The bucket is rated in the wrong units for the job
A bucket is rated by what it holds, heaped, which is loose material. An excavation is measured by the hole, which is bank material. Between the two sits the swell factor, and it is working against you the entire time.
Take the defaults. A 0.75 cubic yard bucket at a 90 percent fill factor picks up 0.675 loose cubic yards. Divide by 1.25 for 25 percent swell and that cycle removed 0.54 cubic yards from the ground — 72 percent of the number stamped on the bucket. Nothing has gone wrong; the two figures are simply in different units, and anyone estimating from the rating alone is 28 percent optimistic before the machine has started.
Where the hours come from
| Step | Default figures |
|---|---|
| Loose in the bucket | 0.75 x 90 percent = 0.675 cu yd |
| Bank per cycle | 0.675 / 1.25 = 0.54 cu yd |
| Cycles per hour | 3,600 / 22 s x 50/60 = 136 |
| Digging rate | 0.54 x 136 = 73.6 bank cu yd per hour |
| 133.3 cu yd of hole | 1.8 hours at the bucket |
Two of those five lines are guesses on most jobs. The cycle time is the one worth measuring, because it moves the answer proportionally and because a stopwatch settles it in four minutes. The working minutes per hour is the one worth being honest about: 50 out of 60 is a machine that is digging steadily with somewhere to put the material, not a machine that is repositioning, trimming to grade and working around a gas line.
Then the trucks take over
The digging rate is a ceiling, not a schedule. At the defaults the machine fills a 12 yard truck in 18 cycles, which is 6.6 minutes under the boom, and the truck is then away for 25 minutes. That is a 31.6 minute turn, so it takes 5 trucks to have one under the boom continuously. With the 3 trucks in the form the machine waits 3.9 minutes between loads and the effective rate drops from 73.6 bank yards an hour to 54.7.
That is the calculation that decides whether hiring a fourth truck saves money or wastes it, and it turns entirely on the round trip time. Double the haul distance and the same machine needs nearly twice the trucks to stay busy. The reverse is worth noting too: if the material is staying on site, there are no trucks in the sum at all and the bucket really is the ceiling.
Why the days round up so hard
The default dig is 2.4 hours of governed work. It is also one full day of machine rental at 450 dollars plus 250 dollars of delivery, because nobody rents an excavator for two and a half hours. That is 700 dollars for 133 bank yards, 5.25 dollars a yard, and every additional hour up to eight is free in the sense that the day is already paid for.
Which is the real argument for lining up the whole day of work before the machine arrives rather than after. The second and third small jobs on the same rental cost the delivery fee nothing and the day rate nothing. This is also why the page reports hours and days separately: the hours tell you how the work goes, the days tell you what the invoice says.
Questions people ask
How many cubic yards can an excavator move in a day?
At the defaults on this page, 73.6 bank cubic yards an hour digging flat out, which is 589 in an eight hour day — if there is somewhere to put every yard of it as fast as the bucket produces it. With three trucks on a 25 minute round trip the governed rate falls to 54.7 an hour and 438 in the day. Both figures rest on a 22 second cycle and 50 working minutes in the hour, and both collapse if the material is sticky, the swing is long or the machine keeps repositioning.
What is a bucket fill factor and what number should I use?
It is the share of the rated heaped capacity the bucket actually picks up in a given material, and it varies from well above 100 percent in loose free-running sand to well under 80 in wet sticky clay that will not release, or in blasted rock that bridges across the bucket. The operator running the machine will have a working figure for the material in front of them. It is not a machine specification and it is not on the data plate, which is why it is an input here rather than a constant.
How many trucks do I need to keep the excavator working?
Divide the total truck turn by the loading time. At the defaults the loading takes 6.6 minutes and the truck is away 25, so the turn is 31.6 minutes and 31.6 divided by 6.6 rounds up to 5 trucks. Fewer than that and the machine waits; more and the trucks queue, which costs money in a different pocket. The sensitivity is all in the round trip, so the same dig with a dump site five minutes away needs two trucks and the same dig an hour away needs eleven.
Why does the calculator charge a whole day for two hours of work?
Because that is how machines are rented. Day rates, week rates and a delivery charge that lands whether the machine works two hours or ten are the normal shape of the bill, so the page rounds the hours up to whole days at whatever hours-per-day you count and prices it that way. It is also the reason the cost per yard on a small job looks alarming: 700 dollars over 133 yards is 5.25 a yard, and half of that is the truck that brought the machine.
Does this cover trenching or loading rather than mass excavation?
The arithmetic is the same for anything that moves material one bucket at a time — the difference is entirely in the cycle time you measure. Trenching to a tight line runs slower per cycle than mass digging because of the trimming and the accuracy. Loading a truck from a stockpile runs faster because the swing is short and the material is already loose, in which case set swell to zero, since the pile is already in loose measure and applying swell twice would understate the rate.