Concrete Calculator
Calculate how much concrete you need in cubic yards, cubic feet, and bags for slabs, columns, and footings. See also Gravel Calculator and Asphalt Calculator.
Shape
Cost Estimate (optional)
How to Calculate Concrete Needed
Calculating concrete volume requires measuring the length, width, and depth of your project area. Concrete is sold by the cubic yard (27 cubic feet), so you need to convert your measurements into cubic yards. For slabs and footings, multiply length by width by depth (in feet). For cylindrical columns, use the formula for the volume of a cylinder. Always order 5-10% extra to account for spillage, uneven subgrade, and form irregularities.
Concrete Volume Formula
Slab/Footing:
Volume (cu ft) = Length (ft) x Width (ft) x Depth (in) / 12
Cubic Yards = Volume (cu ft) / 27
Column (Cylinder):
Volume (cu ft) = PI x (Diameter/2)^2 x Height
(all dimensions in feet)
Bags needed = Volume (cu ft) / bag yield
40 lb bag = 0.30 cu ft
60 lb bag = 0.45 cu ft
80 lb bag = 0.60 cu ft
Example Calculation
Slab: 10 ft x 10 ft x 4 inches
Volume = 10 x 10 x (4/12) = 33.33 cu ft
Cubic yards = 33.33 / 27 = 1.23 yd3
80 lb bags = 33.33 / 0.60 = 56 bags
Ordering and Delivery Tips
- Order 5-10% more than calculated to account for waste and uneven ground.
- Ready-mix trucks typically carry 8-10 cubic yards per load.
- Most suppliers have a minimum order of 1 cubic yard for delivery.
- Short-load fees apply for orders under the truck minimum (usually 3-5 yards).
- Have your forms, rebar, and crew ready before the truck arrives.
- Concrete begins setting within 60-90 minutes of mixing in warm weather.
Curing Time Guidelines
Concrete reaches about 70% of its full strength in 7 days and 99% in 28 days. Keep concrete moist during the first 7 days for optimal curing. Avoid walking on fresh concrete for at least 24 hours, driving on it for 7 days, and placing heavy loads for 28 days. Temperature affects curing — below 50F slows the process significantly.
Concrete Coverage by Thickness
| Thickness | Cu Ft per 100 Sq Ft | Cu Yd per 100 Sq Ft | 80 lb Bags per 100 Sq Ft | Common Use |
|---|---|---|---|---|
| 2 inches | 16.67 | 0.62 | 28 | Overlay / repair |
| 4 inches | 33.33 | 1.23 | 56 | Sidewalks, patios |
| 6 inches | 50.00 | 1.85 | 84 | Driveways, garage floors |
| 8 inches | 66.67 | 2.47 | 112 | Footings, heavy loads |
Frequently Asked Questions
How many bags of concrete do I need for a 10x10 slab?
A 10x10 ft slab at 4 inches thick requires about 1.23 cubic yards or 56 bags of 80 lb concrete mix. At 6 inches thick, you would need 1.85 cubic yards or 84 bags.
How thick should a concrete slab be?
Sidewalks and patios need 4 inches minimum. Driveways should be 5-6 inches for passenger vehicles. Garage floors and areas with heavy vehicles need 6-8 inches. Always check local building codes.
Should I use bags or ready-mix delivery?
For projects under 1 cubic yard (about 45 bags of 80 lb mix), bags are practical. For larger projects, ready-mix delivery is more economical and ensures consistent quality. The break-even point is typically around 1-2 cubic yards.
What PSI concrete do I need?
Standard residential projects use 3,000-4,000 PSI concrete. Driveways and garage floors should be 4,000 PSI minimum. Footings typically require 2,500-3,000 PSI. Commercial applications may need 5,000+ PSI.
How much extra concrete should I order?
Order 5-10% more than your calculated amount. Uneven subgrade, form bulging, and spillage all consume extra material. For complex shapes or sloped ground, order 10-15% extra.
Can I pour concrete in cold weather?
Concrete can be poured in temperatures above 40F, but special precautions are needed below 50F. Use hot water in the mix, insulating blankets, and avoid pouring if temperatures will drop below freezing within 24 hours.
Do I need rebar or wire mesh?
Wire mesh (6x6 W1.4/W1.4) is standard for 4-inch slabs. Rebar (#4 at 12-18 inch spacing) is recommended for driveways, thicker slabs, and any structural application. Fiber mesh can supplement but not replace steel reinforcement.