Differences

• SSDI is a payment for people unable to work because of disability or blindness. Eligibility is dependent on the person's past work history. Payment is usually a fixed amount nationwide. At the age of 65, SSDI ceases and is replaced by Social Security retirement benefits.
  
  SSI is a payment for people on low incomes who are blind, disabled or over age 62. Eligibility is not dependent on the person's past work history. Payment amounts are dependent on income. Some states pay an additional "top-up" amount from state funds.
  
  

Application

• A person can apply for both, either or neither benefits at a particular time. Acceptance may not be immediate, as confirmation that the person meets the definition of disability may take some time; in some cases, the application may be initially rejected and then accepted after an appeal.
  
  This means that even if a person applies for both benefits at the same time, it is possible to be accepted for one while still waiting for a decision on the other. For example, a person aged 63 who is disabled may quickly be accepted for SSI as the age qualification is easy to verify, but may face a wait for SSDI. During this time, the person can receive the SSI payments regardless of the status of the SSDI application.
  
  

Payment Schedule

• SSI is payable immediately from the point the person became eligible -- for example, the onset of disability. Once the person is accepted, the back pay for the period between becoming eligible and being accepted will usually be paid in installments.
  
  SSDI has a five-month waiting period from the point at which the person became disabled or blind. Some or all of this period may have passed by the time the person is accepted and will thus be deducted from any back pay. Back pay for SSDI is usually paid as a lump sum once the person is accepted.
  
  These two systems mean that if a person applies for both benefits and is accepted quickly, it is possible to be receiving SSI while still going through the SSDI waiting period.
  
  

Interaction

• Receiving SSI usually has no affect on a person's SSDI eligibility or payment amount. Unlike SSI, SSDI eligibility is not affected by income unless this comes from paid employment.