Concrete Calculator

How This Calculator Works

Concrete needed is a pour’s volume, converted into the cubic yards ready-mix concrete is actually sold in. Enter your project’s dimensions — a rectangular slab (patio, walkway, shed pad, small driveway) or a round footing (a deck post, fence post, or mailbox post) — and this calculator does more than give you a raw volume. It works like a contractor sketching out the project with you: how much concrete to actually order (with a margin for waste), roughly how much it will weigh, and a few things worth checking before you start.

Use the Shape picker above to switch between the two: Rectangular Slab for a flat pour measured by length/width/depth, or Round Footing / Post for a cylindrical pour (like a sonotube-formed post footing) measured by diameter/depth instead.

The Formula

Rectangular slab:

1. Volume: $\vE{\text{Volume}} = \vA{\text{Length}} \times \vB{\text{Width}} \times \vC{\text{Depth}}$, converted to cubic yards — the unit ready-mix concrete is ordered in.
2. Amount needed: the volume, rounded up to the nearest whole cubic yard, since suppliers sell by the yard and you can’t order a fraction of one.
3. Recommended order: the rounded amount plus a waste margin (10% by default — a standard rule-of-thumb for spillage, subgrade irregularities, and over-excavation, but adjustable in the calculator above if your project needs more or less). Running short mid-pour is a far bigger problem than a small amount of leftover concrete.
4. Estimated weight: the volume multiplied by a typical concrete density of about 4,050 lb per cubic yard (roughly 150 lb per cubic foot) — an estimate, since exact density varies by mix.
5. Bag counts: for smaller jobs, the same volume divided by how much a pre-mixed bag yields once mixed — about 0.60 cubic feet for an 80 lb bag, 0.45 for a 60 lb bag, and 0.30 for a 40 lb bag — rounded up, since you buy whole bags. Bagged concrete is usually more practical than a ready-mix delivery only up to roughly half a cubic yard; above that, the bag count climbs fast and delivery is both cheaper and far less work.

Round footing — the volume of a cylinder:

$$\vE{\text{Volume}} = \pi \times \left(\vD{\text{Diameter}} \div 2\right)^2 \times \vC{\text{Depth}}$$

The same order/weight/bag steps above apply. A footing also needs to reach below the local frost line so seasonal ground freeze-thaw can’t heave it, which is why the calculator flags whether the entered depth clears the commonly-cited 12 in frost-line floor many (not all) climates sit at or above — always check your own local building code, since the real requirement varies significantly by region.

Worked Example

Rectangular slab — a 20 ft × 12 ft patio, poured 4 inches deep:

1. Volume: $\vA{20} \times \vB{12} \times \left(\vC{4} \div 12\right) = \vE{80}$ cubic feet, which is 2.96 cubic yards (dividing by 27 cubic feet per cubic yard).
2. Amount needed: rounded up to 3 cubic yards.
3. Recommended order: $3 \times 1.1 = 3.3$ cubic yards.
4. Estimated weight: $2.96 \times 4{,}050 \approx 12{,}000$ lbs.

That’s a small patio project, at a typical residential 4-inch slab thickness — a moderate DIY project for most homeowners comfortable with basic concrete work, though hiring help for the pour itself is common even on small jobs like this one.

Round footing — a 10-inch diameter sonotube, poured 24 inches deep (a typical deck-post footing):

1. Volume: $\pi \times \left(\vD{10} \div 2\right)^2 \times \vC{24} \approx \vE{1{,}885}$ cubic inches, which is about 0.04 cubic yards.
2. Amount needed: rounded up to 1 cubic yard — ready-mix is still ordered by the whole yard even for a pour this small, which is exactly why the bag-count option above matters more for a footing than a slab.
3. Depth check: 24 inches clears the commonly-cited 12-inch frost-line floor.

A single footing this size is a small enough pour that bagged concrete (a few 80 lb bags) is almost always more practical than ordering a whole cubic yard of ready-mix.

Common Mistakes

• Forgetting a waste margin. Spillage, over-excavation, and uneven forms mean an order sized to the exact calculated volume often comes up short — the waste percentage field above pads the recommended order for exactly this reason.
• Ordering by volume when bags are priced by weight (or vice versa). Bagged concrete is sold by weight (40/60/80 lb bags), not by volume directly — use the bag-count figures above rather than trying to convert cubic yards to bags by hand.
• Skipping the sub-base or reinforcement question entirely. This calculator estimates material volume only — it doesn’t check whether a project needs a compacted gravel base, rebar, or wire mesh, which are separate structural decisions worth confirming before pouring.
• Assuming ready-mix delivery is always cheaper for a small pour. Ready-mix is priced (and often has a minimum order) by the cubic yard — for a small footing or repair, bagged concrete is frequently the more practical and cost-effective choice.