For example, a 20-year-old tree that has a diameter at breast height (dbh) of 10.0 inches has an MAI of 0.5 inches/year. MAI is calculated as M A I = Y ( t ) / t {\displaystyle MAI=Y(t)/t} where Y(t) = yield at time t. Because the typical growth patterns of most trees is sigmoidal, the MAI starts out small, increases to a maximum value as the tree matures, then declines slowly over the remainder of the tree's life.