Concrete Calculator: Volume, Bags & Cost
Estimate concrete for slabs, round columns and round post holes. Add waste, calculate whole bags from product yield and compare ready-mix material costs.
Your inputs
Example inputs are hypothetical. Choose units and assumptions that match your measurements.
Your result
Enter your values, then choose Calculate.
Calculations stay in this browser. Display rounding does not change the underlying calculation.
Calculation breakdown
Method, assumptions and worked example
Choose the correct shape
Rectangular slab volume = length × width × thickness. Round column or hole volume = π × diameter² ÷ 4 × depth. For a round post inside a round hole, subtract the post’s cross-sectional area: π × (hole diameter² − post diameter²) ÷ 4 × depth. Each post is assumed to extend through the full entered concrete depth. Add separate rows for different sections.
Worked example
A slab 3 m long, 2 m wide and 0.1 m thick contains 0.6 m³. A 10% allowance raises the estimate to 0.66 m³. The selected 80 lb QUIKRETE Concrete Mix No. 1101 preset yields approximately 0.60 ft³ per bag, equivalent to 0.0169901 m³. Round 0.66 ÷ 0.0169901 up to 39 whole bags.
Compare material costs
At 6 per bag, 39 bags cost 234. Ready-mix at 150 per m³ plus 100 entered delivery fees costs 199 for the example volume. Supplier minimum orders, short-load charges, labor, tools and reinforcement can change the actual bill. Ready-mix price is always entered per cubic meter, even when dimensions are in feet or inches.
Use your product’s stated yield
The preset applies only to the named product and bag size. Other mixes require their own labeled volume yield; bag weight alone is insufficient. Yield is approximate. Choose an allowance for your site and measurement uncertainty. The result estimates material quantity and does not specify safe slab thickness, reinforcement, footing dimensions or structural suitability.
Frequently asked questions
Does the calculator round bags up?
Yes. After adding your allowance, it rounds the required bag count up to the next whole bag.
Can I calculate square posts inside round holes?
Not directly. The post-hole mode assumes a round post. Use separately calculated geometry for other cross-sections.
Does changing feet to meters change the price unit?
No. Section dimensions convert, while ready-mix price remains per cubic meter. The custom bag-yield unit also stays separate.
Method review and references
Formula and content reviewed on 10 October 2026. Worked examples are hypothetical. Methods and assumptions are explained on each page. Use actual measurements, product yields and applicable school or employment rules. These tools are independent estimates.