Practical Tips for Preliminary Quantity Estimation You Can Use Today

What Is Preliminary Quantity Estimation?

Preliminary quantity estimation refers to the work of calculating the approximate quantities needed for civil engineering and building projects at a stage before detailed design, to support planning and design. It involves estimating approximate earthwork volumes and material quantities from drawings and site conditions to inform early project decisions. For beginners, the ability to estimate quantities at this stage is extremely important.

By understanding approximate quantities early on, you can form a rough outlook on the project's budget and schedule. Conversely, if you proceed without such estimates, you may encounter unexpected issues during construction such as “we had to excavate more than expected” or “extra costs for disposal of surplus soil,” which can arise. Preliminary quantity estimation serves as a foundation to reduce these risks and improve the accuracy of plans.

Basics of Earthwork Quantities (Excavation, Fill, Surplus Soil Disposal, Import)

In civil earthworks, tasks involve excavating and filling soil. Typical quantities handled here include excavation (volume of ground removed), fill (volume of soil placed), surplus soil disposal (volume of excess soil removed off-site), and soil import (volume of soil brought in from outside). These are all measured in cubic meters (m³) and serve as basic indicators of the scale of earthwork.

Surplus soil is the portion of excavated material that cannot be reused for backfilling or filling. Surplus soil must be transported and processed at disposal sites, and the greater the surplus soil disposal volume, the higher the costs and effort. Conversely, if the site lacks enough soil for fill, soil must be imported from elsewhere. Generally, it is desirable to balance excavation and fill to minimize surplus disposal and import, but site conditions often create discrepancies. That is why accurately grasping the approximate quantities for each of these items is important.

Basic Approach to Estimating Quantities from Site Conditions and Design Drawings

To calculate approximate quantities, the basic approach is to identify the difference between the existing terrain and the planned design shape. The difference between the current ground elevation (surface elevation) and the design elevation at completion becomes the depth of excavation or the thickness of fill, and you estimate volumes by considering the extent of these differences.

Specifically, first understand the site conditions. For example, determine whether the site is flat or sloped, and whether there are valleys or bumps, then compare those features with the completed form shown on the design drawings (finished heights and earthworks shapes). Design drawings indicate planned heights for buildings and roads and the areas of cut and fill. From this information, you can roughly determine where you will excavate by how many meters and where you will place how many meters of fill.

At this time, it is easy to organize the work by dividing the site into several areas or sections. For instance, if the terrain is sloped, estimate the average height difference between the high and low sides and multiply that by the area to get the volume. The key is to mentally overlay the current site conditions with the design outcome and visualize “how much soil needs to be moved to achieve the design,” which forms the basic approach to quantity estimation.

Common Volume Calculation Formulas (Trapezoidal and Triangular Sections, Average Cross-Section Method)

Several simple volume calculation formulas are well known for quantity calculation. Among them, the average cross-section method is frequently used in earthwork. This method obtains volume V by multiplying the average of the cross-sectional areas at both ends of a segment (A1 and A2) by the length L of that segment. Expressed as a formula: V = ((A1 + A2) / 2) × L.

As a prerequisite, you need to calculate the cross-sectional areas for excavation or fill. Cross-section shapes can be approximated using simple shapes such as trapezoids or triangles to make calculations easier. For example, a cut section with a flat bottom and sloping sides can be treated as a trapezoid. The area A of a trapezoid, using top width b1, bottom width b2, and height (depth) h, is A = (b1 + b2) / 2 × h. If the bottom width is 3 m, the top width is 7 m, and the depth is 2 m, then A = (3 + 7) / 2 × 2 = 10 m². Also, a section that tapers to a point (such as a wedge-shaped excavation) can be treated as a triangle, and you would use the triangular area formula A = (base × height) / 2 to find the cross-sectional area.

Using these cross-sectional areas, you apply the average cross-section method to calculate the soil volume for each segment. For example, suppose a fill segment has a cross-sectional area A1 = 8 m² at the start and A2 = 12 m² at the end, and the segment length L = 20 m. The volume V is (8 + 12) / 2 × 20 = 200 m³. If the actual terrain is complex, increasing the number of segments and applying the average cross-section method to each segment and summing them will improve accuracy. Although this method includes some error at the preliminary stage, it is widely used as a quick way to approximate soil volumes.

Why Estimates Are Needed Even in Early Design Stages (When Speed Matters More Than Precision)

In the early stages of a project, detailed design drawings and finalized numbers are not yet available. However, it is necessary to grasp approximate quantities early because they are used to produce rough construction cost estimates and schedule estimates, determine project feasibility, and decide design policies.

At this stage, what is required above all is speed. Even if the precision is somewhat rough, producing numbers quickly has value. For example, if you can quickly compare how much earthwork differs between Plan A and Plan B, you can early on consider differences in cost and schedule. Conversely, spending too much time on calculations in the name of precision may delay decision-making and impede the overall project progress.

Of course, a preliminary estimate must not be a random number. The important thing is to produce the most reliable estimate possible in a short time. Input data are limited in early design, but by using rules of thumb and simple calculation methods, you should be able to quickly present a “roughly this much” quantity. That greatly helps decide the project direction.

Making Use of Site Surveys and Existing Information (Survey Maps, Elevation Differences, Terrain Models)

To improve the accuracy of preliminary quantities, it is essential to make the most of available information. First, if you have access to survey maps or topographic maps, read the terrain features and elevations from them. You may obtain existing survey data from government offices or predecessors, and public topographic maps or open terrain data may also be available. Use these resources to understand elevation differences and terrain features of the planned site (direction of slope, locations of valleys or mounds).

