Concrete Calculator

What is concrete calculator?

Ordering the right amount of concrete avoids two costly problems: running short in the middle of a pour or paying for a lot of waste. The volume depends on length, width and thickness, plus an extra allowance for uneven ground and spillage.

This calculator handles flat slabs and footings, and round columns such as post holes and piers. It also shows how many pre mixed bags of different sizes you would need.

How to use the concrete calculator

1. Choose a slab or a round column and your units.
2. Enter the dimensions and a waste allowance. Ten percent is a common choice.
3. Read the volume and the number of bags, or give the volume to a ready mix supplier.

How the concrete calculator works

For a slab, volume equals length times width times thickness. For a round column, volume equals pi times the radius squared times the height. The waste allowance is then added. A cubic yard is 27 cubic feet. Typical bag yields used here are 0.3 cubic feet for a 40 pound bag, 0.45 for a 60 pound bag and 0.6 for an 80 pound bag.

Worked example

Common uses of the concrete calculator

• Planning a patio or garden path.
• Estimating a shed base.
• Fence post holes.
• Footings and steps.
• Comparing bag and ready mix cost.

Tips and common mistakes

Round up and order a little extra. Concrete cannot be added later without a visible joint.

For jobs over about one cubic yard, ready mix delivery is usually easier than bags.

Check bag yields on the product label, since brands differ.

Background: what to know before you rely on it

Concrete is sold by volume, and in the United States the cubic yard is the common unit, while most other countries use cubic meters. Ready mix concrete is delivered by truck in loads and priced per cubic yard or meter, often with a minimum order. Bagged concrete suits small jobs. Because ground is rarely perfectly level and forms can bow, a calculation based on exact dimensions is almost always a little low, which is why an allowance is added. Strength grade, curing and reinforcement are separate design decisions that depend on the use of the slab.

Private, free and fast

This tool runs entirely in your browser. What you type or choose is not uploaded to a server and is not saved after you close the page, which keeps it fast and private. It is free to use as often as you like. See more in all calculators, or browse the full list of tools.

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Results are for information and education. For legal, financial, medical or security decisions, check with a qualified professional. Financial results are estimates and exclude fees and taxes unless the tool says otherwise.