Concrete Calculator
To estimate concrete needed, multiply the slab’s length, width, and thickness (converted to feet) to get cubic feet, divide by 27 for cubic yards, or divide by a bag’s yield (0.30-0.60 cu ft depending on bag size) to get the number of bags.
Quick Answer
To estimate concrete needed, multiply the slab’s length, width, and thickness (converted to feet) to get cubic feet, divide by 27 for cubic yards, or divide by a bag’s yield (0.30-0.60 cu ft depending on bag size) to get the number of bags.
Concrete Calculator
Enter your slab dimensions and bag size below for an instant estimate, then see the formula, worked example, and common mistakes.
Enter your values and click calculate.
How to Use This Calculator
Measure the slab area in feet, and thickness in inches — most residential slabs are 4 inches thick, footings and posts may be deeper.
40, 60, and 80 lb bags yield about 0.30, 0.45, and 0.60 cubic feet respectively — check your specific product label since yield varies slightly by brand and formulation.
A 5-10% buffer covers spillage, uneven subgrade, and minor measurement error.
For pours larger than about 1 cubic yard, ready-mix delivered by the truck is usually more economical and consistent than bagged concrete.
Formula
Cubic feet = Length(ft) x Width(ft) x (Thickness(in) / 12). Cubic yards = Cubic feet / 27. Bags = (Cubic feet x (1 + waste%)) / bag yield (cu ft).
Reference Table: Bagged Concrete Yield
| Bag size | Approx. yield | Bags per cubic yard |
|---|---|---|
| 40 lb | ~0.30 cu ft | ~90 bags |
| 60 lb | ~0.45 cu ft | ~60 bags |
| 80 lb | ~0.60 cu ft | ~45 bags |
Common Mistakes to Avoid
- Forgetting to convert slab thickness from inches to feet before multiplying (using “4” instead of “4/12” inflates the estimate 3x).
- Assuming exact bag-yield numbers apply to every brand — always check the specific bag label, since yield varies slightly by manufacturer and formulation.
- Not adding a waste buffer for spillage, subgrade irregularities, and over-excavation — 5-10% extra is standard practice.
- Confusing cubic feet and cubic yards when ordering ready-mix (delivered by the cubic yard) vs. bagged concrete (yield stated in cubic feet).
When the Estimate May Be Wrong
Actual concrete needs vary with subgrade preparation (uneven excavation can add significant volume), slump/mix design, and real bag yield which can differ slightly from label claims. For structural slabs, footings, or anything load-bearing or code-regulated, verify quantities with a ready-mix supplier or contractor and follow local building code requirements.
FAQs
How many 80 lb bags of concrete are in a cubic yard?
About 45 bags of 80 lb concrete mix are needed to make one cubic yard (27 cubic feet), since each 80 lb bag yields roughly 0.6 cubic feet.
How do I convert cubic feet to cubic yards for concrete?
Divide the cubic feet by 27, since one cubic yard equals 3 ft x 3 ft x 3 ft = 27 cubic feet.
Should I order ready-mix or use bagged concrete?
Ready-mix delivered by the cubic yard is usually more economical and consistent for projects over about 1 cubic yard; bagged concrete is more practical for small jobs like postholes or repairs.
How much extra concrete should I order?
A 5-10% waste buffer is standard to cover spillage, uneven subgrade, and minor measurement error.
Sources and Methodology
Bag-yield figures (40 lb approx. 0.30 cu ft, 60 lb approx. 0.45 cu ft, 80 lb approx. 0.60 cu ft) reflect standard published yields from major bagged-concrete manufacturers (Quikrete, Sakrete) and are consistent across current industry references as of 2026; always confirm against the specific product label since yield can vary slightly by formulation.
