Concrete Slab Calculator

A concrete slab calculator works out how much ready-mix concrete you need to pour a flat rectangular slab — a shed base, a patio, an AC pad or a garage floor. Enter the length, width and thickness and it returns cubic yards and cubic metres, plus the number of 60 and 80 lb bags for smaller pours, with 10% waste built in.

Your measurements

Result

A "slab" and a "pad" are the same pour as far as the maths goes: length × width × thickness. The most common slab thickness is 4 inches; go to 5–6 inches if anything heavy will sit or drive on it.

How it is calculated

Slab volume is length × width × thickness, with the thickness converted from inches to feet (or centimetres to metres). We then convert to cubic yards (×1.308) and to 80 lb and 60 lb bag counts, and add a 10% waste allowance for spillage and an uneven base.

Steps

Measure the slab length and width in feet.
Enter the thickness in inches — 4 inches is standard for patios and shed bases.
Read the cubic-yard figure for a ready-mix order, or the bag count for a small DIY pour.
Order the 10%-waste figure so an uneven sub-base does not leave you short.
Pouring more than about half a yard? Price a ready-mix delivery — it is usually cheaper than bags.

Worked example

A 10 ft × 10 ft shed base at 4 inches is 100 sq ft × 0.33 ft = 33 cubic feet, about 1.23 cubic yards. With waste you would order roughly 1.35 yards, or mix around 57 80 lb bags for a DIY pour.

Common questions

How thick should a concrete slab be?

Four inches is the standard minimum for patios, footpaths and shed floors. For driveways, garages or anything bearing vehicle weight, use 5–6 inches over a compacted gravel base.

How many bags of concrete in a yard?

About 45 80 lb bags or 60 60 lb bags make a cubic yard. That is why anything over half a yard is usually cheaper to order as ready-mix.

Do I need gravel under the slab?

Yes — a compacted gravel sub-base (typically 4 inches) gives drainage and a stable platform so the slab does not crack or settle.

Should I add rebar or mesh?

Most slabs use steel reinforcing mesh; thicker structural slabs use rebar. The calculator sizes the concrete only — reinforcement is sized from the design.