How many trucks does an excavator need? Balancing trucks to the loader

Watch a loading operation for ten minutes and you can usually tell whether it is balanced. Either the excavator is sitting with a full bucket waiting for the next truck, or trucks are lined up idling while the machine loads the one in front. Both cost money. The question is which one costs more on your job, and what number of trucks gets you closest to neither.

How many trucks does an excavator need?

The basic formula is short:

Trucks needed = truck cycle time divided by load time

The truck cycle time is the whole trip, from the moment a truck starts loading until it is back under the bucket for the next load. That includes load, haul, dump, and return. Load time is how long the machine takes to fill one truck.

The logic is simple. While one truck is gone on its trip, the excavator should be filling the others. If a round trip takes six times as long as loading, you need six trucks to keep the machine busy. The answer is almost never a whole number, which is where the real decision starts.

Figuring load time

Load time comes from three things:

• Bucket size. How much the bucket really carries in one pass, in loose yards, in this material. A heaped bucket of dry sand and a bucket of sticky clay are not the same number.
• Passes per truck. The truck's realistic capacity divided by the real bucket load, rounded to what an operator actually does. On heavy material, weight may limit the truck before the box is full. Our article on dump truck capacity covers that.
• Swing cycle. How long one dig, swing, dump, and return takes. It depends on the machine, the operator, the dig depth, and how far the machine swings to reach the truck. A short swing to a truck parked in the right spot is faster than a wide swing to a truck parked wherever the driver felt like.

Passes times swing cycle, plus a little time for the truck to pull in and position, gives you load time. Time it on the job. Spec sheet numbers are best-case numbers.

Count everything in loose yards, not bank yards. Our page on bank vs loose yards explains why the dirt gets bigger when you dig it.

A worked example

These are made-up numbers to show the method. Your machines, operators, and routes will be different.

• The truck holds about 8 bucket loads of this material.
• Each swing cycle takes about 30 seconds.
• Each truck takes about a minute to pull in and position.

Load time is 8 passes at 30 seconds, or 4 minutes, plus a minute to position: 5 minutes per truck.

The trip: 5 minutes loading, 15 minutes hauling, 8 minutes at the dump, and 12 minutes back. The full cycle is 40 minutes.

Trucks needed: 40 divided by 5 is 8 trucks.

Now move the dump 10 minutes farther away in each direction. The cycle becomes 60 minutes, and you need 12 trucks to keep the same machine busy. The excavator did not change at all. The haul distance decided the truck count.

What happens when you're short on trucks?

With fewer trucks than the formula says, the excavator waits. Every truck is fully used, but the loading machine and its operator, often the most expensive hour on the job, sit idle between trucks. Production is limited by the trucks, and the job takes longer than the bid assumed.

Being a truck short is sometimes the right call. If the excavator can do useful work between trucks, like stripping topsoil, sorting material, digging ahead, or grading, the waiting time is not wasted. Plan that work on purpose instead of hoping it happens.

What happens when you have too many?

With more trucks than the formula says, the trucks wait. The excavator runs flat out, but trucks idle in line, and somebody is paying for truck hours that do not move dirt. On hourly trucking, that is you or your customer. On per-load trucking, the truck owners lose money and they notice fast.

A small line is not always bad. One truck waiting means the excavator never stops. The question is how much truck time you are willing to pay for to keep the machine at full speed.

Rounding up or rounding down

When the math gives you 6.4 trucks, you have to pick 6 or 7. A rough rule of thumb:

• Round up when the loading machine and crew cost more per hour than a truck, when the schedule is tight, or when the dump cycle is unpredictable.
• Round down when trucking is the bigger cost, when the excavator has other productive work to fill the gaps, or when the site cannot handle a line of trucks.

Either way, compare what an hour of the whole loading spread costs against what an hour of one truck costs, and let that decide.

Things that throw the math off

• The dump. Lines at the scale, closing hours, and weather shutdowns change the cycle more than anything on your site.
• Traffic. A route that works at 7 a.m. may not work at 3 p.m.
• Site roads. Mud and tight turnarounds add minutes to every cycle.
• Trucks bunching up. Even with the right count, trucks tend to drift into groups, so several arrive at once and then a gap opens. A foreman or dispatcher who staggers them early helps.
• Mixed trucks. Tandems and tri-axles take different numbers of passes. Do the math for each truck type.

Re-check the numbers after the first hour of the job. Real cycle times will tell you quickly whether to call another truck or send one home.

If you run your own trucks, our page for trucking and hauling covers the dispatch side, and scheduling and dispatch shows how TrackOver keeps trucks and crews on one board. You can try it free if that would help.

Once the cycle time is known, turning it into a price for the customer is its own step. We walk through that in how to price trucking and haul-off.

Common questions

What is the formula for how many trucks an excavator needs? Divide the truck's full cycle time, meaning load, haul, dump, and return, by the time it takes the excavator to load one truck. If the cycle is 40 minutes and loading takes 5, you need 8 trucks.

How do you figure excavator load time? Multiply the number of bucket passes it takes to fill a truck by the time for one swing cycle, then add time for the truck to pull in and position. Time it on the job, because spec sheet numbers are best case.

Is it better to have too many trucks or too few? It depends on which hour costs more. Too few trucks leaves the excavator waiting, and too many leaves trucks idling in line. Round up when the loading spread is the expensive part or the schedule is tight, and round down when trucking is the bigger cost or the excavator has other useful work to fill the gaps.

Why does haul distance change the truck count so much? Because the load time stays the same while the cycle gets longer. In the example here, adding 10 minutes each way took the cycle from 40 to 60 minutes and the truck count from 8 to 12.