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Concrete Slab Calculator Guide
Use this calculator for estimating the volume of concrete needed for rectangular slabs, patios and similar flatwork. Enter the slab dimensions and an ordering allowance to see net and order quantities.
How the estimate works
Length × width × thickness gives the slab volume. Thickness is converted from inches to feet, then cubic feet are divided by 27 to obtain cubic yards. The bag estimates use approximate retail yields and should be checked against the exact product bag yield.
Important assumptions
- The calculator estimates rectangular volume; irregular slabs should be split into simple sections and calculated separately.
- Concrete placement can require extra material because of form variation, grade changes, spillage and uneven subgrade.
- For structural work, use the dimensions and specifications shown on the project drawings and confirm the final order with your concrete supplier.
Frequently asked questions
What allowance should I use?
A small allowance can cover normal measuring and placement uncertainty. The appropriate percentage depends on site conditions and supplier practice.
Are the bag counts exact?
No. Retail bag yields vary by product. Use the manufacturer's stated yield for the final purchase quantity.
Is this an engineering calculator?
No. It is a quantity estimator and does not replace structural design, reinforcement details, specifications or local code requirements.
For the calculation assumptions used on this site, see the MaterialYardage methodology.