short-load fee. The per-yard framing you see on aggregator sites is not how plants actually bill it: suppliers who publish a policy charge a flat fee per load, commonly 50 to 175 dollars, sometimes capped at the value of the shortfall. That means a 2-yard patio can carry fees a 10-yard driveway never sees. Figuring your yardage accurately lets you see this coming and decide whether to combine pours, size the job up to a fuller load, or use bagged concrete instead. Our cost-per-yard guide breaks down every fee that lands on the invoice.

Quick reference: common slab sizes

If your project matches one of these, here is the yardage at a 4-inch thickness, before the waste cushion. Add 10 percent to any of these and round up before you order.

Figuring yardage for shapes that are not slabs

Not every pour is a rectangle. Footings, round columns, sonotubes, curbs, and stairs each need the volume worked out a little differently before you divide by 27. A round column, for example, uses π x radius² x height ÷ 27 instead of length times width. Because these are easy to get wrong by hand, they have their own walkthrough in concrete yardage by shape, which covers each one with a worked example.

How to check a contractor's yardage

One of the most useful reasons to know this math is to check someone else's number. When a contractor quotes yardage, you can measure the job yourself, run the formula, add a sensible waste factor, and see whether the two numbers land close. If a quote is far above your figure with no explanation, ask what the extra covers. It might be a thicker pour, a deeper footing, or a genuine allowance for bad subgrade, all of which are fair. But it might also be padding. Being able to figure yardage turns you from a spectator into someone who can ask the right question.

Next steps

Pick the calculator that matches your pour from the full list of concrete calculators, or see the yardage method above and how many bags of concrete are in a yard for the bag math.