How the math works
Volume is area × thickness (a slab) or π × radius² × height (a column). We add your waste allowance, divide by 27 for ready-mix cubic yards, and by the bag yield (about 0.6 cu ft for an 80 lb bag) for the bag count.
Concrete
Concrete calculator
Enter your slab, footing, or column dimensions to get how much concrete you need — bags of mix or ready-mix cubic yards — with a waste allowance and a heads-up on when ready-mix is the better buy.
Concrete needed
—Enter dimensions to estimate concrete.
| Bags (incl. waste) | — |
|---|---|
| Ready-mix (cubic yards) | — |
| Volume (cubic feet) | — |
| Estimated cost | — |
Estimate only.
Includes your waste allowance (concrete can't be reused). Bag yields are typical for standard
mixes; confirm on the bag. Past about 1 cubic yard, ready-mix delivery is usually cheaper and
easier than mixing bags.
Bags vs ready-mix
Bags suit small pours — post holes, a small pad, repairs. Past about a cubic yard the bag count climbs fast, so ready-mix delivery is cheaper and lets you pour in one go. The calculator tells you when you've crossed that line.
Standard volume math + typical bag yields; 1 cu yd = 27 cu ft. Estimates only.
Before you order: common concrete mistakes
Mixing up 60 lb and 80 lb bags
An 80 lb bag yields about 0.6 cu ft; a 60 lb bag only about 0.45, and a 40 lb bag about 0.30. If your bag count assumes 80 lb but the store only stocks 60 lb, you'll be roughly a third short. Check the yield printed on the bag and match it in the calculator before you buy.
Forgetting the sub-base and over-dig
Concrete fills the hole you actually dug, not the neat rectangle on paper. Soft spots, over-excavation, and an uneven sub-base all swallow extra volume. Level and compact the base first, and keep the 10% waste allowance to cover what's left.
Hand-mixing past a cubic yard
Bags are fine for post holes and small pads, but near a cubic yard (about 45–60 bags) mixing by hand gets slow and the concrete can start setting before you finish. That's the point to price ready-mix delivery instead — the calculator flags it once you cross that line.
Ordering exactly what the math says
Concrete can't be paused or reused once mixed, and coming up short mid-pour ruins the finish. Round up and carry a 10% cushion — a leftover bag is far cheaper than a second trip or a cold joint through your slab.
FAQ
How much concrete do I need?
Multiply length × width × thickness for a slab (or π × radius² × height for a column) to get the volume, then add a waste allowance. A 10 × 10 ft slab at 4 inches is about 33 cubic feet; with 10% waste that's roughly 62 × 80 lb bags, or about 1.36 cubic yards of ready-mix.
How many 80 lb bags of concrete in a cubic yard?
An 80 lb bag of concrete mix yields about 0.6 cubic feet, and a cubic yard is 27 cubic feet, so it takes about 45 bags to make a cubic yard. That's why ready-mix usually wins past roughly a cubic yard of pour.
When should I order ready-mix instead of bags?
Roughly past 1 cubic yard (about 45–60 bags), mixing by hand becomes slow and expensive — ready-mix delivery is usually cheaper and gives a single continuous pour. The calculator flags this once your volume crosses that line.
How much waste should I add?
10% is a common allowance for spillage, uneven sub-base, and over-excavation. Concrete can't be reused once mixed, so it's better to have a little extra than to come up short mid-pour.
How thick should a concrete slab be?
Four inches is standard for patios, walkways, and shed floors. Driveways that carry cars usually want 5–6 inches, and anything taking heavy vehicles or equipment more still. Thickness drives the volume directly, so set it before reading the bag count.
How much concrete do I need for a fence post?
Roughly one to two 50 lb bags of fast-setting mix per hole is typical, but it depends on hole size. Take the hole volume (π × radius² × depth), subtract the post, and add waste — switch the calculator to the column/post shape to do it exactly.
How long before I can use a new slab?
You can usually walk on it after 24–48 hours, but wait about 7 days before driving on it and a full 28 days for the concrete to reach its rated strength. Keep it damp while it cures for a stronger result.
Estimate only, based on standard volume math and typical bag yields. Includes your waste allowance. Confirm bag yield on the package and consult a pro for structural work.