The difference between future Social Security obligations and the income rate of the Social Security Trust Fund as of present. The Social Security program is said to be in actuarial deficit if the summarized income rate is less than the summarized cost rate of Social Security for any given valuation period. This situation is commonly referred to as the Social Security System being "insolvent."

Actuarial balance is calculated for 66 different valuation periods, beginning with the upcoming 10 year period and growing with each successive year up to the full 75 year projection. If at any point over the 75 year projection the anticipated costs of Social Security exceed the future value of the trust fund's income, that period would be deemed to be in actuarial deficit.