How to Size an Excavator for Your Job: A Seven-Step Selection Process

Sizing an excavator properly is a sequence, not a judgement call. Working through a fixed process takes about half an hour and removes the two outcomes buyers regret most: a machine that cannot finish the recurring job, and a machine that is expensive to move and rarely used to its capacity. This guide sets out a seven-step process with the specific measurement to take at each step.

Work through the steps in order. Each one either narrows the field or eliminates options, and skipping ahead usually means revisiting an earlier decision later.

Step 1: Define the Recurring Job, Not the Worst Case

Write down the three tasks the machine will do most often. Be specific about depth, width and material. "Digging footings" is not usable; "digging 600 mm wide footings to 1.8 m in clay" is.

Deliberately exclude the one-off extreme job. Sizing a machine for the largest job you can imagine means paying for capability you use once. If that job genuinely recurs several times a year, it belongs on the list; if not, hire a larger machine when it comes up.

Step 2: Establish the Three Hard Numbers

From your job list, extract three figures and write them in metres.

• Required dig depth — the deepest point that must be reached, measured from ground level.
• Required reach — the furthest horizontal distance you must work from a single machine position.
• Narrowest access — the tightest gate, doorway or corridor the machine must pass, measured at its narrowest point and at the height the machine will be at.

Measure these on site with a tape rather than estimating. Access in particular is often narrower than people remember, and it is the constraint that most often forces a change of plan.

Step 3: Apply the Access Filter First

Access eliminates options before performance does. Compare your narrowest access figure against a machine's transport width, and subtract at least 100 mm of total clearance. If the access point is 1.0 m, you need a machine whose retracted transport width is around 0.9 m or less.

Remember that the machine must also fit while being driven through, not just measured statically. Check corners, changes of level, and the width at the machine's full height.

If access eliminates the class you wanted, look for narrower machines within a smaller class rather than assuming no machine will work. Several compact models are built specifically for sub-metre access.

Step 4: Add Margin to Depth and Reach

Once access sets the boundary, use depth and reach to pick within it. Add roughly 20 percent to your required dig depth and look for a machine rated at or above that figure. Working continuously at maximum depth is slow and hard on the linkage; a machine with margin finishes faster and handles the occasional deeper section.

Apply the same reasoning to reach. A machine working at its maximum reach is at its least stable and its slowest. Margin here translates directly into cycle time.

Step 5: Check Transport Honestly

Add the machine's operating weight to the trailer's unladen weight and compare the total against your tow vehicle's rated towing capacity and the trailer's rated payload. Both limits must be satisfied, and the trailer's payload is the one most often overlooked.

Then check the trailer deck length against the machine's transport length, deciding in advance whether the bucket travels on the deck or overhangs. Finally, confirm your driving licence category covers the combination you intend to tow.

If transport rules out the class you chose, either move down a class or budget for a delivery service. Do not proceed assuming it will work out.

Step 6: Check Ground Conditions and Surface Protection

Identify the surfaces the machine will cross and work on. Lawns, paving, driveways and suspended floors all require attention to ground bearing pressure, which is weight distributed over the track contact area.

Rubber tracks are the default choice for finished surfaces. Steel tracks suit rock, demolition debris and rough ground. If the work involves both, consider rubber tracks with track pads, or plan to protect surfaces with mats.

For indoor work on suspended floors, check the floor's rated loading against the machine's operating weight and consult a structural engineer where there is any doubt.

Step 7: Confirm Attachments and Hydraulics

List the attachments you intend to run now and within the next two years. Each has requirements the base machine must satisfy.

• Buckets: check the pin dimensions or coupler type and the width range the machine can handle without instability.
• Hydraulic thumbs and grapples: require an auxiliary hydraulic circuit — confirm the machine has one and that the controls suit it.
• Breakers and augers: require specific flow and pressure. Check the machine's auxiliary flow rating against the attachment's requirement, not just its presence.
• Tilt buckets and rotators: usually need a second auxiliary circuit.

Specifying an auxiliary circuit after purchase is far more expensive than ordering it with the machine. If there is any chance you will run hydraulic attachments, order the circuit at the outset.

Recording the Decision

Put the outcome into a single short specification you can send to dealers. It should read like this: operating weight class, minimum dig depth, minimum reach, maximum transport width, track type, cab or canopy, and required hydraulic circuits. With that list, comparing machines takes minutes rather than hours, and there is no risk of being steered toward a machine that does not fit the actual requirements.

Common Mistakes in the Sizing Process

• Sizing for the rarest, largest job instead of the recurring ones.
• Checking depth but not reach, then discovering the machine must be repositioned constantly.
• Measuring the gate but not the route to it — narrow gates at the end of a wide path are rarely the real constraint.
• Using shipping weight instead of operating weight for transport calculations.
• Assuming an auxiliary circuit can be added later cheaply.
• Ignoring bucket overhang when checking trailer length.
• Choosing on price first and then trying to justify the choice against the job.

When to Get a Second Opinion

If your requirements sit on a class boundary — for example, needing 2.4 m of depth where the two-tonne class tops out — talk to two dealers and ask specifically what happens at that limit. Experienced dealers will tell you honestly whether a machine will cope or struggle, and that answer is worth more than any specification sheet.

Once the process produces a short list, arrange to see the machines in person. Ten minutes standing next to a machine tells you things about size, access and visibility that no table can convey.

Start comparing against your finished specification in the mini excavator collection, and look at the SDLE25 mini excavator and SDLE35 compact excavator as two adjacent class options.