A 10×12 ft slab poured 4 inches thick needs 40 cubic feet of concrete — 1.48 cubic yards, or 1.63 yards with the standard 10% waste cushion. In bags, that's 74 bags at 80 lb (60 lb: 98 bags; 40 lb: 147). Measure your slab in feet, enter the thickness in inches, and this calculator does the conversion and the bag math for you.
How to measure a slab
Three measurements, all before you touch a bag: length and width in feet (measure the inside of your forms, not the hole you dug), and thickness in inches. Thickness is the one people get wrong, because it's quoted in a different unit from the other two — a "four-inch slab" is a third of a foot thick. Multiply length × width × thickness-in-feet and you have cubic feet; divide by 27 for cubic yards, the unit ready-mix is sold in.
The standard thicknesses exist for a reason. Four inches handles patios, walkways, and shed floors — foot traffic and garden loads. Driveways that carry cars want 5–6 inches, and anything holding a hot tub, a loaded truck, or a structure's footing needs an actual specification, not a rule of thumb. When in doubt, the extra inch is cheap insurance: going from 4 to 5 inches on a 10×12 slab adds about a third of a yard, and a slab that's too thin fails by cracking, which no sealer fixes.
Why the waste allowance exists
Concrete math assumes a perfect world: level subgrade, straight forms, no spills. Real pours lose volume to ground that dips an inch here and there, forms that bow under pressure, mix left coating the wheelbarrow, and the last stubborn corner of the slab. Running short is the one unforgivable error — fresh concrete can't wait while you drive to the store, and a second batch landing on a setting first one leaves a cold joint, a permanent weak seam.
Ten percent is the trade's standard cushion for slabs on prepared, compacted ground. Hand-dug post holes earn more — 15–20% — because a hole dug "12 inches wide" is really 12 inches wide in some places and 15 in others, and a cylinder estimate believes your tape measure completely. The waste allowance in this calculator is applied once, on top of the exact volume, before bags are counted — the same order a contractor estimates in.
Bag yields: the only table you need
| Bag size | Yield per bag | Bags per cubic yard | Weight of a full yard |
|---|---|---|---|
| 80 lb | 0.60 cu ft | 45 | 3,600 lb |
| 60 lb | 0.45 cu ft | 60 | 3,600 lb |
| 40 lb | 0.30 cu ft | 90 | 3,600 lb |
Yields are QUIKRETE's published figures for its standard concrete mixes. Notice the last column: whichever bag you choose, a cubic yard of dry mix weighs about 3,600 lb. Bag size doesn't change how much concrete you move — it changes how many trips you make and how much each lift weighs. Eighty-pound bags are the cheapest per cubic foot and the hardest on your back; forty-pound bags cost more per yard and are the only ones many people should be lifting by the fortieth bag.
When bags stop making sense: the ~1 yard crossover
Somewhere around one cubic yard, the economics and the physics both flip. Below a yard — a few post holes, a small pad, a mailbox post — bags win: no delivery fee, no minimum order, mix only what you need. Above a yard, ready-mix usually wins on price alone, because a yard of truck-delivered concrete typically costs far less than the 45 eighty-pound bags that replace it — and the truck also does the mixing, arrives consistent, and pours in minutes instead of hours.
The honest caveat is the short-load fee. Many ready-mix plants charge extra on orders under 3–4 yards, and that fee can narrow the gap considerably on a 1.5-yard pour. That's why the cost check in this calculator uses your local delivered price per yard: enter it, and the verdict compares real numbers — bags at your store's price against the truck at your plant's price — instead of a generic rule.
Worked example
A patio slab is 10 ft × 12 ft, poured 4 inches thick, with a 10% waste allowance:
- Volume: 10 × 12 × (4 ÷ 12) = 40.00 cu ft = 1.48 cu yd
- With waste: 44.00 cu ft = 1.63 cu yd to actually order
- Bags: 80 lb → 74 bags; 60 lb → 98 bags; 40 lb → 147 bags
- Cost check at $6.50 per 80 lb bag: 74 × $6.50 = $481.00 bagged
- Ready-mix at $165 per cubic yard: 1.62963 × $165 = $268.89 (cubic yards unrounded — 44.00 ÷ 27) — the truck is cheaper by $212.11, before any short-load fee
Post holes work the same way. Four fence-post holes at 12 inches wide and 36 inches deep hold 2.36 cu ft each — 9.42 cu ft total, 10.37 cu ft with waste — which is 18 bags of 80 lb. Small job, bags win, no truck needed.