Construction material calculators
Concrete Calculator
Estimate concrete volume for slabs and cylindrical pours. Calculate cubic yards, add an optional allowance and work out whole bags from your product’s yield.
- Slabs & cylinders
- Bags & ready-mix
- Imperial & metric
Estimates material quantity only. It does not design reinforcement, set structural thickness or confirm code compliance.
Method
How to calculate concrete volume
Worked example: a 12 ft × 9 ft slab at 4 in with an optional 10% allowance.
Slab
yd³ = L (ft) × W (ft) × T (in) ÷ 324
Bags
Whole bags = ceiling(ft³ ÷ yield per bag)
- Step 1 Surface area 12 ft × 9 ft = 108 sq ft
- Step 2 Thickness to feet 4 in ÷ 12 = 0.3333 ft
- Step 3 Base volume 108 × 0.3333 = 36.00 ft³ (1.33 yd³)
- Step 4 Optional 10% allowance 36.00 × 1.10 = 39.60 ft³ (1.47 yd³)
- Step 5 Bags at 0.60 ft³ yield 39.60 ÷ 0.60 = 66 bags
- Step 6 Bags at 0.50 ft³ yield 39.60 ÷ 0.50 = 79.2 → 80 bags 0.4 ft³ more than needed.
Cylinders
Calculating a solid cylinder
Volume of a cylinder = π × radius² × height.
- Diameter 12 in → radius 0.5 ft
- Height 36 in = 3 ft
- π × 0.5² × 3 = 2.3562 ft³ = 0.0873 yd³
This assumes the whole cylinder is concrete. Nothing is subtracted for a post, sleeve or reinforcement.
Buying
Bags or ready-mix?
The right choice depends on volume, site access, labor and your quotes.
Bagged concrete
- Mix at your own pace, in small batches.
- Works where a truck cannot reach.
- Every bag must be carried and mixed — the bag count shows the labor involved.
Ready-mix delivery
- Arrives mixed, so the pour can be placed in one go.
- Suppliers may set a minimum order, sell in increments and charge extra for small loads.
- The truck needs access to the site — ask about reach and ground protection.
Before you order: checklist
- 1Recheck the form dimensions and thickness against your plan.
- 2Confirm the mix your project specifies with your contractor or designer.
- 3Decide whether to add an allowance, and how much.
- 4Ask the ready-mix supplier for their minimum order, increment and any short-load charges.
- 5Check how the concrete will reach the forms (chute, wheelbarrows or pump).
- 6Make sure you have enough people and tools to place and finish the concrete.
Next steps
Related tools and measurement guides
Calculators
Questions
Concrete estimating questions
How do I calculate concrete in cubic yards?
Multiply length (ft) × width (ft) × thickness (ft) for cubic feet, then divide by 27. With thickness in inches, use length × width × thickness (in) ÷ 324.
How many bags of concrete do I need?
Divide the volume in cubic feet by the yield printed for that exact product, then round up. QUIKRETE’s data sheet for Concrete Mix 1101 lists approximate yields of 0.60 ft³ for an 80 lb bag and 0.45 ft³ for a 60 lb bag — so one cubic yard (27 ft³) needs 45 bags of 80 lb or 60 bags of 60 lb.
Why does the product yield matter?
Bag weight is not a volume. Different mixes with the same bag weight can produce different volumes, so use the yield stated by the manufacturer for the product you are buying. If your product is not listed, choose Custom yield and enter it from the bag.
Should I add an extra allowance?
That is a decision for you or your contractor; the default is 0%. If you add one, it is shown separately and used for both bags and ready-mix.
Can I calculate several pours together?
Yes, use Add another pour, or enter a quantity for identical slabs or cylinders. Only combine pours that use the same mix — estimate different mixes separately.
Does the cylinder mode allow for a post?
No. Cylinder mode calculates a solid cylinder. It does not subtract a post, sleeve or reinforcement, so it is not a post-hole calculator. Follow the product instructions and local codes for footings.
Does this calculator tell me how thick my slab should be?
No. It estimates material quantity for the dimensions you enter. Thickness, strength, reinforcement and code compliance are design decisions for a qualified professional.