The bucket capacity required for a job depends on the hourly production requirement, the density of the material, how full the bucket can be loaded for a particular type of material, called the "fill factor," and the cycle time. The hourly production requirement is set by project management. The number of tons of material per square yard is taken from standard reference tables. Cycle time is the time between when a bucket is filled, and when it is filled again. The type of material is crucial, since that determines the weight per cubic yard and the fill factor varies by bucket manufacturer.
Read the excavating project specifications to determine the nature of the material to be moved. Determine the material weight from a standard reference table, such as the "Weights Of Building Materials, Agricultural Commodities, and Floor Loads For Buildings" produced by Penn State University or "The Building Estimator's Reference Book." If the material to be moved is crushed stone, the Building Estimator's Reference Book says crushed stone weighs 2,500 lbs. (1.25 tons) per cu. yd. If the material is crushed stone, look up "crushed stone" on the Fill Factor Data Sheet provided with the bucket. The fill factor will specify, as a range, how fully the bucket should be loaded with that material. If the bucket should be loaded not more than 85 percent full with crushed stone, that's the Fill Factor for that material in that bucket.
Time a loading operation with a stopwatch. Start timing when the excavator bucket begins to "dig in" and stop timing when the bucket again begins to dig in. Divide 60 by the cycle time to learn the number of cycles per hour. If the cycle time is .5 minute, 60 / 0.5 = 120 cycles per hour.
Divide the hourly production requirement by the cycles per hour with your calculator. If management requires 800 tons to be moved per hour, and the machine and operator average 120 cycles per hour, then 800 / 120 = 6.67 tons moved per cycle.
Divide the per cycle payload (6.67 tons) by the material density (1.25 tons/cu. yd.) to determine the nominal bucket capacity: 6.67 / 1.25 = 5.28 cu yds.
Divide the nominal bucket capacity by the fill factor to find the required bucket capacity: 5.28 / 0.85 = 6.21 cu. yds.