CONCRETE CALCULATOR
How it works
A 10 ft × 10 ft slab, 4 inches thick, with 10% added for waste, mixed from 80 lb bags.
- Convert the thickness to feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 10 × 10 × 0.333 = 33.33 ft³, which is 1.235 cubic yards.
- Add 10% waste: 33.33 × 1.10 = 36.67 ft³.
- Bags: 36.67 ÷ 0.6 ft³ per 80 lb bag = 61.1, which rounds up to 62.
- A part bag cannot be bought, so bags always round up.
62 bags of 80 lb concrete — 1.36 cubic yards with waste included.
Constants
| Constant | Value |
|---|---|
| 80 lb bag yield | 0.6 ft³ |
| 60 lb bag yield | 0.45 ft³ |
| Wet concrete weight | 150 lb/ft³ |
| Ready-mix increment | 0.25 yd³ |
| Cubic feet per cubic yard | 27 ft³ |
SOURCE — yield per 80 lb bag: 0.6 ft³ (45 bags per cubic yard). Quikrete TDS #1101-10 and Sakrete published data; 80 ÷ 133 lb/ft³ = 0.6015.
SOURCE — yield per 60 lb bag: 0.45 ft³ (60 bags per cubic yard). Quikrete TDS #1101-10 and Sakrete published data.
SOURCE — wet mixed concrete ≈ 150 lb/ft³ (≈ 4,050 lb per cubic yard).
SOURCE — plants batch by ¼ yd³; many have a 1 yd³ minimum. Confirm with dispatch.
SOURCE — 1 yd³ = 27 ft³, by definition.
Quick answers
A 10 ft × 10 ft slab at 4 inches thick needs 62 bags of 80 lb concrete with 10% waste, or 56 bags with none.
There are 45 bags of 80 lb concrete in a cubic yard, and 60 bags of the 60 lb size. Each 80 lb bag yields 0.6 cubic feet of mixed concrete.
One cubic yard of concrete is 27 cubic feet and weighs about 4,050 lb wet, at roughly 150 lb per cubic foot.
Bags stop making sense at about half a cubic yard — 23 bags of 80 lb — beyond which a ready-mix truck is cheaper, faster and far more consistent.
Ready-mix plants batch in quarter-yard increments and many hold a one-yard minimum, so a 1.36-yard order will be written as 1.5 yards.
FAQ
How many bags of concrete are in a yard?
Forty-five bags of the 80 lb size, or 60 bags of the 60 lb size. An 80 lb bag yields 0.6 cubic feet of mixed concrete and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 = 45.
How many bags of concrete for a 10 × 10 slab?
Fifty-six bags of 80 lb at 4 inches thick with no waste, or 62 bags with 10% added. In 60 lb bags that is 75 and 82 respectively. The slab is 1.24 cubic yards before waste.
How much concrete do I need for a 4-inch slab?
One cubic yard covers 81 square feet at 4 inches thick. Multiply your square footage by 0.0123 to get cubic yards, or divide by 81.
How many bags of concrete for a post hole?
A 12-inch diameter hole 4 feet deep is 3.14 cubic feet, which is 6 bags of 80 lb. An 8-inch hole 3 feet deep is 1.05 cubic feet, or 2 bags. Always round up — post holes are never as neat as the arithmetic.
Bags or a ready-mix truck?
Bags are practical up to about half a cubic yard, which is 23 bags of 80 lb — roughly 1,840 lb of material to carry and mix. Past that a truck costs less per yard, arrives consistently mixed, and saves hours of work.
How much does a yard of concrete weigh?
About 4,050 lb wet, at 150 lb per cubic foot. That is why a full ready-mix truck carrying 8 to 10 yards weighs over 30 tons loaded and cannot cross a soft lawn.
How much extra concrete should I order?
Five to ten percent. Subgrades are never perfectly level, forms bulge slightly, and running short mid-pour leaves a cold joint that will always be visible and is a genuine weak point.
How many bags of concrete for a footing?
Work out length × width × depth in feet, then divide by the bag yield. A 20 ft footing 16 inches wide and 8 inches deep is 17.8 cubic feet — 30 bags of 80 lb, or 0.66 cubic yards.