Concrete Calculators

A slab takes its width times its length times its thickness, divided by 27 to get cubic yards — and then about 10% more, because the ground under a slab is never as flat as the drawing.

That last part is where estimates go wrong in a way that costs a Saturday. Order the exact calculated volume and you will be short, because the subgrade dips, the forms bow outward under wet concrete, and some of it ends up on the ground.

Bags or a truck

The crossover sits very close to one cubic yard. Below it, bagged mix is cheaper even counting your time. Above it, a ready-mix delivery beats bags decisively — a yard of concrete is about 45 bags of 80 lb mix, which is 3,600 lb of material to carry, open and mix while the first batch is already setting.

Most suppliers have a one-yard minimum plus a short-load fee. That fee looks expensive until you price the alternative in bags.

What thickness actually buys you

Four inches is the standard for a residential slab carrying foot traffic and furniture. Five gives real margin for a vehicle. Six is for a driveway that will see a loaded truck.

Going thicker is not how you stop cracks. Concrete cracks because it shrinks as it cures, and it will do that at any thickness. Control joints — cut or tooled to a quarter of the slab depth, spaced no more than 24–30 times the thickness in feet — decide where it cracks. Skipping them does not prevent cracking; it just means the crack goes wherever it likes.

The part no calculator can tell you

Whether the ground underneath is any good. A slab poured on compacted gravel over undisturbed soil lasts decades. The same slab on soft fill settles and breaks in half, and the concrete had nothing to do with it.