How to Figure Concrete Yardage

Using 3-D Volume Equations

Familiarize yourself with how concrete volume is measured. Concrete's volume (the amount of physical space it takes up) is typically measured in either cubic yards (yds) or cubic meters (m). A cubic yard is a 3-dimensional cube that is 3 feet long in each dimension, while a cubic meter is a cube that is 1 meter long in each dimension. Typically, bags of dry concrete mix will specify the volume of "wet" concrete each bag will make when properly mixed with water. Below are rough estimates for how many bags of dry concrete mix it takes to make one cubic meter of wet concrete: 40 kg bag: 56 bags to make 1 cubic meter 32 kg bag: 71 " " " 1 cubic meter 26 kg bag: 86 " " " 1 cubic meter

Break your concrete project up into different rectangular prisms. Compared to other 3-D shapes, a rectangular prism's volume is relatively easy to calculate, so, if possible, it is best to break your entire project up into one or more rectangular prisms. For example, if your project calls for you to pour a single rectangular slab-on-grade, that slab would be your only prism. However, if you need to pour a slab along with 4 straight walls, each wall would be its own prism, resulting in a total of 5 prisms. A rectangular prism is a three-dimensional shape with six faces, all of which are rectangles; opposite faces in a rectangular prism are parallel to each other. In layman's terms, a rectangular prism can be thought of as any "boxy" shape with straight edges.

Calculate the volume of each prism. A rectangular prism's volume can be found by multiplying its length times its width times its height. For example, in the next few steps, let's imagine that we're pouring a slab that is 10 feet (3.05 m) long, 12 feet wide (3.66 m), and 4 inches (10.16 cm) deep.

Convert all measurements to the same units. Our slab's length and width are given in feet and meters, but its height is given in inches and centimeters. For our volume equation to work, all measurements must be in the same units, so we must convert. Since there are 12 inches per foot, we should divide our inch value by 12 to get a value for feet. The slab is 4 / 12 = 0.33 feet deep. To convert from centimeters to meters, simply divide a centimeter value by 100. A slab that is 10.16 cm deep is 10.16 / 100 = 0.10 meters deep. To convert back to centimeters, multiply by 100.

Find the volume of the prism using the formula: Volume = Length * Width * Height. Multiply the 3 dimensions together to arrive at the prism's volume. In our example, the slab's volume is 10 ft × 12 ft × 0.33 ft = 39.6 cubic feet. To find the metric equivalent, we use our meter measurements instead of our feet measurements. 3.05 m × 3.66 m × 0.10 m = 1.12 cubic meters.

Convert the volume into cubic yards or cubic meters as needed. The slab's volume above is expressed as 39.6 cubic feet, but, unfortunately, concrete is usually measured in cubic yards. There are 27 cubic feet in a cubic yard, so, to convert to cubic yards, we can divide by our cubic feet value by 27. The slab's volume is 39.6 / 27 = 1.47 cubic yards. Alternatively, since there are three feet in a yard, you can divide each individual foot measurement by 3 to get yard values and then multiply these together to get the same answer. Concrete is also often measured in cubic meters. In our example problem, we already found this value. However, in the case that you need to convert between cubic yards and cubic meters, know that: 1 cubic yard = 0.764554858 cubic meters 1 cubic meter = 1.30795062 cubic yards

Find additional prism volumes as above. If you have more than one prism in your concrete pour, calculate each one's volume individually using the above method. Finally, add all of the volumes together to arrive at your total volume for your project. Be careful that none of your prisms overlap, or you will end up counting some of the concrete twice, which can lead to you buying more than you need.

