Three numbers pick the machine, and only one of them is about digging. The deepest cut you have to make sets the floor, the narrowest gap the machine has to pass sets the ceiling, and what your trailer can legally carry often overrules both. A one ton mini reaches around 6 ft down, a three ton near 10 ft, and an eight ton past 14 ft.
Work back from the deepest cut
Take the deepest thing on the job, a footing, a septic line, a pool shell, and add a foot for the spoil you stand on and the slope you cut back. That figure sets the smallest machine that can finish without you standing in a hole hand digging the last foot, which is where a day turns into three.
The narrowest gap sets the ceiling
Measure the gate, the side passage, the soft landscaping and any overhead wire or eaves on the route in. A machine that cannot reach the work is not a smaller machine, it is no machine. Retracting undercarriages let some minis narrow to squeeze through and widen again to dig, and that feature is worth asking for by name.
The trailer usually overrules the digging
Collecting the machine yourself means the combined weight of excavator, attachments and trailer has to sit inside your vehicle's rating and your licence. That arithmetic caps most private hires at the smallest classes. Taking delivery removes the cap entirely, and the transport charge is often less than a day of fighting an undersized machine.
A bigger machine is usually fewer days
The class above costs more per day and moves considerably more per hour, so on anything volume driven it finishes sooner and often cheaper overall. The exception is finish work, where a large bucket on a heavy machine damages what you are trying to protect and a small unit with an operator who can feather it wins.
Questions people ask about what size excavator do i need
What size excavator digs a footing for a house extension?
A three to five ton machine handles most residential footings comfortably, reaching around ten to twelve feet and lifting the trench boxes and materials that go with the work. Drop to a one ton only if access forces it, and expect the job to take noticeably longer.
Will a mini excavator remove a tree stump?
A one ton machine will worry a small stump out over an afternoon. Anything with a real root plate wants three tons or more, and often a thumb style grapple to pull rather than only dig. Tell the yard the trunk diameter and the species, since the answer changes with both.
Does a deeper trench need shoring as well as a bigger machine?
Beyond five feet a trench needs a protective system unless it is cut entirely in stable rock, and that is a federal requirement rather than a preference. Trench boxes and shoring are hired from the same counter as the excavator, so order both in one call.