Concrete Calculator

What the calculator assumes, and what it does not

Ordering concrete is not the same as measuring it. The volume of the slab is the easy part; what you actually buy has to allow for what gets lost. This calculator makes two assumptions about that, and both are worth knowing before you order.

• A waste allowance, set to 10 percent by default. It is a field, not a fixed rule, so you can change it or set it to zero. Ten percent is a common starting figure for a straightforward slab on flat, well prepared ground. Uneven subgrade, an awkward shape or a lot of edge work all argue for more, and 15 to 20 percent is not unusual on a difficult pour.
• Bags are rounded up to whole bags. You cannot buy 61.4 bags, so the count you see is always the next whole number above what the arithmetic gives. That rounding is applied after the waste allowance, not instead of it.

What it does not do: it does not add a buffer of its own on top of the figure you set, it does not know your subgrade, and it does not account for spillage, an over dug edge, or a form that bows under load. It also assumes a rectangular slab of uniform thickness. If your pour is none of those things, the number here is a floor to start from rather than an order to place.

Working a slab out by hand

A slab 10 feet by 10 feet at 4 inches thick, the most common small pour there is.

1. Put the thickness in feet. 4 inches is 4 divided by 12, which is 0.3333 feet.
2. Multiply the three dimensions. 10 × 10 × 0.3333 = 33.33 cubic feet.
3. For cubic yards, divide by 27, because a cubic yard is 3 × 3 × 3 feet. 33.33 ÷ 27 = 1.23 cubic yards.
4. For bags, divide by the yield of the bag you are buying. An 80 lb bag makes about 0.6 cubic feet, so 33.33 ÷ 0.6 = 55.6, which rounds up to 56 bags with no waste allowance at all.
5. Add the 10 percent allowance and it becomes 61.1, which rounds up to 62 bags. That difference, six bags, is what the allowance is buying you.

The bag yields this page uses are 0.6 cubic feet for an 80 lb bag, 0.45 for a 60 lb bag and 0.3 for a 40 lb bag. Those are the figures printed on the common mixes; check your own bag, because a different mix can differ slightly.

Edge cases, and what the answer means

• Rectangular slabs only. The tool multiplies length by width by thickness. An L shape, a circle or a stepped footing needs to be broken into rectangles and added up, or measured another way.
• Thickness is in inches, everything else in feet. That mixture is deliberate, because it is how slabs are specified, but it is also the most common place to slip. A 4 entered where 4 inches was meant is right; a 0.33 is right too, but only if you meant feet.
• Bag counts and yardage answer different questions. Below about half a cubic yard, bags are usually the practical choice. Above roughly a cubic yard, mixing that many bags by hand stops being realistic and ready mix is normally cheaper and better. The calculator gives you both so you can see which side of that line you are on.
• A waste allowance of zero is allowed. If you set it to zero the tool believes you, and the bag count becomes the bare minimum with nothing spare. That is rarely what you want on a real pour.

Your measurements stay in the page

Every dimension you enter is processed by this page in your browser. The slab size, the waste allowance and the resulting bag count are not sent to a server, not stored after you close the tab, and not visible to anyone else. There is no account and nothing to sign up for.

Bags of concrete for common slab sizes (4-inch slab, 80 lb bags)

Below is a handy lookup table for common concrete slab sizes computed at a standard 4-inch thickness using 80 lb bags (0.60 cubic feet yield per bag, rounded up to the nearest whole bag, assuming no waste):

Note: Always add 5–10% for waste to cover uneven subgrades, edge overpour, and spillage. The calculator above includes a customizable waste allowance automatically.

How to calculate concrete in cubic yards

Calculating concrete volume involves finding the three-dimensional space of your slab form and converting it into cubic yards (the standard industrial ordering unit):

How many bags per cubic yard

If you are using pre-mixed bagged concrete, you must divide one cubic yard (27 cubic feet) by the specific yield of the bag weight you purchase:

• 80 lb bags (yields 0.60 ft³): 27 ÷ 0.60 = 45 bags per cubic yard.
• 60 lb bags (yields 0.45 ft³): 27 ÷ 0.45 = 60 bags per cubic yard.
• 40 lb bags (yields 0.30 ft³): 27 ÷ 0.30 = 90 bags per cubic yard.

Bags vs ready-mix

For small pads, fence posts, deck footings, or minor repair patches, mixing concrete bags by hand is cost-effective and practical. However, at around 1 cubic yard or more (which requires 45 of the heavy 80 lb bags), ordering ready-mix concrete from a delivery truck is highly recommended. Ready-mix saves extensive manual labor, guarantees a uniform cure, and is often priced competitively once you factor in delivery and aggregate consistency.

How thick should a slab be

Slab thickness depends directly on the load it will support:

• 4 inches: The standard minimum thickness for walkways, garden paths, backyard patios, and small shed pads.
• 5 to 6 inches: Required for residential driveways, garage floors, or parking pads designed to hold passenger vehicles and light trucks.
• Local standards: Always check municipal building codes and soil compaction profiles, as local frost lines and sandy soils often dictate thicker edges or rebar reinforcement.

Common mistakes

• Entering thickness in feet: Entering "4" for thickness directly into a cubic-feet formula without dividing by 12. This treats the slab as 4 feet thick instead of 4 inches, multiplying your material estimate by 12.
• Skipping the waste allowance: Ordering only the exact mathematical volume. Uneven subgrades and minor spills always happen; failing to add 5–10% extra can leave you short on a pour.
• Confusing concrete with cement: Cement is simply the gray powder binder (composed of limestone and calcium). Concrete is the final structural aggregate mixture of cement, sand, gravel, and water.

Related home measurement calculators

Two neighbours cover the loose materials rather than the mix: gravel for a sub base or a driveway, and sand for bedding under pavers. Both work the same way as this page, from an area and a depth, and both report a weight as well as a volume because that is how those materials are sold.

The Sakrete concrete calculator runs the same arithmetic as this one, with the same bag yields, the same waste allowance and the same rounding. It exists for people searching by brand name. If you are not looking for a particular brand, either page gives you the same answer.