How Much Concrete Do I Need?
Ordering concrete comes down to one number: cubic yards. Get it right and the pour goes smoothly; get it wrong and you either pay for concrete you throw away or — worse — run short halfway through. Here is exactly how to work it out.
The formula
Concrete is sold by the cubic yard, but jobs are measured in feet and inches — so every estimate is a unit conversion. Convert thickness to feet (4 inches = 4 ÷ 12 = 0.333 ft), multiply the three dimensions for cubic feet, then divide by 27. The only judgment call is the waste allowance.
Slabs, driveways and footings
A slab is length × width × thickness. A driveway is thicker (4–6 in) and long, so small thickness changes move the yardage a lot. Footings are narrow and deep, often priced per linear foot; columns use π × radius² × height. Break complex shapes into rectangles and add them up.
Bags vs ready-mix
For a small pad, a 60 lb bag yields ~0.45 cubic feet and an 80 lb bag ~0.6. Divide total cubic feet by the bag yield. Past roughly a cubic yard (~45–60 bags), a ready-mix truck is cheaper, faster and more consistent. Keep your local bag and ready-mix prices in your pricebook rather than guessing.
From volume to a priced estimate
Knowing the yardage is half the job — a client wants a price. The Trowla concrete calculator gives the volume, then lets you add your cost per yard and labor rate for a full job total, save it to a project, and send a branded quote. No sign-up; your prices stay on your device.
FAQ
How do I calculate how much concrete I need?
Multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 for cubic yards. A 10 × 10 ft slab at 4 inches is about 1.23 cubic yards. Add 5–10% for waste.
How many bags of concrete do I need?
A 60 lb bag yields about 0.45 cubic feet and an 80 lb bag about 0.6. Divide total cubic feet by the bag yield. Above about 1 cubic yard, ready-mix is cheaper and faster.
How much does concrete cost per yard?
Ready-mix typically runs a few hundred dollars per cubic yard delivered, before labor, forms and finishing. Prices vary by region and mix, so price your own supplier — the calculator lets you enter your real cost per yard.
How much extra concrete should I order for waste?
Add 5–10%. Uneven subgrade, form spillage and over-excavation all consume extra, and running short on a pour is far more expensive than a little overage.