A concrete job lives or dies on the order quantity. Too little and you get a cold joint and a second delivery fee; too much and you pay to haul away what you already paid to bring in. This calculator works out the exact volume of your slab, footing, column or round slab, adds the waste allowance you choose, and shows the same volume as ready-mix (cubic yards or metres) and as pre-mix bags.
The formulas
Every shape is just a volume. A slab is length × width × thickness. A footing is the same idea stretched out: length × width × depth of the trench. A column or post hole is a cylinder: π × radius² × height. A circular slab is a very flat cylinder. One cubic yard is 27 cubic feet, and one cubic metre is about 35.31 cubic feet — the calculator converts everything both ways so you can order in whichever unit your supplier uses.
Why the waste allowance matters
The math assumes perfect forms and a perfectly level base. Real ground is neither: subgrade dips, forms bow slightly, and some concrete stays in the wheelbarrow. Ten percent extra is the standard planning allowance for flatwork; use 5% for simple, well-formed shapes and up to 15% for hand-dug footings and post holes, which are never the size you planned.
Bags vs ready-mix
Bag yields are printed on every sack: an 80 lb bag makes about 0.6 ft³ of concrete, a 60 lb bag 0.45 ft³ and a 40 lb bag 0.3 ft³. As a rule of thumb, once a job passes roughly one cubic yard — about 45 × 80 lb bags — ready-mix is usually cheaper and far less work than mixing on site, and the concrete is more consistent. Below that, bags win on convenience.
Worked example
A standard driveway slab, 10 ft × 10 ft and 4 inches thick: volume = 10 × 10 × (4/12) = 33.33 ft³ = 1.23 yd³ exact. With a 10% allowance you order 1.36 yd³, or 62 × 80 lb bags (36.67 ft³ ÷ 0.6 ft³ per bag, rounded up).
How to use this tool
- Choose your units and the shape that matches your project.
- Enter the dimensions — thickness and depth go in inches (or cm), everything else in feet (or metres).
- Set the quantity if you are pouring several identical pieces, and keep the 10% waste allowance unless your forms are exact.
- Read the “to order” volume for ready-mix, or the bag table if you are mixing on site.
- If you have a supplier quote, enter the price per yard or metre to see the ready-mix cost.
Ordering tips that save money
Ask about minimum-load and short-load fees before you assume a small order is cheap — a 1.4 yd³ order can cost nearly as much as 2 yd³. Order for a morning pour in hot weather, have your forms and help ready before the truck arrives, and confirm the slump and any fibre or rebar requirements with your supplier when strength matters (driveways typically want at least 3,500–4,000 psi mix).