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How Do I Calculate How Many Yards of Concrete Do I Need?

It's never fun to be in the middle of a DIY project and realize you have to make a run to the store because you don't have enough materials. Not only is it annoying, it could ruin your project. This is particularly true when you're working with concrete, which requires precision timing to ensure a clean finish. To determine the amount of concrete needed for your project, you need to know three things: the length, width, and depth of the area.

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Measuring the Base

The ground on which you'll pour the concrete is known as the base or subgrade. The base must be both evenly graded and compacted before you even take your measurements, much less start pouring concrete. This helps ensure accurate measurements and a smooth finish.

If your project calls for a 4" slab, preparing the base means correcting areas where the subgrade dropped below 4" deep. Larger projects are more likely to have this issue, but it's common on any job, so measure subgrade depth carefully.

An accurate measurement of the amount of concrete needed requires measuring the length, width, and depth of each section. Complete your measurements twice for greater accuracy.

The most common measuring tools are:

  • A folding rule, which usually measures 6' and is perfect for smaller projects
  • A 30' measuring tape for mid-sized jobs
  • A reel measuring tape - either open or closed - for large slabs

On smaller jobs, you can usually get accurate measurements even when working alone. For larger slabs, though, you may want assistance.

How Is Concrete Measured?

Concrete is measured and sold by the cubic yard. Large jobs are typically categorized as those requiring four or more cubic yards of concrete. For this amount of concrete, you should request delivery by a ready-mix concrete truck. There are also trailers that deliver two cubic yards. Or, you can rent a concrete mixer. If the area is very small, you can buy bags of ready-mix concrete; just add water.

Calculating How Much Concrete You Need

The calculation is fairly simple:

  • Length x Width x Depth = Area
  • Area / 27' = Cubic Yards of Concrete Needed
  • However, it does require all of your measurements to be in feet, which can get confusing since depth is nearly always in inches. In that case, simply divide 12 by the depth:

  • 4" / 12 = 0.3333

If you like, you can also use the following guide, which tells you how many feet a cubic yard of concrete covers at various depths. For example, one cubic yard of concrete 4" deep covers 81'.

Then, take the perimeter of your project (Length x Width) and divide it by the number in our guide.

For example, a slab measuring 10' wide by 10' long has a perimeter of 100':

  • 10 x 10 = 100
  • Divide the perimeter by 81 (the number of feet covered by one cubic yard of concrete), and you get 1.234:

  • 100 / 81 = 1.234

So, your slab requires 1.234 cubic yards of concrete.

Guide:

  • One cubic yard of concrete 4" thick covers 81'
  • One cubic yard of concrete 5" thick covers 65'
  • One cubic yard of concrete 6" thick covers 54'
  • One cubic yard of concrete 7" thick covers 46'
  • One cubic yard of concrete 8" thick covers 41'
  • One cubic yard of concrete 10" thick covers 32'
  • One cubic yard of concrete 12" thick covers 27'

If your project is not a square or rectangle, create a scale drawing and divide it into rectangles. Then, calculate the volume of each section and add them together, using the formulas provided above, to determine the amount of concrete required.

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How Much Concrete Should You Order?

The guide is just that: a guide. You want to order more concrete than your measurements indicate to account for an uneven subgrade, spillage, and other factors that can't be calculated. Contractors usually add around 10 percent to their calculation to ensure they have enough concrete. To figure that, multiply your final number by 1.10. Using our 100' perimeter calculation above, that looks like:

  • 1.234 x 1.1 = 1.3574

So, you need 1.3574 cubic yards of concrete to cover a 10' by 10' slab that's 4" deep.

When ordering concrete, you always round up, never down. Suppliers sell concrete in varying increments. If the supplier you choose sells by the quarter-yard, you would order 1.5 cubic yards based on the above calculation. This is also what you'd order if they sell by the half-yard. If they sell by the yard, you'd order 2 cubic yards, since 1 wouldn't be enough.