Cement Bag Calculator
A cement bag calculator tells you exactly how many bags of pre-mixed concrete you need for your project. Whether you are pouring a patio, a shed foundation, concrete footings, or setting fence posts, buying the right quantity saves you money and ensures you do not run short mid-pour. This tool accepts the length, width, and depth of your concrete area, calculates the total volume in cubic feet and cubic yards, and divides by the yield of your selected bag size to give you the number of bags required. A waste factor is applied to cover spillage and slight yield variation. Enter your project dimensions and select your bag size below.
Concrete volume and bag quantity formula
Volume (cu ft) = Length x Width x (Depth / 12)
Volume (cu yd) = Volume (cu ft) / 27
Bags needed = ceiling( Volume (cu ft) x (1 + Waste/100) / Bag yield )
The ceiling function rounds up to the nearest whole bag. You can never buy a fraction of a bag, so always round up.
Standard bag yields
- 40-pound bag: approximately 0.30 cubic feet of mixed concrete
- 60-pound bag: approximately 0.45 cubic feet of mixed concrete
- 80-pound bag: approximately 0.60 cubic feet of mixed concrete
- 1 cubic yard = 27 cubic feet (requires about 45 x 80-lb bags)
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab 4 inches thick?
A 10x10 slab at 4 inches thick is 33.33 cubic feet or about 1.23 cubic yards. An 80-pound bag of pre-mix concrete yields approximately 0.60 cubic feet. You would need about 56 bags. Add 10% for waste, bringing the total to about 62 bags.
What does an 80-pound bag of concrete yield?
An 80-pound bag of pre-mix concrete (such as Quikrete or Sakrete) yields approximately 0.60 cubic feet of mixed concrete. A 60-pound bag yields about 0.45 cubic feet, and a 40-pound bag yields about 0.30 cubic feet.
When should I use bags versus ready-mix truck concrete?
For projects under about 0.5 cubic yards (13-14 cubic feet), bagged concrete is practical. For larger pours, ordering ready-mix concrete by the yard from a plant is significantly more economical and ensures consistent quality. Most concrete plants have a 1-yard minimum order.
How thick should a concrete slab be?
Residential driveways: 4 inches minimum. Sidewalks and patios: 3-4 inches. Garage floors with vehicle traffic: 4-6 inches. Slabs supporting heavy equipment or vehicles: 6 inches or more. Foundation walls and structural slabs require engineering design.
Should I add extra bags to my order?
Yes. Always order 5-10% more concrete than calculated to account for spillage, uneven sub-base, and slight mix yield variations. Running short mid-pour is problematic because freshly placed concrete must be finished before it sets.
Official sources
- Quikrete Companies: Quikrete Concrete Calculator - Bag Yield Data.
- American Concrete Institute: ACI 301 Specifications for Structural Concrete.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.