Concrete Bags Calculator

How the calculation works

The volume of a slab is length times width times depth, with the depth converted from inches to feet. A 10 by 10 foot slab at 4 inches deep is 33.33 cubic feet, or 1.23 cubic yards. Bagged concrete is sold by weight, and each bag size yields a known volume: an 80 lb bag yields about 0.60 cubic feet, a 60 lb bag about 0.45 and a 40 lb bag about 0.30. Divide the volume by the yield per bag and round up — that is the number of bags. The waste allowance covers spillage, uneven subgrade and the concrete that stays in the mixer, and 10% is the usual figure for a flat slab.

When bags stop making sense

Bagged concrete is convenient for small jobs, but the economics flip quickly. A 10 by 10 slab at 4 inches needs about 62 eighty-pound bags — roughly 5,000 pounds of material to mix by hand. Once a job passes about 1 cubic yard, or roughly 45 eighty-pound bags, a ready-mix delivery is usually cheaper per cubic yard and far less work, and the concrete arrives consistent. For anything structural — footings, foundations, load-bearing slabs — ready-mix with a specified strength is the right choice regardless of size.

Tips for a good pour

Measure the depth at several points: subgrade is rarely as flat as it looks, and a slab that is 5 inches deep in one corner needs more concrete than the drawing says. Have all the bags on site before you start, and mix in small batches so no batch sits while you finish the last one. If the day is hot, keep the bags and the mixing water in the shade — concrete that sets too fast is hard to finish and weaker when cured.

Frequently asked questions

How many 80 lb bags of concrete do I need for a 10×10 slab?

At 4 inches deep, a 10×10 ft slab is 33.33 cubic feet. With a 10% waste allowance that is about 36.7 cubic feet, or 62 eighty-pound bags (each yields ~0.60 cu ft). Enter your exact dimensions above for your job.

How much concrete does one 80 lb bag make?

About 0.60 cubic feet, or 0.022 cubic yards. A 60 lb bag makes about 0.45 cubic feet and a 40 lb bag about 0.30 cubic feet. These are the manufacturers' nominal yields.

Should I use bags or ready-mix?

Bags suit small jobs under about 1 cubic yard — repairs, fence posts, small pads. Above that, ready-mix delivery is usually cheaper and the mix is more consistent. For structural work, always use ready-mix with a specified strength.