If this is the case, a formula can be used to determine the non-marital portion of that account. You can simply multiply the average yearly contribution times the number of years worked prior to the marriage. The problem with doing this is that it really is an imperfect method. The premarital portion can easily be overestimated, especially in long-term marriages, as we all know as people age their incomes usually increase along with their contributions to a 401(k) or an IRA. This average can kind of be skewed and often over-value that premarital portion.