Table 1 on page 4 of the publication linked below gives number of apples per tree based on apple growth habit and trunk cross sectional area. Follow that and you won't be guessing like you are with most rules of thumb.

Here is a link that might be useful: apple crop load by tree diameter and growth habit

What a useful article! I'm going out first thing in the morning to measure my trees and calculate fruit load!

Interesting... My trees have been dropping fruit the past few days...Cortland and McIntosh and both are 7 years old. I'll do my measurements ...

I guess I'd view those crop load recommendations as valid if you want commercial quality fruit. I'd probably reduce them by 10-20% because I want highest fruit quality. But at least it's a good starting point that should give annual production if the thinning is done early.

So I measured the trees, and they're 1, 3 and 5" in diameter. Which puts me well within the limits of the chart, I'm happy to see.

otoh, I don't want to grow as many apples on these trees as that high-intensity program calls for, so I think I've got it about right. Particularly with the 3" tree that I was concerned about.

I've been spending the last couple weeks thinning fruit with my small crew- mostly them while I try to find something interesting to do. However, today it occured to me that there are variables that trump tree diameter in terms of determining how much fruit to leave.

Larger apples fewer apples, of course- you'd leave more Jonathon on similar sized tree than Jonagold, for instance. But also, some varieties produce rather sparse canopies and these varieties can't produce as many quality apples as other types in ratio to their trunk diameters.

Like most things in horticulture, there is no single infallible recipe that can provide completely accurate instructions for fruit thinning.

I do agree with fruitnut about leaving fewer apples in home orchards, not just because you want better quality fruit but also because home orchards often don't get dawn to dusk sun as most commercial orchards do. Length of exposure- there's still another variable to consider.

Also because how many apples do you need? I take several loads to the food pantry, but in the end it's only a few people here to eat them and limited space to store them. Every year I end up throwing some out.

So why stress the tree to grow more?

People who make money selling apples have other priorities than the home grower.