Concrete calculator
Concrete Calculator
Enter slab dimensions and thickness to estimate ready-mix cubic yards and 40, 60, or 80 lb bags. A 10% material allowance is included by default and can be changed.
Planning estimate, not a contractor quote. Review the assumptions and replace presets with local prices when available.
Method
How this calculator works
Concrete orders are volume calculations. The slab’s thickness must be converted from inches to feet, and the result must then be converted from cubic feet to cubic yards. Bag counts use nominal bag yield, so check the exact package and product data before buying.
Volume = length × width × thickness. Adjusted volume = volume × (1 + waste percentage). Ready-mix is rounded up to 0.25 yd³. Bags = adjusted cubic feet ÷ nominal bag yield, rounded up to a whole bag.
Worked example
A realistic starting point
A 10 ft by 10 ft slab at 4 in thick is 33.33 ft³. With 10% allowance it needs 36.67 ft³, or 1.36 yd³. Plan on 1.50 yd³ of ready-mix or about 62 nominal 80 lb bags.
What changes the answer
Factors worth checking
Measure true thickness
A small thickness change affects the entire slab volume. Measure forms and compacted base instead of assuming the excavation depth.
Separate complex sections
Calculate slabs, footings, piers, and thickened edges individually. Add their adjusted volumes at the end.
Bag yield varies
Nominal yields differ by mix and manufacturer. The package data is the final authority for bag count.
Ready-mix minimums
A supplier may charge a short-load fee below its preferred truck volume. Ask about minimums, unloading time, and access.
Common questions
Concrete volume FAQ
How many bags equal one cubic yard?
Using nominal yields, one cubic yard is about 90 40 lb bags, 60 60 lb bags, or 45 80 lb bags before waste.
How thick should a slab be?
Thickness depends on use, soil, loads, reinforcement, and local requirements. This calculator estimates volume only; use the project design thickness.
Is 10% waste always necessary?
No. It is a planning default for grade variation, spillage, and measurement uncertainty. Adjust it to the site and supplier guidance.
When is ready-mix better than bags?
Large bag counts require substantial mixing and handling. Compare delivered cost, access, crew size, set time, and short-load fees.
Evidence
Sources & review
- QUIKRETE CalculatorManufacturer reference for bagged concrete planning and nominal package quantities.
- NRMCA — Concrete in PracticeIndustry technical references for ready-mixed concrete.
Published by Calcstead. Report a correction.
Method and source links reviewed September 4, 2026. Price presets are editable planning inputs and should be replaced with current local information.