Construction

Concrete Calculator — Yards

Enter your slab dimensions to get cubic yards, cubic feet and how many bags of pre-mixed concrete you need.

Lengthft

Widthft

Thicknessin

1.23

cubic yards of concrete

Cubic feet

33.3

80-lb bags

60-lb bags

Concrete is unforgiving once it sets — order 5–10% extra to cover spillage, uneven subgrade and over-excavation.

Price per cubic yard (optional)

How the concrete calculator works

This concrete calculator turns a slab's footprint and depth into the volume of concrete you need, expressed in cubic yards — the unit ready-mix trucks are sold by in the US. The only catch is that thickness is measured in inches while length and width are in feet, so the inches must be converted to feet (divide by 12) before multiplying.

The formula is:

Cubic feet = length (ft) × width (ft) × (thickness in ÷ 12)
Cubic yards = cubic feet ÷ 27 (there are 27 cubic feet in a cubic yard)
Bags = cubic feet ÷ bag yield — an 80-lb bag yields about 0.60 ft³, a 60-lb bag about 0.45 ft³, and a 40-lb bag about 0.30 ft³ (always rounded up)

Worked example: a 10 × 10 ft, 4-inch slab

A 10 ft by 10 ft patio poured 4 inches deep needs 10 × 10 × (4 ÷ 12) = 33.3 cubic feet of concrete. Dividing by 27 gives 1.23 cubic yards. If you are bagging it instead of ordering a truck, that is 33.3 ÷ 0.60 = 56 bags of 80-lb mix, or 74 bags of 60-lb mix. For anything larger than a few cubic yards, ready-mix delivery is almost always cheaper and faster than bags.

Concrete bag yields reference

Bag size	Approx. yield	Bags per cubic yard
40 lb	0.30 ft³	90
60 lb	0.45 ft³	60
80 lb	0.60 ft³	45

Yields vary slightly by brand, so treat bag counts as a close estimate and round up. Adding a 5–10% allowance on top protects you from running short mid-pour, which can leave a visible cold joint where fresh concrete meets concrete that has already begun to cure.

Ready-mix or bags?

This is the decision the cubic-yard figure is really for, and the crossover is sharper than most people expect. One cubic yard is 45 bags of 80-lb mix — each of which has to be carried, opened, and mixed. Mixing that by hand is a full day's work; with a small mixer it is still several hours, and every batch is a chance for the water ratio to drift.

The rough guide:

Under ½ cubic yard (about 20 bags) — bag it. Post holes, small pads, setting a fence line.
½ to 1 cubic yard — genuinely marginal. Bags if you have a mixer and time, ready-mix if you value the afternoon.
Over 1 cubic yard — order ready-mix. It arrives consistent, it is cheaper per yard, and a slab poured in one go is stronger than one assembled from dozens of separate batches.

Watch for the short-load fee. Most suppliers have a minimum, often around 1 cubic yard, and charge a premium below it — which can make 0.8 yards cost nearly as much as 1.5. If you are close to the line, it is often cheaper to order the minimum and find something useful to do with the extra.

Choosing a mix

Bagged concrete comes in several formulations and they are not interchangeable. Standard mix suits most slabs and footings. High-strength (often 5000 psi) is worth it for driveways and anything carrying vehicle loads. Fast-setting mix goes off in under an hour and can be poured dry into post holes, which is excellent for fence posts and useless for a slab — you will not have time to screed it. Fibre-reinforced mix adds strands that reduce surface cracking, a cheap upgrade for a patio.

For anything structural or load-bearing, reinforcement matters more than the bag on the label: steel mesh or rebar carries tension that concrete alone cannot, and a 4-inch slab under a car should have it.

Curing is not the same as drying

Concrete does not dry out — it undergoes a chemical reaction that needs water to continue. Let the surface dry too quickly and that reaction stops early, leaving a slab that is weaker and more prone to surface cracking, regardless of how carefully you calculated the volume.

Keep it damp for the first week: cover with plastic sheeting, or mist it periodically in warm weather. Concrete reaches roughly 70% of its strength in 7 days and its nominal full strength at 28 days. It will take foot traffic after a day or two, but keep vehicles off a new driveway for a week at minimum. In cold weather the reaction slows dramatically, and below about 5 °C it effectively stalls — which is why pouring in a hard frost is a wasted delivery.

Frequently asked questions

How many cubic yards of concrete do I need for a 10×10 slab?

A 10 ft × 10 ft slab at 4 inches thick needs about 1.23 cubic yards (33.3 cubic feet). Change the thickness above to see how it scales — a 6-inch pour of the same footprint needs about 1.85 cubic yards.

How many 80-lb bags are in a cubic yard of concrete?

About 45 bags, since each 80-lb bag yields roughly 0.60 cubic feet and a cubic yard is 27 cubic feet. A 60-lb bag yields about 0.45 ft³ (≈60 bags per yard).

Why should I order extra concrete?

Subgrades are rarely perfectly level, forms bow slightly, and some concrete is always lost to spillage. Ordering 5–10% extra avoids running short mid-pour, which is far worse than having a little left over.

Does this calculator store my measurements?

No. The calculation runs entirely in your browser — your input isn't uploaded or saved.

Should I use ready-mix or bags?

Under about half a cubic yard (20 bags), bag it. Over one cubic yard, order ready-mix — it's cheaper per yard, more consistent, and a slab poured in one go is stronger than one built from dozens of separate batches.

How long does concrete take to cure?

It reaches about 70% strength in 7 days and its nominal full strength at 28 days. Keep it damp for the first week — concrete cures by chemical reaction, not by drying out, and letting the surface dry too fast leaves it weaker.