Concrete Calculator
Concrete Doesn't Forgive Guesswork
Order too little ready-mix and the truck has to make a second trip while the first pour starts to set at the cold joint. Order too much and you're paying to have leftover concrete hauled away. Whether you're pouring a rectangular slab, a footing, or a set of cylindrical columns, the volume calculation is simple geometry — the trick is doing it in the right units and adding a realistic overage before you call the supplier.
The Formula
For a rectangular slab or footing, volume is length × width × thickness. For a cylindrical column, it's the area of the circular cross-section times height, multiplied by however many columns you're pouring:
Column: Volume = π × r² × Height × Count
Length and width are entered in feet (or meters for metric), while thickness or diameter is entered in inches (or centimeters) — the calculator handles the unit conversion internally before dividing the result by 27 to convert cubic feet to cubic yards, the unit ready-mix is sold in.
Where This Calculation Matters
- Ordering ready-mix — concrete plants sell by the cubic yard, and most have a minimum order plus a short-load surcharge below it, so getting close to the real number avoids both waste and extra trips.
- Bagged concrete for small pours — footings, fence post holes, and small pads are often done with 60 lb or 80 lb pre-mix bags rather than a truck; knowing the cubic footage tells you exactly how many bags to buy.
- Column and pier work — deck footings and support piers use the cylindrical formula, where even a small error in diameter is squared in the volume calculation.
Slab Volume by Thickness (10 ft × 10 ft Pour)
| Thickness | Volume | 60 lb bags | 80 lb bags |
|---|---|---|---|
| 4 in | 1.36 cu yd | 75 | 56 |
| 5 in | 1.70 cu yd | 93 | 70 |
| 6 in | 2.04 cu yd | 112 | 84 |
| 8 in | 2.72 cu yd | 149 | 112 |
Bag counts assume 60 lb bags yield about 0.45 cu ft and 80 lb bags about 0.60 cu ft of mixed concrete each — standard figures printed on most bagged-concrete packaging.
How to Use This Calculator
- Choose the shape — Slab / Footing for rectangular pours, or Column for cylindrical piers and posts.
- Select Imperial or Metric units.
- For a slab, enter Length, Width, and Thickness. For a column, enter Diameter, Height, and Number of Columns.
- Select Calculate to see the total volume in cubic yards (or cubic meters), plus an estimated bag count for bagged concrete where applicable.
Related Calculations
Once you know the volume, the Cement Bags Calculator can break a wet volume down by mix ratio, and the Rebar Calculator covers the reinforcing grid that typically goes in before the pour.