
It does depend a lot of course as to what tree it is as some are fast growing and some very slow, but if you estimate a tree growth ring is about 1/8 inch thick, so twice that for the diameter (1/4 in).
Our 100 year old example tree would then be 100 x 0.25 = 25 inches in diameter.
The formula for circumference in pi * d, so 3.14 x 25 = 78.5 ins, so anything with a circumference of about 78.5 inches or 2m would be the answer.
You would need to measure the trunk at about waist height I would have thought ... sounds like fun
Mind you, I could well be wrong
