Free and simple, type the size of your slab, footing or round column and get cubic yards and the number of bags right away. The answer already includes 10% extra, so you do not run short. No sign-up, no email.
Step 1: Measure your area: length and width in feet, and how thick the concrete will be in inches. Most home slabs are 4 inches thick.
Step 2: Pick the shape above, slab, footing, or round column, and type your numbers.
Step 3: Press Calculate. You get the exact amount, plus the amount to actually order with 10% extra, in cubic yards and in bags.
Concrete (cubic feet) = Length (ft) × Width (ft) × Thickness (ft)
Thickness is in inches, so divide it by 12 first. Example: 4 inches ÷ 12 = 0.333 feet.
Example: a 10 × 10 slab, 4 inches thick.
10 × 10 × 0.333 = 33.33 cubic feet = 1.23 cubic yards (divide cubic feet by 27).
For a round column: Cubic Feet = 3.1416 × (Diameter ÷ 2) × (Diameter ÷ 2) × Depth.
Bag guide: one 80 lb bag makes about 0.6 cubic feet, one 60 lb bag about 0.45, and one 40 lb bag about 0.30.
| Slab Size | Cubic Yards to Order | 80 lb Bags | Common Use |
|---|---|---|---|
| 10 × 10 ft | 1.36 yd³ | 62 bags | Shed floor, small patio |
| 12 × 12 ft | 1.96 yd³ | 88 bags | Patio, hot tub pad |
| 10 × 20 ft | 2.72 yd³ | 123 bags | One-car driveway part |
| 20 × 20 ft | 5.43 yd³ | 245 bags | Garage floor, big patio |
Small job (under about 1 cubic yard)? Bags are usually easier. Bigger job? Ordering ready-mix by the cubic yard is usually cheaper and much less work than mixing 60+ bags.
1. Forgetting the 10% extra. Ground is never perfectly flat, and some concrete always spills or stays in the wheelbarrow. This calculator already adds 10% to the order amount.
2. Mixing inches and feet. Thickness is almost always in inches (like 4). If you type 4 into a feet box by mistake, the answer will be 12 times too big. Here, thickness has its own inches box, so this cannot happen.
3. Ordering bags for a big job. A 20 × 20 slab needs about 245 bags. Mixing that by hand is a full day of very hard work, ready-mix delivery is the smarter choice at that size.
4. Measuring only once. Measure length and width twice. A small mistake in size becomes a big mistake in concrete, and extra delivery trips cost money.
At 4 inches thick: 1.23 cubic yards exact, or 1.36 cubic yards with 10% extra. In bags, that is about 62 bags of 80 lb mix.
About 45 bags. One cubic yard is 27 cubic feet, and one 80 lb bag makes about 0.6 cubic feet: 27 ÷ 0.6 = 45 bags.
4 inches is normal for patios, walkways and shed floors. Driveways and garage floors that hold cars are often 4–6 inches. This is a general guide, heavy vehicles or soft ground may need more.
Under about 1 cubic yard, bags are simple and cheap. Above that, ready-mix delivered by truck is usually better value and far less physical work.
Real ground is uneven, forms are never perfect, and some concrete is always lost. Ordering 10% extra is the standard way to make sure you do not run short in the middle of a pour, running short is much worse than having a little left over.
Both numbers are used, for different jobs. 5% extra is usually enough for a neat slab poured inside forms on flat, firm ground. 10% extra is safer when the ground is uneven, there are no forms, or the shape is not a simple rectangle, more is spilled and the base has low spots. Example, a 10 × 10 slab at 4 inches: exact 1.23 cubic yards, 1.30 yards with 5% extra, 1.36 yards with 10% extra. This calculator uses 10%, because running short in the middle of a pour is much worse than having a little left over.