How to Calculate Excavation Volume
Calculate the bank volume first: length times width times depth divided by 27 gives cubic yards of undisturbed soil. Then multiply by the swell factor to get the loose volume that will actually be loaded into trucks. For a 30x20 foot excavation that is 4 feet deep: 30 x 20 x 4 / 27 = 88.9 bank cubic yards. With a 1.25 swell factor, the loose volume is 111.1 cubic yards. This requires about 9 tandem dump truck loads (12 CY each) and costs $5,000 at $45 per loose cubic yard.
For trenches, calculate the linear volume: length times width times depth. A 100-foot trench that is 2 feet wide and 5 feet deep contains 100 x 2 x 5 / 27 = 37 bank cubic yards, or about 46 loose cubic yards. Sloped excavations (required for safety in deep digs) increase the volume significantly. A 10-foot deep excavation with 1:1 side slopes requires about 50% more volume than a straight-walled pit of the same base dimensions. For backfill after utility installation, use our gravel calculator to estimate the structural fill material.
Excavation Cost Factors
The base cost for excavation is $30-60 per cubic yard for digging and hauling on accessible, flat terrain. Several factors increase this cost significantly. Rocky terrain requires specialized equipment (hydraulic hammers, ripper attachments) and costs $60-120 per cubic yard. High water table requires dewatering pumps and adds $500-2,000 per day to the project. Tight access (narrow side yards, overhead wires, limited staging) limits equipment size and adds 30-50% to the cost. Contaminated soil requires special handling, transport to approved facilities, and can cost $100-300 per cubic yard.
Minimum charges are common: most excavators have a $500-1,500 minimum regardless of project size because mobilizing equipment costs $300-800 per delivery. For small projects (under 20 CY), the per-yard cost is effectively much higher than the quoted rate because the minimum dominates. Plan your project to combine multiple excavation needs (foundation, utility trenches, grading) into one mobilization to minimize per-yard costs. For the concrete slab that goes into the excavated area, our sand calculator helps estimate the base material.
Excavation Safety and Regulations
OSHA requires that all excavations deeper than 5 feet use protective systems: sloping (cutting back the trench wall at an angle), shoring (installing hydraulic or timber supports), or shielding (using a trench box). The slope angle depends on soil type: Type A (stable clay) allows 3/4:1 slope, Type B (silt, sandy loam) requires 1:1, and Type C (sand, gravel) requires 1.5:1. A competent person must inspect the excavation daily before workers enter. All utilities must be marked by calling 811 before any digging begins.
Keep heavy equipment and spoil piles at least 2 feet back from the edge of any excavation. The weight of equipment or piled soil near the edge can cause a cave-in. Provide a ladder, ramp, or other means of egress within 25 feet of travel for any worker in a trench deeper than 4 feet. Never enter an unprotected trench. Soil classification should be performed by the competent person using at least one visual and one manual test (pocket penetrometer, thumb penetration, or ribbon test). For the building that goes above the excavation, our wall framing calculator helps plan the framing stage.
Soil Types and Excavation Difficulty
Sand and gravel are the easiest materials to excavate — they dig fast and load easily but require shoring for any trench work because they collapse readily. Loam and topsoil are moderately easy and are the most common materials in residential excavation. Clay is difficult because it sticks to buckets and truck beds, requiring frequent cleaning. It also swells significantly when dug (up to 40%), increasing hauling costs. Solid rock is the most expensive, requiring hydraulic hammers ($150-300/hour) or blasting ($3-8 per cubic yard for the explosive work alone).
Before excavation, order a geotechnical soil report ($1,500-3,000) for any structural project. The report identifies soil types, water table depth, bearing capacity, and potential problem conditions like expansive clay or underground springs. This information is essential for foundation design and prevents costly surprises during excavation. If you hit unexpected rock or water during the dig, stop work and consult the engineer. Continuing to excavate in unknown conditions can destabilize adjacent structures and create dangerous situations. For fill material after excavation, our gravel driveway calculator helps estimate the structural base.
Common Excavation Mistakes
Not calling 811 before digging is the most dangerous mistake. Hitting a gas line can cause an explosion; hitting an electrical line can be fatal; hitting a water main causes flooding and expensive repairs. The call is free and required by law in most states. Allow at least 48 business hours for utilities to be marked. The second mistake is excavating too close to existing structures without underpinning. Digging alongside an existing foundation without proper shoring can cause the adjacent building to settle and crack.
Underestimating the swell factor leads to insufficient hauling capacity. If you order 8 trucks based on bank volume but the loose volume requires 10 trucks, the last loads sit on site while you arrange additional trucks at premium rates. Not planning for dewatering is another common oversight. If the excavation is below the water table, you need pumps running continuously, which adds $500-2,000 per day to the project. Finally, not stockpiling topsoil separately wastes a valuable resource. Topsoil costs $20-40 per cubic yard to buy, but you already have it on site if you strip it and stockpile it before the main excavation begins.