SSDI and SSI
If you have filed for Social Security disability, the agency screens your application for both SSDI (disability insurance) and SSI. If you don't qualify for SSDI because you don't have enough work credits paid in, you may qualify for SSI, although SSI is means-tested and Social Security limits your income as well as your resources (savings, property, valuables, etc.).
If you win SSDI or SSI, you are eligible for back benefits, which are disability payments retroactive to the onset of your disability, whenever the agency determines that to be. Nearly everyone who wins disability is owed back benefits, as the application process takes several months, at least, and the vast majority of applicants are adjudged disabled before their application dates.
Limits on Back Benefits
For SSDI, back benefits are payable to the onset date, up to twelve months before your application was filed. For SSI, back benefits are payable to the onset date, up to the date of your application.
SSDI Waiting Period
Social Security imposes a five-month waiting period after the onset of your disability before benefits are payable, in the case of SSDI benefits. Thus, if your disability began a year or more before you applied, the maximum amount of back benefits would be seven months before the application up to and including the month in which you were approved.
For SSI benefits, there is no waiting period. The maximum amount of back benefits would be those paid retroactive to the date of your application. If you applied two years ago and are approved today, and your onset date pre-dates your application (as it almost always does), then you are owed 24 months of SSI benefits as back pay.