What waste factor should I use?

Use 10% for typical slab and wall pours. Bump it to 15% for hand-dug footings, soft or sloped subgrade, or irregular forms. It is far cheaper to have a little left over than to stop a pour waiting on a second truck.

How many bags of concrete in a cubic yard?

It takes 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb to make one cubic yard. Over about one yard, ready-mix is usually faster and cheaper than bagging.

Do I measure in feet or inches?

Measure however you read the tape, but convert everything to feet before you run the formula. The most common mistake is leaving depth in inches — divide it by 12 first.

How to Calculate Concrete

The whole job comes down to one formula and one conversion most people get wrong. Here is how to figure cubic yards, bags, and a waste factor the way a contractor actually does it.

Calculating concrete is not complicated, but it is unforgiving — a single missed unit conversion is the difference between the right truck and a cold joint. Concrete is ordered in cubic yards, and the only formula you really need is length times width times depth, divided by 27. The catch is that every measurement has to be in feet before you divide, and depth is almost always given in inches. Get that conversion right and the rest is arithmetic.

Use the Concrete CalculatorCubic yards, bags, and cost for any concrete pour.

The core formula

Cubic yards equals length (ft) times width (ft) times depth (ft), divided by 27. There are 27 cubic feet in a cubic yard, which is where the 27 comes from. The reason pros divide by 27 and not by anything else is that ready-mix is batched, sold, and delivered by the cubic yard — so that is the unit your order has to be in.

Convert inches to feet first

This is the step that sinks most takeoffs. Depth is given in inches — 4", 5", 6" — but the formula needs feet. Divide the inch value by 12 before you multiply.

4 inches = 0.333 ft
5 inches = 0.417 ft
6 inches = 0.5 ft
8 inches = 0.667 ft

Worked example: a garage slab

Take a 24 ft by 30 ft garage slab poured at 5 inches thick. Convert depth: 5 / 12 = 0.417 ft. Multiply: 24 x 30 x 0.417 = 300.2 cubic feet. Divide by 27: 300.2 / 27 = 11.12 cubic yards. Add a 10% waste factor: 11.12 x 1.10 = 12.23. You order 12.25 cubic yards.

Always add a waste factor

The calculated number is a perfect-world number, and pours are not perfect. Subgrade is uneven, forms bulge, and some concrete is always lost to spillage and consolidation. Add 10% on most slab and wall pours, and 15% on hand-dug footings or soft, sloped ground.

Bags vs ready-mix

For small work you can bag it: one cubic yard takes 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb. Anything over about a yard is usually faster and cheaper as ready-mix. Watch for the short-load fee most plants charge under 3–4 yards.

Measure the subgrade, not the form

A 4-inch slab over a rutted base eats 4.5–5 inches in the low spots. Use your real average depth, not the form height, or you will come up short.

Round up to the quarter yard

Plants batch in quarter-yard increments. Always round up — running short mid-pour costs far more than a little extra concrete.

Total the whole foundation at once

Footings, walls, and slab are separate calculations but one order day. Keep a running total so nothing gets left off the truck schedule.

Frequently asked questions

Because there are 27 cubic feet in one cubic yard (3 ft x 3 ft x 3 ft). Concrete is ordered in cubic yards, so after you find the volume in cubic feet you divide by 27 to get the number you actually put on the order.

Put this into practice

Open the concrete calculator and run your numbers. Save the job and keep every area together.

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