Disability claims taken either under SSI or SSDI use the same criteria for medical evaluation. Read more on that here. However, what you receive under each program depends solely on how much Social Security Taxes you paid under your wages. The Social Security Administration, SSA uses two basic formulas to calculate your SSDI benefits.
Average Indexed Monthly Earnings
Indexing brings nominal earnings up to near current levels. For example, if you had earned an average of $10,000 in the year 1979 while he was born in 1953 then the wage index factor will determine the wage he could be earning today equalling the same wage he earned in 1979, in this case, it would be around $40,000. The total indexed wage earnings are then divided by the total indexed factors to calculate an estimated AIME. You can calculate or have a rough estimate of your AIME through this chart here.
The SSA will use up to 35 of your earning years in the calculation. The SSA will take the years with the highest indexed earnings, add them up together and divide them by the total months in the indexed years.
Primary Insurance Amount
The primary insurance amount, PIA is the base amount of your benefits calculated through your lifetime wages. The SSA will use a total of the three fixed percentages of your AIME to calculate your PIA. The SSA changes the bend points (dollar amounts resulting from averaging the three AIME) each year to reflect an average national indexed wages. You can see a table of bend points here.
Your income statement
The easiest way to estimate your benefits is to check your income statement if you have been paying social security taxes. You can see the amount of earnable disability benefits estimated on your income statement bill. Or you can go to the government website here to check your statement online to determine how much benefits you will earn if you became disabled this year. Your income statement