Concrete Calculator
Calculate cubic yards of concrete needed for slabs, footings, and driveways, plus how many bags to buy.
How to Use the Concrete Calculator
- Enter the length and width of your slab in feet.
- Select the thickness: 4 inches for standard slabs and patios, 6 inches for driveways.
- Click Calculate to see cubic yards, cubic feet, and bag counts (60 lb and 80 lb).
- Order with the 10% waste margin shown — running short mid-pour is expensive.
Formula & Method
Cubic Feet = Length(ft) x Width(ft) x Thickness(in) / 12
Cubic Yards = Cubic Feet / 27
Bag yields: one 80 lb bag makes about 0.6 cubic feet; one 60 lb bag makes about 0.45 cubic feet. Always add 5-10% extra for spillage, over-excavation, and uneven subgrade.
Examples
| Project | Size | Concrete | 80 lb Bags |
|---|---|---|---|
| Patio slab | 10 x 10 ft, 4 in | 1.23 yd3 | 56 |
| Driveway section | 20 x 10 ft, 6 in | 3.70 yd3 | 167 |
| Walkway | 20 x 3 ft, 4 in | 0.74 yd3 | 34 |
| Shed base | 12 x 8 ft, 4 in | 1.19 yd3 | 54 |
Frequently Asked Questions
How much concrete do I need for a 10x10 slab?
A 10 x 10 ft slab at 4 inches thick needs 1.23 cubic yards (33.3 cubic feet) — about 56 bags of 80 lb mix, or a 1.5 yard ready-mix order with margin.
Is it cheaper to buy bags or ready-mix?
Bags cost roughly 2x more per yard but have no delivery fee. Under 1 yard, bags usually win. Over 2 yards, ready-mix is almost always cheaper and far less labor.
How thick should a concrete slab be?
4 inches for patios, walkways, and shed bases. 5-6 inches for driveways with vehicles. 6-8 inches for heavy trucks or RVs.
How many 80 lb bags make a yard?
About 45 bags of 80 lb concrete mix make one cubic yard (each bag yields roughly 0.6 cubic feet, and a yard is 27 cubic feet).
How long does concrete take to cure?
Concrete sets in 24-48 hours for foot traffic, handles vehicles after 7 days, and reaches full design strength at 28 days. Keep it moist for the first week.
People Also Ask
How much does a yard of concrete cover?
One cubic yard covers 81 square feet at 4 inches thick, 65 sq ft at 5 inches, and 54 sq ft at 6 inches thick.
How many bags of concrete for a 10x10 slab?
For a 10x10 slab at 4 inches: about 56 bags of 80 lb mix or 74 bags of 60 lb mix. Add 10% extra for waste.
What is the difference between concrete and cement?
Cement is just one ingredient — the binder. Concrete is cement plus sand, gravel, and water. You buy concrete mix, not pure cement, for slabs.