Dig Depth vs Reach vs Dump Height: Choosing the Right Dig Envelope
Excavator specification sheets publish a dozen geometry figures, and buyers usually read exactly one of them: maximum dig depth. That is understandable, because depth determines whether a job is possible at all. But the two figures that determine whether the job is efficient — and whether the machine works comfortably inside its stability envelope — are reach and dump height.
Together, depth, reach and dump height describe the machine's dig envelope: the three-dimensional space in which the bucket can work. Choosing a machine on the shape of that envelope rather than on one number is what separates a machine that suits your work from one that merely copes with it.
The Three Dimensions of the Dig Envelope
| Dimension | What it measures | What it determines | When it is the deciding figure |
|---|---|---|---|
| Maximum dig depth | How far below the machine's ground line the bucket reaches | Whether the job is possible | Trenches, foundations, drainage |
| Maximum reach at ground level | How far horizontally from the machine the bucket lands | How often you reposition | Long runs, wide areas, working back from an edge |
| Maximum dump height | How high the bucket can discharge material | Whether you can load out | Loading skips, trucks, stockpiling over a bund |
Also worth checking: maximum digging height, minimum front swing radius, and vertical wall digging depth — the last one matters if you need a clean vertical face rather than a sloped one.
How the Envelope Changes With Size
The three dimensions do not grow at the same rate. Reach and dump height grow faster than depth, which is why larger machines feel disproportionately more productive.
| Class | Max depth | Reach at ground | Dump height | Practical consequence |
|---|---|---|---|---|
| 0.8–1 t | 1.6–2.1 m | 2.8–3.4 m | 1.8–2.4 m | Loads low skips only; frequent repositioning |
| 1.5–2 t | 2.1–2.6 m | 3.5–4.2 m | 2.3–2.9 m | Standard skips; moderate repositioning |
| 2.5–3.5 t | 2.5–3.2 m | 4.2–5.3 m | 2.9–3.7 m | High-sided skips and small trucks |
| 5–8 t | 3.5–4.5 m | 6–8 m | 4–5.5 m | Loads trucks directly; works a wide face |
A contractor loading skips will feel the dump height difference far more than the depth difference. One loading trench boxes will feel depth. One cutting a long drainage run will feel reach. The right machine depends on which of the three your work is actually constrained by.
Reach: The Figure Most Often Ignored
Reach determines how much work the machine does from one position. A machine with 500 mm more reach completes a long trench with noticeably fewer repositionings — and repositioning, not digging, is where much of the time goes on linear work.
There is an important caveat. Reach is quoted as a maximum, usually at ground level with the arm fully extended. At maximum extension the machine is at the edge of its stability envelope and its lift capacity is at its lowest. Working routinely at maximum reach is slow, unstable and hard on the machine.
The practical approach: size reach so that your routine work is done at roughly 70–80% of maximum, keeping reserve capacity for the occasional long placement.
Dump Height: The Loading Constraint
Dump height decides whether the machine can discharge into whatever is receiving the material. Check it against:
- Skip height — standard builders' skips are around 1.2 m; high-sided and enclosed skips are considerably taller.
- Truck body height — a tipper body is much higher than a skip, and usually rules out the compact classes entirely.
- Stockpile height — discharging onto a growing pile needs clearance above the pile, not above the ground.
- Reach at that height — the machine must still be far enough away to work safely beside the receiving container.
Dump height is usually quoted at maximum, but what matters is dump height at the horizontal distance you need. Ask for both, or for a working-range diagram.
Reading a Working Range Diagram
Most manufacturers publish a side-on diagram showing the envelope as a curve. Learn to read three things from it:
- The depth figure at the bottom of the curve.
- The reach figure at the far side.
- The height figure at the top, and how far out it occurs.
Then look at where the curve narrows. A machine whose envelope is wide and shallow suits bulk excavation and loading; one that is narrow and deep suits trenching. Two machines with identical maximum depth can have quite different envelopes.
Matching Envelope to Job
| Work type | Priority dimension | Second | What to avoid |
|---|---|---|---|
| Service trenches | Depth | Reach along the run | Machines with depth only at minimum radius |
| Foundation dig | Depth | Reach for spoil placement | Insufficient reach to place spoil clear of the edge |
| Skip loading | Dump height | Reach at height | Marginal dump height needing a ramp |
| Bulk excavation | Reach | Bucket capacity | Short reach causing constant repositioning |
| Working near structures | Minimum swing radius | Tail swing | Conventional tail swing in confined space |
| Clean vertical faces | Vertical wall digging depth | Depth | Machines that can only dig a sloped face |
Stability Inside the Envelope
The envelope describes what the machine can reach, not what it can safely do at every point. Keep in mind:
- Lift capacity falls as radius increases, and it is lowest over the side.
- A heavy load at maximum reach is the classic tipping condition.
- Tracks retracted for access reduce stability precisely when digging at depth.
- The dozer blade, lowered, materially widens the effective support base.
- Uneven ground moves the envelope in ways the diagram does not show.
A Short Specification Routine
- Identify which of the three dimensions governs your most common job.
- Add margin — around 200–300 mm on depth, and 20–30% reserve on reach and dump height.
- Check the working range diagram, not just the headline figures.
- Confirm the arm configuration the quoted machine actually has.
- Confirm the bucket that ships with it.
- Verify stability arrangements: track gauge range, blade, counterweight options.
Depth tells you whether the machine can do the job. Reach tells you how quickly. Dump height tells you whether you can get rid of what you dig. Compare all three and the choice usually becomes obvious.
The mini excavator collection publishes dig depth, reach and dump height for every machine, so whole envelopes can be compared — including between the SDLE25 and the SDLE42.
