How to calculate concrete
Concrete is measured by volume. Multiply your slab's length, width, and thickness — all converted to feet — to get the volume in cubic feet, then divide by 27 to get cubic yards, the unit ready-mix is ordered in:
Cubic yards = (Length ft × Width ft × Thickness ft) ÷ 27
Thickness is usually given in inches, so divide it by 12 first. A 10 ft × 10 ft slab poured 4 inches thick is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, or about 1.23 cubic yards.
Bags vs. ready-mix
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | ~90 |
| 60 lb | 0.45 cu ft | ~60 |
| 80 lb | 0.60 cu ft | ~45 |
Bags are fine for small jobs, but the count climbs fast — even a single cubic yard is about 45 80-lb bags. Past roughly one cubic yard, ordering ready-mix delivery is usually cheaper and far less work than mixing by hand.
Always order a little extra
Add 5–10% for spillage, an uneven subgrade, and slight over-excavation. The waste field above defaults to 5%. Running out partway through a pour creates a weak cold joint, so it is always better to finish with a little left over.
Frequently asked questions
How do I calculate how much concrete I need?
Multiply the length by the width by the thickness, with every measurement in feet, to get the volume in cubic feet. Divide by 27 to convert to cubic yards, which is how ready-mix concrete is ordered. For a 10 ft × 10 ft slab that is 4 inches thick, the volume is 10 × 10 × 0.33 = 33.3 cubic feet, or about 1.23 cubic yards.
How many bags of concrete are in a cubic yard?
It depends on the bag size. An 80-lb bag yields about 0.60 cubic feet, a 60-lb bag about 0.45 cubic feet, and a 40-lb bag about 0.30 cubic feet. Since a cubic yard is 27 cubic feet, that works out to roughly 45 80-lb bags, 60 60-lb bags, or 90 40-lb bags per cubic yard.
Should I use bags or order ready-mix concrete?
Bagged concrete is practical for small jobs up to about half a cubic yard — fence posts, small pads, repairs. Once you pass roughly 1 cubic yard, mixing dozens of bags by hand becomes impractical and ready-mix delivery is usually cheaper and far less work.
How much extra concrete should I order?
Add 5–10% to your calculated volume to allow for spillage, uneven subgrade, and over-excavation. Running short mid-pour creates a weak cold joint, so it is always better to have a little extra than to come up short.
How thick should a concrete slab be?
Four inches is standard for patios, walkways, and shed floors. Driveways and slabs that carry vehicles or heavy loads are usually 5–6 inches. Always check your local building code, which may set minimum thicknesses and reinforcement requirements.
Disclaimer: Bag yields vary slightly by brand and mix, and results are estimates for planning. Confirm quantities with your supplier and follow local building codes for slab thickness and reinforcement.