How to Calculate Cut and Fill for a Construction Site

Before you can build an office building, house, shopping mall, parking lot, or any other structure, the site must be leveled.

Estimating the amounts of cut and fill required to level the construction site is one of the most complicated and tedious processes in construction estimating. Calculation of construction materials, areas to be painted, number of doors and windows, etc. It’s child’s play compared to estimating cut and fill amounts for a construction site. The reason for this is that the unimproved site almost always has an irregular surface. Because of this, it can be very difficult to determine how much dirt needs to be removed from the unimproved site or brought in.

There are three main ways to calculate cut and fill for the construction site. The first two are “by hand” and require a construction plan, ruler, pencil, calculator and hours of time. The third method requires digging software, a computer, and some computer skills.

The first “by hand” method uses the grid method to calculate the cut and fill for the construction site. In this method, the site is divided into grids of equal size. Grid squares are typically 1 inch to 2 inches in size on the paper plane. For each corner of each grid square, both the existing elevation and the proposed or final elevation are calculated and noted on the plan. Once all elevations are determined, the difference between the existing elevation and the proposed elevation at each corner is also noted on the plan. Then, for each grid square, these elevation differences for each of the four corners are added and averaged by dividing the sum by 4. If the average of the elevation differences for a grid square is negative, then this is a cutting area. In cut areas, existing elevations must be reduced to the proposed elevations or “cut”. If the average elevation difference is positive, that means the existing elevations should be raised to the proposed elevations or “filled in.” Then all the negative numbers are added together and added together to determine the total depths of cut. The positive numbers are then added and totaled as the total fill depths. Then each of these cut and fill totals is multiplied by the area of ​​the grid squares to determine the number of cubic feet of cut and cubic feet of fill. And finally, these cut or fill volumes are divided by 27 to determine the total number of cut cubic yards and fill cubic yards for the site (excavators work in cubic yards instead of cubic feet). Finally, the difference between the total cut volume and the total fill volume is calculated. If there is more cut than fill, then the soil must be removed from the construction site and transported by truck. If there is more fill than cut, then soil must be imported to the construction site. The time required to calculate cut and fill using the grid method is about eight hours for a single sheet site plan of moderate complexity. Accuracy with this method is +/-20%.

The second method used to calculate cut and fill is called the cross section method. In this method, the estimator draws a set of equally spaced horizontal lines along the site plan 1 to 2 inches apart. Then, for each cross section line, the estimator plots, on graph paper, both the existing and the proposed vs. the distance along the cross section. Then count the number of grid squares where the existing surface is above the proposed surface. This is the cut area for that cross section. Then count the number of grid squares where the existing surface is below the proposed surface. This is the padding area for the cross section. Repeat these steps until all cross sections of the site plan are complete. It then averages the cut area between each pair of cross sections and multiplies it by the distance between the cross sections and divides that number by 27. This is the cut volume between the adjacent cross sections. Then repeat these steps for the infill areas to determine the infill volume between all adjacent cross sections. Then add all the cut volumes and all the fill volumes to determine the cut and fill total for the construction site. If there is more cut than fill, then construction site dirt should be removed. If there is more fill than cut, the soil must be imported to the construction site. The time required to calculate cut and fill with the cross section method is 20-30% longer than with the grid method. Accuracy with this method is approximately +/-15%.

The third and final method of calculating construction site cut and fill is to use excavation software. In this method, elevation data is traced from the paper site plan using a large format digitizer, traced on screen using a PDF of the construction site plan, or imported directly from an AutoCAD file. The computer then analyzes the drawing to build both existing and proposed surfaces. Then, for hundreds of thousands of points throughout the site, the software calculates the difference between the existing elevations and the proposed elevations to determine the total cut or fill volumes for the construction site. The time required to calculate cut and fill using the excavation software is approximately one quarter of the time required for the grid method. Accuracy with this method is approximately +/-5%.

The advantages of both the grid method and the cross section method for calculating cut and fill is that they both use inexpensive and commonly available supplies. The downside is that both require many hours of work and are moderately accurate.

The advantage of using excavation software to calculate cut and fill is much faster estimation speed and much higher accuracy. The disadvantages are the cost that can range from several thousand dollars to several tens of thousands of dollars for software and hardware and requires the estimator to have some computer skills.

The appropriate method for your company to calculate cut and fill for your bids depends on the number of excavation lifts you need to perform each year. If you’re only doing a few digging runs a year and you or your appraiser aren’t terribly busy, then the “by hand” methods are probably adequate. If you are an excavator or general contractor who does a lot of cut and fill calculations, purchasing the necessary excavation hardware and software will greatly increase your ability and accuracy in bidding on jobs that require excavation.