How Much Concrete Do I Need?

Quick answer: concrete is sold by the cubic yard, so the job is to find your pour's volume in cubic feet, divide by 27, and add about 10% for waste. For a slab that is length × width × thickness (all in feet). The concrete calculator does this instantly and also tells you how many 60 lb or 80 lb bags to buy.

The one formula you need

Every concrete estimate is just a volume. Measure your pour in feet, multiply the three dimensions, then convert to cubic yards (the unit ready-mix is sold in):

cubic yards = (length ft × width ft × thickness ft) ÷ 27

The trick most people miss: thickness is usually given in inches, so convert it to feet first by dividing by 12. A 4-inch slab is 4 ÷ 12 = 0.333 ft thick.

Worked example: a 10 × 12 ft slab

For a patio slab 10 ft by 12 ft, poured 4 inches thick:

• Thickness in feet: 4 ÷ 12 = 0.333 ft
• Volume: 10 × 12 × 0.333 = 40 cubic feet
• Cubic yards: 40 ÷ 27 = 1.48 cubic yards
• Add 10% waste: 1.48 × 1.10 ≈ 1.63 cubic yards

So you would order about 1.65 cubic yards of ready-mix, or round up to where your supplier sells in quarter-yard increments.

How many bags of concrete?

For small jobs, bagged concrete is easier than ordering a truck. Each bag lists its yield in cubic feet:

• An 80 lb bag yields about 0.60 cubic feet
• A 60 lb bag yields about 0.45 cubic feet
• A 40 lb bag yields about 0.30 cubic feet

Divide your total cubic feet by the bag yield. For the 40 cubic-foot slab above: 40 ÷ 0.60 ≈ 67 bags of 80 lb concrete. That is a lot of mixing by hand — past roughly 25–30 bags, a ready-mix delivery is usually cheaper and far less work.

Footings and round post holes

Footings are just long, narrow slabs — use the same length × width × depth formula. Round post holes use the cylinder formula instead:

cubic feet = π × (diameter ft ÷ 2)² × depth ft

For a 12-inch-wide hole 3 ft deep: 3.14 × (0.5)² × 3 = 2.36 cubic feet per hole. Multiply by the number of holes, and remember to subtract the volume the post itself takes up if it is large.

Why add a waste factor?

Real-world pours never match the math exactly: the subgrade is uneven, forms bow slightly, and some concrete is always lost to spillage and the wheelbarrow. Adding 5–10% protects you from the worst outcome in concrete work — running short mid-pour, which can leave a cold joint or a weak seam. Slightly over is cheap insurance; short is a real problem.

Frequently asked questions

How much does a yard of concrete cover? One cubic yard covers about 81 square feet at 4 inches thick, 65 square feet at 5 inches, or 54 square feet at 6 inches.

Should I order a little extra? Yes — most pros add 5–10%. It is far better to have a little left over than to stop a pour to mix more.

What thickness should my slab be? 4 inches is standard for patios and walkways; use 5–6 inches for driveways or anything that carries vehicle weight.