Concrete Calculator

Enter the length, width and thickness of your slab to get the exact concrete volume in cubic yards and cubic feet — plus how many pre-mix bags you need if you are mixing it yourself instead of ordering a truck.

Bag counts use the standard published yields for pre-mix concrete: one 80-lb bag makes about 0.6 cubic feet and one 60-lb bag about 0.45 cubic feet. Always add waste (10% is the usual allowance) for spillage, uneven ground and a slightly thicker-than-planned slab, and note that bag counts are always rounded UP to whole bags — you cannot buy part of a bag.

The formula

Volume (cu ft) = Length × Width × (Thickness ÷ 12); Cubic yards = Cu ft ÷ 27; Bags = ceil(Volume × (1 + waste%) ÷ yield per bag)

Worked example

A 10 × 10 ft slab, 4 inches thick, is 33.3 cu ft = 1.235 cu yd before waste; with 10% waste you order 1.358 cu yd, or 62 bags of 80-lb pre-mix.

The example is pre-loaded in the result panel — press Calculate with your own numbers to replace it.

Frequently asked questions

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

At 4 inches thick that slab is 33.3 cu ft. Dividing by the 0.6 cu ft yield of an 80-lb bag gives 56 bags before waste, or 62 bags with a 10% waste allowance. At that quantity a ready-mix truck (1.36 cu yd) is usually cheaper and far less work.

Why add 10% waste?

Real slabs are never perfect: the subgrade dips, forms bow slightly and some concrete spills. Ordering 5–10% extra is far cheaper than a second delivery or a cold joint partway through the pour.

How thick should a slab be?

4 inches is standard for patios, walkways and sheds. Driveways and garage floors typically use 4–6 inches, often with rebar or wire mesh. Heavy vehicles or poor soil call for 6 inches or more — check local code.

Category: Construction & Home calculators