A fast, private concrete calculator
Planning a construction or DIY project always starts with a critical question: how much concrete do I need? Ordering too little means a stressful mid-pour scramble, while ordering too much wastes money. This concrete calculator gives you precise volume requirements and bag estimates for slabs, footings, and columns in seconds.
Because the tool runs entirely in your web browser using JavaScript, your project dimensions are never uploaded to a server. You can quickly toggle between shapes and units, tweak your waste margins, and get instant results safely and privately.
Using the concrete slab calculator
Whether you are pouring a patio, a driveway, or setting fence posts, the math changes depending on the shape of your pour. The calculator simplifies this by adapting its inputs to your specific project:
Slabs and Footings: These are rectangular pours. To make data entry as natural as possible, the calculator uses mixed units. If you select Imperial, you will enter the length and width in feet, but the thickness in inches (since most slabs are measured in inches, like a standard 4-inch patio). If you select Metric, you will enter length and width in meters, and thickness in centimeters.
Columns and Posts: For cylindrical pours like deck footings or fence posts, select the "Column" shape. You will be prompted for the diameter (in inches or centimeters) and the height or depth of the hole (in feet or meters).
If you are pouring multiple identical items—such as ten fence post holes or four deck footings—simply adjust the Quantity field. The tool will multiply the final volume automatically.
Understanding the outputs
Once you enter your valid dimensions, the calculator provides a breakdown of your required concrete in several formats:
Total Volume: The primary output is the total volume in cubic meters (m³). Below it, you will see the exact conversion into cubic yards of concrete (yd³) and cubic feet (ft³). If you are ordering ready-mix concrete from a truck, the dispatch company will almost always ask for your requirement in cubic yards (or cubic meters, depending on your region).
Concrete Bag Calculator: For smaller DIY projects, you will likely buy premixed bags from a hardware store. The calculator estimates exactly how many bags you need to buy based on standard industry yields:
- 80 lb bags: Yields approximately 0.60 cubic feet (0.017 m³)
- 60 lb bags: Yields approximately 0.45 cubic feet (0.0127 m³)
- 40 kg bags: Yields approximately 0.019 m³
- 25 kg bags: Yields approximately 0.012 m³
Note: Bag yields can vary slightly based on the exact mix design (e.g., high-strength vs. fast-setting) and the amount of water added. It is always wise to buy a few extra bags.
Why you must add a waste margin
In a perfect mathematical world, a 10x10 slab at exactly 4 inches thick requires 1.23 cubic yards. In the real world, you should never order exactly 1.23 cubic yards.
The calculator includes a Waste percentage field, which defaults to 10%. This is standard practice in concrete estimating for several reasons:
- Uneven subgrades: The ground or gravel base beneath your pour is rarely perfectly level. A dip of just half an inch over a large area requires significantly more concrete.
- Form bowing: Wooden forms can bow slightly outward under the immense weight and pressure of wet concrete.
- Spillage and settling: Some concrete is inevitably lost during the mixing and pouring process, or settles into the gravel base.
For perfectly formed, simple shapes on a highly compacted, flat subgrade, you might reduce the waste margin to 5%. For uneven trenches, rocky post holes, or complex forms, leave it at 10% or higher.
How the volume formulas work
If you prefer to check the math manually, the underlying formulas are straightforward geometry.
For a rectangular slab or footing, the formula is Length × Width × Thickness. The trick is ensuring all measurements are in the same unit before multiplying. For example, a 4-inch thickness must be converted to 0.333 feet before multiplying by a length and width in feet. If you need help converting other measurements, try our Unit Converter or the Square Feet to Square Meters tool.
For a column, the formula is the volume of a cylinder: π × r² × h (Pi multiplied by the radius squared, multiplied by the height). Again, the radius (half of the diameter) must be converted to match the height's unit before calculating the final volume.