Calculate the volume of any irregularly-shaped forms. Not every concrete job can be easily split up into rectangular prisms. If you have a spread footing in your concrete design, for example, you can't accurately approximate its shape using rectangular prisms. To find the volume of an irregular form, first find the area of a cross-section of the form. Then, multiply that area times the length of the form. For example, if a spread footing is 3 yards (2.74 m) long and has a cross-sectional area of 0.25 square yards (0.21 square meters), then the volume is 3 × 0.25 = 0.75 cubic yards (or 2.74 × 0.21 = 0.58 cubic meters). Additionally, some common non-rectangular shapes have convenient equations to find their volume. Below are a few of the most common: Cylinders: Volume = (Pi)r × h, where "r" is the radius of the circle at either end of the cylinder and "h" is the height of the cylinder. Triangular Prisms : Volume = 1/2bh1 × l, where "b" is the length of the base of one of the triangular faces, "h1" is its height, and "l" is the length of the prism. Spheres: Volume = (4/3)(Pi)r, where "r" is the radius of the circle that represents the sphere's circumference. While it's unlikely that you'll ever have to pour a perfect sphere, note that many dome-like shapes are just spheres cut in half.

Buy a little more concrete than you need. A good rule of thumb is to add 10 to 15 percent to your figured volume to account for spillage, waste, or over-excavation. Because you can't reasonably expect to use your concrete with 100 percent efficiency, be sure to order more than you actually need. For instance, if you have figured a total volume of 20 cubic yards (15.3 cubic meters), then you should order 1.05 × 20 = 21 cubic yards (or 1.05 × 15.3 = 16.1 cubic meters). If you are pouring steel-reinforced concrete, the steel reinforcing will displace some of your concrete's volume. Usually, you don't need to consider this in your calculations. This will keep your figures on the conservative side.

Convert your volume figure to weight, if necessary. Truck-mixed concrete is sold by volume, but bags of concrete mix are sold in retail establishments by weight. Often, dry mix will have information on the bag indicating the "wet" weight or volume each bag produces. Concrete weighs about 4000 pounds per cubic yard (2400 kg per cubic meter). So, if you need 2 cubic yards (1.53 cubic meters) of concrete, then you need (2 * 4000) or 8000 pounds, or (1.53 * 2400) or 3672 kg of concrete. As noted above, buying more dry concrete mix than is needed is usually much safer than buying less concrete mix than is needed - unused mix can always be used in the future.

Quick Method for Basic Slabs

Ensure your concrete pour is a rectangular prism. Contractors have devised a quick, easy system of determining how much concrete yardage is needed for a given pour. This method doesn't require you to use any volume equations - however, there are two stipulations for its use. First, it only works for rectangular prisms (basic "box-shaped" pours). This method is easiest for relatively shallow pours, but it's possible for all rectangular prisms. Second, it requires that your pour area's length, and width measurements be in feet and that its depth measurement be in inches. To convert any of your measurements to feet, use the conversion factors below: 1 yard = 3 feet 12 inches = 1 foot 1 meter = 3.28 feet 30.48 centimeters = 1 foot

Find the square footage of the area you're pouring. Square footage (written "sq. ft" or ft) is a measure of 2-dimensional area typically used to describe floor space. To determine your concrete job's square footage, simply multiply the length and width of the area you're pouring into without taking into account its depth. For example, let's say we're pouring into a rectangular prism that is 10 ft wide, 5 feet long, and 6 inches (0.5 feet) deep. Its square footage would be 10 × 5 = 50 sq. feet. We don't need to worry about its depth yet. Remember, this method only works for rectangular prisms. In other words, your pour area must have straight vertical edges.

Divide your square footage by a numerical coefficient. Now that you've found your square footage, all you need to do to find your project's concrete yardage is to divide the square footage by a certain number - the thicker your project, the smaller the number; the thinner your project, the larger the number. Below are coefficients for a few common thicknesses. If your project's thickness isn't listed below, don't worry - you'll learn how to easily figure out your coefficient in the next step. If your project is 4 inches thick, divide your square footage by 81 to determine your yardage. If your project is 6 inches thick, " " " " " 54. If your project is 8 inches thick, " " " " " 40. If your project is 12 inches (1 foot) thick, " " " " " 27.

Determine odd coefficients manually. If the thickness of your concrete pour doesn't match any of the examples above, you can quickly calculate it by dividing 324 by the thickness of your concrete project (in inches). Then, divide your square footage by your answer to find your project's yardage. For example, let's say our 50 sq. ft concrete pour is 7 inches deep. We would find our concrete yardage as follows: 324/7 = 46.28 50/46.28 = 1.08 cubic yards.