How many yards of ready-mix to order
Work out the volume, add the waste, and round up to the step your supplier sells in. The slab this page opens with — 20 by 24 ft, 4 in thick — is 20 × 24 × 4/12 = 160 cubic feet, which is 160 ÷ 27 = 5.93 cubic yards. With the 10% waste in the box it is 6.52, so the order is 6.75 yards in quarter-yard steps. A supplier who sells in half yards would take an order for 7.0, and one who sells whole yards also 7 — the step select beside the truck's price changes it, and the page says how much a whole-yard step leaves over.
Round once, and round up. 6.52 yards ordered as 6.5 leaves you a wheelbarrow short at the far end of the pour, and a second truck for the last quarter yard costs a short-load fee on its own. Rounding 6.52 straight up to a whole 7 is not wrong either, as long as it is your supplier's step and not the calculator's habit — at a reference $150 a yard, a whole-yard step on a small order is real money.
Why order more than the math
The volume is the inside of the forms on a flat, level base. A real base is neither: an inch too deep across a 20 by 24 ft slab is another 1.48 yards, spread where nobody sees it. Some spills, some stays in the chute and the wheelbarrow, and forms bow a little under the load. The waste box starts at 10%, which sits inside the 5 to 10% most suppliers suggest, and it is applied once — the result shows the yards before and after it.
Yards for common slabs
Before waste:
| Slab | 4 in thick | 5 in thick | 6 in thick |
|---|---|---|---|
| 10 × 10 ft | 1.23 | 1.54 | 1.85 |
| 12 × 12 ft | 1.78 | 2.22 | 2.67 |
| 12 × 16 ft | 2.37 | 2.96 | 3.56 |
| 16 × 20 ft | 3.95 | 4.94 | 5.93 |
| 20 × 20 ft | 4.94 | 6.17 | 7.41 |
| 20 × 24 ft | 5.93 | 7.41 | 8.89 |
| 24 × 24 ft | 7.11 | 8.89 | 10.67 |
A yard of concrete covers 81 square feet at 4 in, 65 at 5 in and 54 at 6 in. For anything that is not one rectangle — an L-shaped patio, a slab with a footing round its edge, a run of post holes — add parts above and the order covers all of them.
Minimum orders and short loads
A truck costs more than its yards. Most suppliers bill a minimum, often 2 yards whatever you take, and add a short-load fee per yard to an order under a set size. This page starts from a published bid sheet's figures — a 2 yard minimum and a fee under 5 yards — and from $150 a yard, near the middle of the $125 to $165 range commonly quoted. All of them are boxes: type your supplier's own quote over them, and a delivery charge if they add one, and the order, the cost and the comparison with bags all move.
The opening slab is over 5 yards, so it carries no short-load fee: 6.75 yards at $150 is $1,012.50. Under the minimum, the comparison block says from how many yards the truck becomes cheaper than bags — below that, a pickup and a mixer win.
Common questions
How many yards of concrete do I need for a 20 by 24 slab?
5.93 cubic yards at 4 in thick, 6.52 with 10% waste, ordered as 6.75 in quarter-yard steps. At 6 in thick it is 8.89 before waste.
Should I round up to a whole yard?
Only if your supplier sells whole yards. Many sell in quarter or half yards, and rounding 1.23 up to 2 to be safe buys 0.77 yards you pay for and have to get rid of. Choose the step in the select and the page shows what it leaves over.
How much does a yard of ready-mix cost?
The page starts at $150, inside a commonly quoted $125 to $165, before the minimum, the short-load fee and delivery. It is a reference, not a quote: prices move by region and by the mix, so type in what your supplier says.
Is it cheaper to buy bags?
Under a yard or two, usually yes — the truck's minimum and fees weigh on a small order. Above that, bags lose on price and on labor: the opening slab is 294 bags of 80 lb, 23,520 lb to lift and mix. The comparison under the shopping list puts both on one line.
Is the waste added twice?
No. Waste is multiplied in exactly one place in this site, and the result shows the yards before and after it. Set the box to 0 and the two lines agree.