Concrete

How Concrete Quantity Is Calculated

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.

Key Factors to Consider

• Curing time and pour temperature both affect the final strength, not just the mix design. Concrete needs time (commonly weeks, not days) to reach its full design strength, and pouring in very hot or very cold weather requires adjustments (curing blankets, admixtures) this calculator doesn’t account for — check your local conditions and your concrete supplier’s own guidance.
• A compacted sub-base is what keeps a slab from cracking over time, separate from the concrete volume itself. A slab poured directly onto unprepared or poorly compacted ground is far more prone to settling and cracking than one poured over a properly compacted gravel base — this is a structural prep step this calculator doesn’t estimate material for, but is worth planning before ordering concrete.
• Rebar or wire mesh reinforcement is a separate decision from the concrete volume. Whether a slab needs reinforcement depends on its use (a walkway vs. a driveway that carries vehicle weight) and local code — this calculator estimates concrete material only, not reinforcement.
• Ready-mix delivery often has a minimum order and delivery fee. Ordering a small amount of ready-mix concrete can still trigger a minimum charge — factor in delivery minimums when deciding between ready-mix and bagged concrete for a small job.

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.

Useful to Know

Weather on pour day matters more than most first-time DIYers expect: concrete needs to stay above roughly 40°F (4°C) for its first several days to cure properly, and a hard freeze before it’s gained enough strength can cause permanent damage that shows up later as cracking or scaling. On the other end, pouring in direct summer sun or very dry heat can make the surface cure too fast relative to the concrete underneath, which also invites cracking — many pros schedule a hot-day pour for early morning and keep the surface damp for the first day or two. None of this changes the volume this calculator computes, but it’s worth planning around before choosing a pour date.