How many yards of concrete do I need? The math, the waste factor, and the truck count

Every flatwork job comes down to one question you have to answer before you order: how many yards? Get it wrong on the low side and you are calling the plant mid-pour with a screed full of concrete going off. Get it wrong on the high side and you are paying for material that goes home in the drum. Here is the arithmetic, the allowance that keeps you out of trouble, and the truck-count rule that quietly decides whether a job is profitable.

The formula

Concrete is sold by the cubic yard. Your slab is measured in square feet and inches. So the conversion is two steps:

1. Turn thickness in inches into feet: divide by 12.
2. Turn cubic feet into cubic yards: divide by 27.

Put together:

cubic yards = square feet times (thickness in inches divided by 12) divided by 27

Take a 20 by 30 driveway, so 600 square feet, at 4 inches:

• 600 times (4 divided by 12) equals 200 cubic feet
• 200 divided by 27 equals 7.41 cubic yards

That is your theoretical volume. It is not what you order.

Why you add a waste allowance

Nobody pours the theoretical number. Real slabs eat more than the calculator says:

• Subgrade is never perfectly flat. Low spots, ruts from the skid steer, and a base that dips half an inch across a wide pour all add volume.
• Forms bow. Even braced forms push out slightly under the weight of wet concrete.
• Edges and thickened perimeters. Many slabs get a thickened edge that never shows up in a flat area calculation.
• Spillage and what stays in the drum. Some goes on the ground, some stays in the truck.

A 10 percent allowance covers normal conditions on residential flatwork. On a rough subgrade or a pour with hand-dug edges, you may want more. The point is that ordering the bare theoretical number is how you end up short.

Back to the 600 square foot driveway at 4 inches:

• 7.41 cubic yards plus 10 percent equals 8.15 cubic yards

The truck count is the number that costs you money

Here is where yardage stops being trivia and starts being margin. Ready-mix trucks typically carry around 10 cubic yards. Your 8.15 yard driveway is one truck. Comfortable.

Now the homeowner asks for 5 inches instead of 4, because they want to park a truck on it:

• 600 times (5 divided by 12) equals 250 cubic feet
• 250 divided by 27 equals 9.26 cubic yards
• plus 10 percent equals 10.19 cubic yards

That extra inch just pushed you past 10 yards. You are now ordering a second truck for 0.19 of a yard.

That second truck is not a rounding error. It is another delivery, possibly a short-load fee on the partial, and more time on site. If you priced the upgrade as "one more inch, add a dollar a foot" and did not look at the truck count, the upgrade may have cost you more than you charged for it.

This is the single most useful reason to watch yardage while you are still building the quote rather than after you win it. When you can see the pour cross 10 yards as you type, you have options: talk the customer into 4 inches with rebar instead of 5, adjust the dimensions slightly, or price the second truck honestly into the number.

Common pours, roughly

Useful reference points for residential work, already including a 10 percent allowance:

• 10 by 10 patio (100 square feet) at 4 inches: about 1.4 cubic yards
• 4 foot sidewalk, 40 feet long (160 square feet) at 4 inches: about 2.2 cubic yards
• 20 by 20 garage slab (400 square feet) at 4 inches: about 5.4 cubic yards
• 20 by 30 driveway (600 square feet) at 5 inches: about 10.2 cubic yards, which is two trucks
• 24 by 40 driveway (960 square feet) at 5 inches: about 16.3 cubic yards, which is two trucks

Notice that the 600 foot driveway and the 960 foot driveway are both two trucks. The bigger job carries its second truck far better than the smaller one does, which is exactly why the small job that barely tips over a truck boundary is the one to watch.

Steps, curbs, and the volume people forget

Flat area is only part of a job. Anything with a rise has volume that a square foot calculation misses entirely:

• Steps. A riser is a block of concrete, not a surface. Three risers add real yardage.
• Curbs and mow strips. Priced by the linear foot, but they still consume material.
• Thickened edges and footings. A perimeter that drops to 8 or 12 inches adds up fast on a large slab.

If you quote steps and curbs as line items but only calculate yardage from your flat areas, you will under-order. Count them.

Do the yardage while you quote, not after

Most contractors do this math twice: roughly in their head at the driveway, then properly the night before the pour with a tape measure sketch and a calculator. The gap between those two numbers is where the surprises live.

CementQuote runs it continuously. As you enter each pour, the bottom bar shows a live yardage ticker: cubic yards with the waste allowance already applied, and the number of trucks that implies. Add an inch of thickness and you watch the truck count tick from one to two immediately, while you can still do something about it. The job plan also totals your forming linear feet and crew hours, so the operational side of the job is visible before you send the price rather than after you are committed to it.

The math is not hard. Remembering to do it before you commit to a number is the part that pays.