Concrete Calculator
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.
Calculate Concrete
How to Use: 3 Easy Steps
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 Formula: How to Calculate Cubic Yards
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.
Ready Answers for Common Slabs (4 inches thick, with 10% extra included)
| 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.
Common Mistakes to Avoid
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.
Questions People Ask
How much concrete do I need for a 10×10 slab?
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.
How many 80 lb bags make one cubic yard?
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.
How thick should my concrete slab be?
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.
Should I buy bags or order ready-mix?
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.
Why does the calculator add 10% extra?
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.
How much extra concrete should I order, 5% or 10%?
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.
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