minimum charge, charge both by the hour and by the inch and even by the square inch.
Minimum charge is for just loading up and traveling to the site. You have to get at least XXX
Charge by the hour for jobs with multiple small stumps that would be ground out in seconds each.
Charge by the inch for stumps up to a certain diameter
Charge by the square inch for large stumps. For instance, a 12" diameter stump ground out for $3/inch comes to $36 but a 48" stump at that rate would be $144. However, comparing the area you grind, the 12" stump is 113 sq. inches while the 48" stump is 1808 sq.inches, or about 16 times the area for 4 times the price. CRAZY
For stumps over 20", I charge by the square inch. $3/inch up to 20 inches, after 20" I charge 20 cents per square inch.
The math shows diameter and area cross at 20" at those rates. You could also charge $4/inch diameter and then 25 cents per square inch based on 20 inches and still have the same cross.
based on diameter, 20" x $3/in = $60
based on by the sq. inch, (pi x r squared based on 20") 3.14 x (10x10)=314 x .20 cents = $62.80
so by that math, a 12" stump costs $36 by diameter while a 48" stump costs $361.73