Tolerance Zone

The tolerance zone is defined by state law and represents the area around a marked utility where the risk of contact during excavation is highest. Within this zone, mechanical excavation with backhoes, excavators, and other powered equipment is prohibited. Only non-destructive methods such as hydrovac excavation, air excavation, or hand digging are permitted.

Tolerance zone widths vary by state. Some states define the zone as 18 inches from the marked centerline, others as 24 inches from the outer edge of the utility, and some use different measurements for different utility types. It is the excavator's responsibility to know the tolerance zone requirements in the state where they are working and to comply with them.

The tolerance zone exists because utility locate marks represent the approximate position of the utility, not its exact location. Locating equipment, ground conditions, and human factors all introduce potential error into the marking process. The tolerance zone provides a safety buffer to account for this uncertainty. Hydrovac excavation within the tolerance zone is the most reliable way to determine the utility's exact position before proceeding with mechanical excavation outside the zone.