Concrete Calculator
Estimate concrete volume, compare ready-mix and bag quantities, and plan an order that accounts for waste.
- 2026 figures, checked October 26, 2026
- Private: runs in your browser
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What is a concrete calculator?
A concrete calculator estimates the material needed for a slab, footing or column. Enter the project dimensions and a waste allowance to see the volume in cubic feet and cubic yards, plus estimated quantities for 80, 60 and 40 lb bags.
A concrete slab calculator uses length, width and thickness. The tool can also help with footings and columns. People sometimes search for a cement calculator or concrete bag calculator, but cement is only one ingredient in concrete; this calculator estimates the finished concrete mix.
Why concrete estimates matter
An estimate helps you decide whether to order ready-mix or buy bagged concrete, compare costs, and avoid ordering an amount that does not cover the project. A shortfall can interrupt the pour, while excess concrete can add cost and leave you with material to manage.
The estimate also helps you check a contractor’s quantity or plan a home project before you buy. Use the ready-mix price field to estimate material cost when you have a price per cubic yard. The result is a planning estimate, not a quote; confirm dimensions, delivery details and product yield with your supplier.
How to use this concrete calculator
- Choose the pour type: slab or patio, footing, or column.
- Enter the project dimensions using the units shown by the calculator. For a slab, enter length and width in feet and thickness in inches. For a footing, width and depth are entered in inches. For a column, use the dimensions requested by the calculator.
- Enter an extra percentage for waste. Include a reasonable allowance for uneven ground, spillage or small measurement differences.
- Enter the ready-mix price per cubic yard if you want a material-cost estimate. Leave it at $0 if you do not have a price.
- Review the cubic feet and cubic yards, then compare the bag quantities with the ready-mix amount. Confirm the bag yield and order requirements on the product packaging or with your supplier.
Concrete calculator example
Worked example
For a slab or patio measuring 12 ft long by 10 ft wide and 4 in thick, with 10% extra and a ready-mix price of $0 per cubic yard, the calculator estimates 44 cubic feet including the extra, or 1.63 cu yd. It also estimates 74 80 lb bags, 98 60 lb bags, or 147 40 lb bags. The base slab volume is length × width × thickness ÷ 12; adding the waste allowance gives 44 cubic feet. Dividing by 27 gives about 1.63 cubic yards.
How to read your concrete estimate
Cubic feet show the estimated volume before conversion to cubic yards. Cubic yards are the ready-mix quantity unit used in the calculator. Bag counts use the listed yields: 0.60 cubic feet for an 80 lb bag, 0.45 for a 60 lb bag, and 0.30 for a 40 lb bag. Because bag counts are whole packages, the calculator rounds them up.
The waste allowance is included in the displayed estimate, so the total is higher than the measured project volume. Treat the result as a buying estimate. Check the form, ground conditions, product instructions and supplier requirements before placing an order.
How to improve your concrete estimate
- 01
Measure the form, not just the plan
Measure the actual inside dimensions of the form or excavation. Check that the slab thickness is consistent and that the footing width and depth match the calculator’s unit labels.
- 02
Break an irregular pour into sections
Estimate each rectangular section separately, then add the volumes. For curved or unusual forms, ask your supplier or contractor to check the measurements before ordering.
- 03
Use a realistic waste allowance
Account for uneven ground, spillage and material left in equipment. Avoid adding extra without a reason; compare the estimate with the site conditions and supplier guidance.
- 04
Confirm bag yield and ready-mix pricing
Bag yields can differ by product. Check the package label before relying on the estimated count. For ready-mix, confirm the quoted price and any delivery or minimum-order terms with the supplier.
Common concrete calculator mistakes
Mixing feet and inches
Enter each dimension in the unit shown beside that field. Using feet where inches are requested, or the reverse, can change the estimate substantially.
Leaving out the waste allowance
A calculation based only on the finished dimensions may not cover spillage or an uneven base. Set an allowance that fits the actual job.
Treating bag counts as exact
The calculator uses stated yields to estimate how many bags you need. Verify the yield on the product you plan to buy, and check whether your project conditions may affect coverage.
Confusing concrete with cement
Concrete is a mix, while cement is an ingredient. Check that the product you buy is concrete mix suitable for the pour, rather than cement alone.
The concrete formula
Slab: length x width x thickness / 12. Footing: length x width / 12 x depth / 12. Column: pi x radius squared x height. Cubic yards = cubic feet / 27.
Key terms
- Cubic foot
- A unit of volume used to express the space filled by concrete.
- Cubic yard
- A larger volume unit. The calculator converts cubic feet to cubic yards by dividing by 27.
- Yield
- The volume of mixed concrete produced by a bag, as stated on the product or used in the calculator estimate.
- Waste allowance
- Extra volume added to account for spillage, uneven surfaces or measurement differences.
Concrete questions
How many 80 lb bags in a yard?
About 45, since each bag yields about 0.6 cubic feet.
Does the estimate include waste?
Yes. Enter an extra percentage in the waste field, and the displayed volume and quantities include that allowance.
Can I use the calculator for a column?
Yes. Choose column and enter the dimensions requested. The calculator uses the column formula: pi × radius squared × height.
Should I order the exact amount shown?
Use the result to plan, then confirm your measurements, product yield and order with your supplier. Site conditions and delivery terms can affect the amount you need.
Before you act on this result. Cordwell calculators and guides are educational estimates, not legal, tax, investment, lending or other professional advice. They rely on the numbers you enter and on simplified rules, and they can't see your full situation. Before you file, sign, borrow, invest, hire, change your withholding or form a business based on anything here, meet with a licensed contractor, engineer or the local building department who can review your specific facts and confirm the numbers. Rules change, and official agency guidance always controls.