How does Social Security Determine your Social Security Disability Benefits?
It can be difficult to determine the amount of benefits you could receive. The steps require that you find your average monthly wage and then primary insurance account.
First, Social Security takes the all your lifetime earning into consideration. Your lifetime earnings are made up of your “wages, compensation, self-employment income, and deemed military wage credits…”.
After deciding your lifetime earnings, Social Security will need to decide what the time range is relevant for your situation. For purposes of Social Security Disability, the time frame begins from the year you became 21 and goes until you became unable to work. Any parts of the year, or years, are not counted, unless doing so would raise your average monthly wage. In that situation, all years within the time frame of disability would be considered. This number would be considered your elapsed years. The next step would be to subtract five from the elapsed years total. That number is now your benefit computation years. The years with the highest earnings are then used which could include no earnings, since you cannot have fewer than two benefit years.
Finally, each step is put together to find the average monthly wage. By taking the creditable earnings from the benefit computation years, using the years with the highest earnings, adding these earnings together, dividing by the total months in the benefit computation years, and rounding to the lower whole dollar amount.
This number is your average monthly wage that Social Security would use to determine your primary insurance account. After that you use the chart posted by Social Security (linked here) to find your primary insurance amount. You would use the average monthly wage column that you just calculated to find your primary insurance amount. When you find your average monthly wage, you look to the left and that number is your primary insurance amount. The primary insurance amount is your base benefit amount, however, can change for a few reasons, such as current earnings.
This can also be done using the Social Security calculator posted below.
 C.F.R. § 404.221(b)
 C.F.R. § 404.221(c)(1)
 C.F.R. § 404.221(3)
 C.F.R. § 404.317
The citations above are all from https://www.ssa.gov/OP_Home/cfr20/cfrdoc.htm