Concrete Slab Calculator

The formula

Volume = Length × Width × Thickness. Convert to cubic yards (1 yd³ = 27 ft³). Bags = volume in ft³ ÷ bag yield (an 80-lb bag ≈ 0.6 ft³, 60-lb ≈ 0.45 ft³, 40-lb ≈ 0.3 ft³).

How to estimate concrete for a concrete slab

A concrete slab is the simplest pour to estimate: length times width times thickness gives the volume, and everything else is conversion. The one thing people get wrong is the thickness — a typical patio or shed floor is 4 inches, a driveway is 4–6 inches, and a slab carrying heavy vehicles is 6 inches or more. Because thickness is in inches and the footprint is in feet, divide the thickness by 12 before multiplying, or just let the calculator handle the units.

Decide between bags and ready-mix before you order. Bagged concrete makes sense up to roughly half a cubic yard — about a 3×3-foot slab at 4 inches, or a handful of post settings. Past that, the bag count climbs fast (a 10×10 slab at 4 inches is over 60 eighty-pound bags) and ready-mix delivered by the cubic yard is cheaper and far less work. The calculator shows both: cubic yards for the ready-mix truck and the bag count for each common bag size.

Always order about 10% extra. Concrete is unforgiving — you cannot pause a pour to run to the store, and the subgrade is never perfectly flat, so a little spillage and over-dig is normal. Order the gravel base separately (4 inches of compacted base under most slabs); the Gravel and Crushed Stone calculators size that.

Frequently asked questions

How much concrete do I need for a slab?+

Multiply length × width × thickness (in feet) for cubic feet, then divide by 27 for cubic yards. A 10 × 10-foot slab at 4 inches is about 1.23 cubic yards, or roughly 1.4 with a 10% overage. Enter your dimensions above and the calculator gives both yards and bag counts.

How many bags of concrete for a slab?+

It depends on bag size: an 80-lb bag yields about 0.6 cubic feet, so a 10 × 10-foot slab at 4 inches (≈33 ft³) needs about 62 eighty-pound bags with waste — or 82 sixty-pound, or 123 forty-pound. The calculator shows all three.

How thick should a concrete slab be?+

Four inches is standard for a patio, shed floor, or walkway. Use 4–6 inches for a driveway, and 6 inches or more where heavy vehicles or loads are involved. Thicker slabs and any structural slab usually need rebar or wire mesh.

How many cubic yards in a 10x10 slab?+

About 1.23 cubic yards at 4 inches thick (1.85 at 6 inches). Add about 10% for waste, so order roughly 1.4 cubic yards for a 4-inch 10×10 slab.

Is it cheaper to use bags or ready-mix concrete?+

Bags are convenient up to about half a cubic yard. Beyond that, ready-mix delivered by the cubic yard is cheaper per yard and much less labor — mixing 60+ bags by hand for a 10×10 slab is a hard day. The calculator shows both so you can compare.

How much does a yard of concrete cover?+

One cubic yard covers about 81 square feet at 4 inches deep, 65 sq ft at 5 inches, or 54 sq ft at 6 inches. Coverage drops as the slab gets thicker.

How long does a bag of concrete take to set?+

Most standard mixes are walkable in 24–48 hours and reach usable strength in about a week, but full cure takes around 28 days. Fast-setting mixes set in 20–40 minutes. Always check the bag — this calculator estimates quantity, not cure time.

Do I need gravel under a concrete slab?+

Yes — a 4-inch compacted gravel base under the slab spreads load and drains, which prevents cracking and heaving. Size it with the Gravel or Crushed Stone calculator, and compact it before you pour.

Should I order extra concrete?+

Yes — about 10%. You cannot stop a pour to get more, and the subgrade is never perfectly level, so over-dig and spillage are normal. The calculator includes an adjustable overage.

Is this concrete slab calculator free?+

Yes — free, no sign-up, and it runs entirely in your browser, so nothing you enter is stored or sent anywhere.