How to Calculate Concrete Needed
Calculating the amount of concrete for a rectangular slab is straightforward. Measure the length and width in feet, and the depth in inches. This calculator converts these measurements into cubic yards — the standard unit for ordering ready-mix concrete.
Concrete Volume Formula
Volume (cubic yards) = Length (ft) x Width (ft) x Depth (in) / 12 / 27
First, depth is converted from inches to feet by dividing by 12. Then the volume in cubic feet is divided by 27 (since there are 27 cubic feet in a cubic yard).
How to Use This Calculator
- Measure and enter the length of the area in feet
- Measure and enter the width in feet
- Enter the desired depth/thickness in inches (4" is standard for most slabs)
- The results show cubic yards needed, including a 10% waste factor
Concrete Bag Coverage
| Bag Size | Coverage (cu ft) | Bags per Cubic Yard |
|---|---|---|
| 60-lb bag | 0.45 | 60 |
| 80-lb bag | 0.60 | 45 |
Concrete Calculator FAQ
How do I calculate cubic yards of concrete?
Multiply length (feet) x width (feet) x depth (inches / 12) to get cubic feet, then divide by 27 to get cubic yards. For example, a 10x10 foot slab that is 4 inches thick: 10 x 10 x (4/12) = 33.3 cubic feet / 27 = 1.23 cubic yards.
How much extra concrete should I order?
Order 10% more than calculated to account for waste, spillage, uneven ground, and over-excavation. For small projects, you may want 15% extra since there is no way to order a partial bag from a ready-mix supplier.
How thick should a concrete slab be?
Standard slab thickness: 4 inches for patios and sidewalks, 4-6 inches for driveways, 6-8 inches for garage floors, and 8-12 inches for foundations. Heavier loads and poor soil conditions may require thicker slabs or reinforcement.
Should I use bags or ready-mix concrete?
For small projects under 1 cubic yard, bags (60-lb or 80-lb) are practical and cost-effective. For larger projects, ready-mix truck delivery is much cheaper per yard and saves significant labor. The break-even point is typically around 1-2 cubic yards.
How long does concrete take to cure?
Concrete reaches about 70% of its final strength in 7 days and full strength in 28 days. Keep it moist for the first 7 days for optimal curing. Avoid heavy loads for at least 7 days. Temperature affects curing time — colder weather slows the process.