If you can visit the site, a simple field survey is also effective. Measuring elevation differences of key points with a laser level and staff or measuring distances in the field can reveal terrain conditions not apparent from drawings. You can also check approximate elevations using smartphone map apps or GPS functions (note accuracy limitations). Recently, drone photogrammetry and 3D laser scanners are also used to obtain terrain models, but such advanced methods are not always necessary for simple preliminary estimates. What matters is getting a big-picture understanding such as how many meters the high and low points differ and the overall slope. This provides realistic backing for quantity calculations based on design drawings.

Relationship Between Preliminary Quantities and Construction Cost/Schedule (Export Volume, Surplus Soil Disposal Cost, Fill Height)

Estimated earthwork quantities directly affect construction costs and schedules. In particular, the volume to be hauled off-site (export volume or surplus soil) and the volume of soil to be imported are major cost factors. Disposing of surplus soil requires truck transport and disposal fees at waste sites, increasing surplus soil disposal costs. Importing soil incurs material purchase and transport costs. Grasping preliminary quantities is directly linked to understanding the scale of these costs.

Earthwork quantities also influence the schedule. For example, if 1,000 m³ of surplus soil is generated, that corresponds to about 100 truckloads with large 10 m³ dump trucks. Transporting that amount of soil will require several days of truck movement and increase the operating days for heavy equipment. Conversely, if a large amount of fill is needed, importing and spreading that soil will take time. Because manpower, equipment, and days required vary greatly with quantity, grasping quantities at the preliminary stage is important for preparing a rough schedule estimate.

Also, the fill height (how much you raise or lower the site elevation) is directly tied to cost. For example, raising a 1,000 m² site by an average of 1 m requires about 1,000 m³ of fill. Even a 10 cm difference in height can lead to tens of cubic meters of volume difference over a large area. Such height differences translate directly into additional truckloads and increased construction costs, so attention should be paid to them from the design stage. Preliminary quantity estimation helps determine appropriate height settings and soil balance to prevent unnecessary costs and schedule delays.

Simple Examples of Takeoff Methods Using Spreadsheet Software or Hand Sketches

Recently, spreadsheet software is often used for preliminary quantity calculations. For example, list ground and design elevations at regular intervals (e.g., every 10 m) and calculate cross-sectional areas in the spreadsheet to automatically obtain segment volumes using the average cross-section method. Summing the volumes of each segment produces a quick overall soil volume estimate. Spreadsheets allow instant adjustments to numbers, making them convenient for comparing design options and adjusting parameters.

On the other hand, hand-drawn sketches are sufficient when a computer is not available. A simple method is to roughly draw the site cross-section on paper and visualize the difference between the existing ground and the design elevation. On the cross-section, outline cut and fill areas and divide them into simple shapes such as triangles and trapezoids to calculate areas. Multiply each area by the depth (the width that the cross-section represents) to get segment volumes. Summing them yields the total preliminary soil volume. With just a sketch and a calculator, you can pick up quantities quickly if you focus on the key points.

Checkpoints to Reduce Errors (Slope Units, Relationship with Area, Unit Conversion)

To prevent common mistakes when producing preliminary quantities, pay attention to the following points:

• Slope notation units: Slopes in design drawings may be shown as 1:◯◯ or as ◯◯%. For example, 1:2 means a vertical change of 1 m for a horizontal distance of 2 m, and a 50% slope refers to the same gradient. Mixing these notations can lead to errors in section dimensions and incorrect volume calculations. Confirm which unit the drawing’s slope notation uses and apply it correctly in calculations.

• Relationship between area and height: Volume is generally calculated as “horizontal area × vertical height.” When using cross-sectional or plan areas, confirm that the area corresponds to the vertical height. For example, when calculating fill volume, multiply the projected horizontal area, not the sloped surface area, by the fill height. Also, when calculating volumes from cross-sections, ensure that cross-sections are taken perpendicular to the design alignment.

• Unit consistency and conversion: Keep length and area units consistent. Dimensions on design drawings may be given in cm or mm, but soil volume calculations should be converted to meters (m) before applying formulas. For example, “500 cm” becomes “5.0 m” and “50 mm” becomes “0.05 m.” Also, standardize volume units to m³ and avoid unit-mismatch errors. Neglecting unit conversion can lead to errors of 100× or 1,000×, so be vigilant.

A Strong Ally for Beginners: Simple Surveying and Volume Calculation with LRTK

Finally, as a powerful tool to support preliminary quantity estimation, we introduce [LRTK](https://www.lrtk.lefixea.com). LRTK is a simple surveying system using smartphones or tablets designed to let beginners easily measure site terrain and calculate earthwork volumes. By attaching a dedicated device to a smartphone and walking the site, you can capture surrounding terrain as 3D point cloud data. The app can immediately perform volume calculations from the data, allowing you to check fill and surplus soil volumes on the spot.

What used to require surveying expertise and effort can now be completed in real time on site with LRTK. This makes it possible to obtain accurate quantities quickly even in the early design stage. Instead of relying on intuition because “it’s just a rough estimate,” you can use technology to visualize the numbers. LRTK is especially helpful for beginners. If you feel uncertain about calculating preliminary quantities, consider trying smartphone surveying with LRTK. You should be able to grasp soil volumes quickly and accurately, greatly improving the accuracy of planning and schedule management.