WorldCalcs

Concrete Calculator

How much concrete do I need? Volume and bag counts for any slab.

Reviewed by the WorldCalcs team · Methodology · Last reviewed: June 2026

Concrete volume

1.23 cu yd

33.33 cu ft · 0.94 m³

80 lb bags
56
60 lb bags
75
40 lb bags
112

Yields are nominal - add 5-10% extra for spillage and uneven subgrade.

These results are estimates only. Coverage rates and bag yields vary by product, surface, and conditions - always round up, buy a little extra, and confirm quantities with your supplier. See our disclaimer.

What is a concrete calculator?

A concrete calculator works out how much concrete you need to fill a slab, footing or pad. You enter the length, width and thickness, and it returns the volume in cubic yards, cubic feet and cubic meters - then converts that volume into the number of 40, 60 or 80 lb bags, or tells you how much ready-mix to order. It takes the guesswork out of a pour you really don't want to get wrong.

How concrete volume is calculated

Volume is length x width x thickness, with every measurement in the same unit. In imperial we take length and width in feet and thickness in inches (dividing inches by 12 to convert to feet), which gives cubic feet; dividing by 27 converts to cubic yards. To find bags, we divide the cubic feet by each bag's yield - an 80 lb bag makes about 0.60 cu ft of mixed concrete, a 60 lb bag about 0.45 cu ft, and a 40 lb bag about 0.30 cu ft - and round up. Because mixing and uneven ground waste a little, add 5-10% extra.

Example

A 10 ft x 10 ft slab poured 4 inches thick is 33.3 cu ft, or 1.23 cubic yards (0.94 m3). Filling that from bags takes about 56 bags of 80 lb concrete, 75 bags of 60 lb, or 112 bags of 40 lb. For a slab this size, ordering ready-mix is usually cheaper and faster than mixing bags by hand.

For round fence-post holes, use our Cylinder Calculator. To get the slab's surface area, try the Square Footage Calculator. Other home projects: Paint Calculator and Tile Calculator.

All calculations happen in your browser. Nothing is sent, stored, or tracked.

Results are estimates and may contain errors — for general information only, not professional advice. Always verify before relying on them. Disclaimer

How to use

Choose imperial (feet for length/width, inches for thickness) or metric (meters for everything). Enter the slab's length, width, thickness, and how many identical slabs you're pouring.

The result returns the volume in cubic yards, cubic feet and cubic meters, plus the number of 40, 60 and 80 lb bags you'd need for that volume. Yields are nominal — add 5–10% extra for spillage and uneven subgrade.

Frequently asked questions

How much concrete do I need for a 10x10 slab?+

A 10 ft x 10 ft slab at 4 inches thick needs about 1.23 cubic yards (33.3 cu ft) of concrete. That's roughly 56 bags of 80 lb mix - though ready-mix is usually more practical at this size.

How many 80 lb bags of concrete in a cubic yard?+

Each 80 lb bag yields about 0.60 cu ft, and a cubic yard is 27 cu ft, so you need about 45 bags of 80 lb mix per cubic yard. For 60 lb bags it's about 60, and for 40 lb bags about 90.

Should I use bags or ready-mix?+

Bags suit small jobs - footings, posts, a small pad. Once you pass about half a cubic yard, mixing by hand becomes hard work; ordering ready-mix delivered is usually cheaper and far quicker.

How thick should a concrete slab be?+

Four inches is typical for patios, walkways and shed bases. Driveways and areas carrying vehicles are usually 5-6 inches. Always follow local code and any engineer's spec for your project.

Should I add extra concrete for waste?+

Yes - add about 5-10%. Subgrade is rarely perfectly level, some concrete sticks to the mixer and wheelbarrow, and running short mid-pour creates a weak cold joint. Buying a little extra is cheap insurance.

How do I get cubic meters?+

Switch the calculator to metric and enter your measurements in meters. It returns the volume in cubic meters alongside cubic yards and cubic feet.

How much does a cubic yard of concrete cover?+

One cubic yard covers about 81 sq ft at 4 inches thick, 65 sq ft at 5 inches, or 54 sq ft at 6 inches. Thicker slabs cover less area per yard.

How much concrete for fence posts?+

Each round hole's volume is pi x radius squared x depth. Work out one hole's volume with our cylinder calculator, multiply by the number of posts, then subtract the post's own volume.

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