Concrete Calculator
Estimate concrete volume for slabs, post holes, and stairs. Know exactly how many bags or cubic yards to order.
How to Calculate Concrete
Concrete is measured in cubic yards or cubic feet. For a rectangular slab, multiply the length by the width by the thickness (in feet). For cylindrical post holes, use the formula for the volume of a cylinder: pi times the radius squared times the depth. Always add 10% extra to account for waste, spillage, and uneven ground.
Concrete Bag Coverage
| Bag Size | Covers (cubic feet) | Bags per Cubic Yard |
|---|---|---|
| 80 lb bag | 0.60 cu ft | 45 bags |
| 60 lb bag | 0.45 cu ft | 60 bags |
| 40 lb bag | 0.30 cu ft | 90 bags |
Note: 1 cubic yard = 27 cubic feet.
Ready-Mix vs. Bags: When to Choose Which
For projects requiring less than 1 cubic yard of concrete, mixing your own from bags is usually the most cost-effective option. For anything over 1 cubic yard, ordering ready-mix delivery is faster and often cheaper. Most ready-mix companies have a minimum order of 1 cubic yard and charge a short-load fee for orders under their minimum (typically 3–5 yards). For very large projects (5+ yards), ready-mix is almost always the way to go.
Tips for Working with Concrete
- Always prepare your forms and base material before the concrete arrives or before you start mixing.
- Concrete begins to set within 30–60 minutes, so have everything ready and work quickly.
- For slabs, a gravel base (4–6 inches) improves drainage and prevents cracking.
- In hot weather, dampen the forms and subgrade to prevent the concrete from drying too fast.
- Cure concrete for at least 7 days by keeping it moist — this dramatically improves strength.
Frequently Asked Questions
A 10x10-foot slab at the standard 4-inch thickness requires about 1.23 cubic yards of concrete. With 10% waste added, that equals approximately 56 80-lb bags or 74 60-lb bags. For a project this size, you might also consider ordering a cubic yard of ready-mix concrete, which can be more economical.
Standard concrete slabs for patios and sidewalks should be 4 inches thick. Driveways for passenger vehicles need at least 4-6 inches. Garage floors and areas with heavy vehicles should be 5-6 inches. Foundation footings typically require 8-12 inches. Always check local building codes for specific requirements for your project type.
Concrete reaches about 70% of its final strength within 7 days and 99% at 28 days. You can walk on it after 24-48 hours and drive on it after 7 days. Full curing takes 28 days. Keep new concrete moist during the first 7 days by misting with water or covering with plastic sheeting, especially in hot or windy conditions.
The concrete mix inside is the same — the difference is simply the amount per bag. An 80-lb bag yields 0.6 cubic feet, while a 60-lb bag yields 0.45 cubic feet. Use 80-lb bags for larger projects to reduce the number of bags you need. Use 60-lb bags when you prefer lighter lifting or have a smaller project.
For projects under 1 cubic yard (about 45 80-lb bags), mixing your own is usually more practical and economical. For anything over 1 cubic yard, ready-mix delivery saves significant time and labor. Most ready-mix companies have a minimum order around 1 yard and may charge a short-load fee for orders under 3-5 yards.