About this tool
Concrete Calculator is a free, in-browser tool that estimates how much concrete a slab, footing or pad needs. Enter the length and width in feet, the thickness in inches and how many identical slabs you are pouring, and it returns the volume plus the number of pre-mixed bags to buy. Nothing is uploaded — every figure is computed locally as you type.
The volume in cubic feet is length × width × (thickness ÷ 12) × quantity × (1 + waste ÷ 100). Cubic yards divide that by 27 and cubic meters multiply by 0.0283168. Bag counts assume a 60 lb bag yields about 0.45 ft³ and an 80 lb bag about 0.60 ft³, so bags = ceil(volume ÷ yield), always rounded up because you cannot buy a partial bag.
Use the waste field (5% by default) to cover spillage, uneven subgrade and over-excavation. For a ready-mix truck delivery, the cubic yards tile is the number to order; for a small job, the bag tiles tell you how many to carry home.
Frequently asked questions
How many 60 lb or 80 lb bags do I need?
A 60 lb bag of concrete mix makes roughly 0.45 ft³ and an 80 lb bag about 0.60 ft³. The tool divides your total cubic feet by that yield and rounds up, since partial bags are not sold. Larger pours are usually cheaper as ready-mix by the cubic yard.
Why add a waste percentage?
Real pours lose material to spillage, an uneven or over-dug subgrade and forms that are not perfectly square. A 5–10% allowance means you are not stuck short mid-pour. Set it to 0 if you want the exact geometric volume.
What thickness should I use for a slab?
Four inches is typical for patios, walkways and shed floors; driveways and load-bearing slabs are often 5–6 inches. Enter thickness in inches — the tool converts it to feet internally by dividing by 12.
Is anything I enter sent to a server?
No. All dimensions and the resulting volumes and bag counts are computed entirely in your browser; nothing is uploaded and the tool works offline.
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