If you search the site for pier constructions the calculations have been posted many times, you have to start at the roof inc snow loads, upper floors and or lofts, main floors etc etc and then look at the total load of the house divided by the amount of piers.......

The area the piers each sit on.....and then look at the soil properties as well, as every site is diferent.

Hence engineers are commonly used.

Ball park calculations and worst case scenarios would be fine for a shed....but if your building a house then the foundation is worth investing in !

Many, me included looked at piers first and then moved on to full perimeter foundations ...or in my case a huge monolithic bath tub as my soils and location were bad.