Question 1

How do I calculate concrete for a slab?

Accepted Answer

For a rectangular slab, multiply length × width × thickness (in the same unit) to get volume, then convert to cubic yards (divide ft³ by 27) or cubic meters. The calculator adds a 10% safety margin to account for spillage, uneven subgrade and over-excavation.

Question 2

How many 80 lb bags of concrete make a yard?

Accepted Answer

A standard 80 lb bag yields about 0.6 cubic feet of cured concrete. There are 27 cubic feet in a cubic yard, so you need 45 bags to make one full yard. 60 lb bags need 60 per yard, and 40 lb bags need 90 per yard.

Question 3

Should I order ready-mix or use bags?

Accepted Answer

For projects under 1 cubic yard, bagged concrete is usually cheaper and easier. For projects over 1 yard, ready-mix delivery is more cost-effective and saves significant labor. Most DIY patios (10x10x4 inch slab = ~1.24 yards) sit right at the threshold.

Question 4

What is the safety margin included?

Accepted Answer

The calculator automatically adds 10% extra to the volume to account for real-world conditions: subgrade settling, spillage, slight over-excavation, and the fact that concrete is ordered by the full yard — you cannot buy half a yard from most suppliers.

Pro tip