Order too little concrete and a pour stalls with a cold joint; order too much and you've paid for waste. A few minutes of math up front avoids both.
The core formula
For a slab or footing, multiply length × width × thickness — all in feet — to get cubic feet, then divide by 27 for cubic yards (a cubic yard is 27 cubic feet). Remember to convert a thickness given in inches to feet first: a 4-inch slab is 0.33 ft thick.
Bags or ready-mix?
Bagged mix is convenient for small jobs. It takes about 45 bags of 80 lb mix (or 60 bags of 60 lb) to make one cubic yard, so anything beyond a yard or two is usually cheaper and faster with ready-mix delivery.
The Concrete Calculator handles slabs, footings, and round columns, converts to yards, and estimates bag counts with a waste allowance.
Don't forget waste
Add 5–10% to your figure to cover uneven subgrade, spillage, and over-excavation. Suppliers round up to the nearest quarter or full yard, so confirm their minimum.