Measure the length and width
Measure the rectangular slab, pad, or other area you plan to fill with concrete.
Surface area = length × width
Concrete & masonry
Concrete Calculator
Calculate how much concrete you need for a rectangular slab or pad, including waste, cubic yards, bags, and optional material cost.
Calculation method
How to calculate how much concrete you need
Measure the length and width of the pour, choose the concrete thickness, then convert the result into cubic feet, cubic yards, or cubic meters. You can also add a waste allowance and estimate the number of bags required.
Account for concrete thickness
Concrete is measured by volume, so the slab thickness must be included. The calculator converts thickness from inches to feet before calculating volume.
Volume = length × width × thickness
Add a waste allowance
Extra concrete can help account for uneven excavation, variations in depth, spillage, and other differences between planned and actual conditions.
Required volume = base volume × (1 + waste %)
Convert to yards or bags
Concrete for larger pours is commonly ordered by volume. For smaller projects using bagged concrete, enter the yield printed on one bag and the calculator will round up to a complete number of bags.
Bags = required volume ÷ yield per bag
Worked example
Suppose you are pouring a10 ft × 10 ft slab that is4 inches thick.
Four inches is approximately 0.333 ft, so the base concrete volume is approximately:
10 × 10 × 0.333 ≈ 33.33 ft³
Adding a 10% waste allowance gives approximately36.67 ft³, which is about1.36 yd³ or 1.04 m³.
If one bag yields 0.5 ft³, then 36.67 ÷ 0.5 ≈ 73.34, so you would need to round up to74 bags.
At $6.50 per bag, the estimated material cost would be $481.
How much extra concrete should you allow?
The appropriate allowance depends on the project. A carefully formed slab on a well-prepared base may require less extra material than a pour with uneven excavation or uncertain depth.
- 5% — a smaller buffer when dimensions and depth are well controlled.
- 10% — a practical general allowance when some variation or waste is expected.
- 15% — a larger buffer for projects with greater uncertainty or irregular conditions.
Use the project plans, supplier guidance, or contractor's recommended quantity when they specify a different allowance.
Bagged concrete vs. ready-mix concrete
Bagged concrete can be convenient for smaller pours and repairs. The amount produced by one bag depends on the bag size and product, so use the yield printed on the packaging.
Larger pours may be more practical to order as ready-mix concrete by cubic yard or cubic meter instead of mixing a large number of individual bags.
The calculator therefore shows the required volume even when you leave the bag-yield field empty.
Concrete calculator FAQ
How do I calculate cubic yards of concrete?
Calculate the concrete volume in cubic feet, then divide by 27 because one cubic yard contains 27 cubic feet.
How many bags of concrete do I need?
Divide the required concrete volume by the yield of one bag, then round up to the next complete bag. Use the actual yield printed on the product you plan to purchase.
Does slab thickness affect how much concrete I need?
Yes. Concrete quantity is based on volume, so even a small change in thickness can significantly change the total amount required.
Can I use this calculator for irregular shapes?
This calculator currently estimates rectangular slabs and pads. For an irregular project, divide the area into simpler rectangular sections and calculate each section separately.
Does the estimated cost include labor?
No. The cost estimate uses only the price per bag you enter. It does not include delivery, labor, reinforcement, forms, equipment, base material, or other project costs.
What this estimate assumes
- The entered length, width, and thickness are accurate.
- The project is treated as a rectangular volume.
- The waste percentage represents the additional material you want included.
- Bag yield matches the specific concrete product you plan to purchase.
- Bag quantities are rounded up to complete bags.
- Estimated cost is based only on the price per bag entered.
Before purchasing: verify actual project dimensions, concrete thickness, bag yield or ready-mix order quantity, structural requirements, and any applicable project specifications.