How SSDI benefits are computed
Social Security Disability Insurance pays benefits to workers who have paid into the system through FICA taxes and who can no longer perform substantial gainful activity due to a medically determinable impairment expected to last at least 12 months or result in death. The benefit amount is calculated using the same PIA formula as retirement benefits, but with two important differences.
The 5-month waiting period
SSDI does not pay benefits for the first five full months after the established onset date of disability. If your disability began January 1, your first payable month would be July. This waiting period exists in statute (42 U.S.C. § 423(c)(2)) and applies even if your application takes two years to be approved — the SSA pays retroactively to the sixth month, but never to months one through five.
SGA and trial work period
To remain eligible, your monthly earnings must stay below the SGA limit. For 2025, that limit is $1,620 per month for non-blind workers and $2,700 for statutorily blind workers. SSDI also offers a Trial Work Period of 9 months during which you can earn any amount without losing benefits — useful for testing whether you can return to work.
Medicare after 24 months
SSDI recipients become eligible for Medicare after receiving benefits for 24 months. The clock starts from the first month of SSDI entitlement (the sixth month after onset), so Medicare typically begins 30 months after the disability began. There is no way to accelerate this, even for serious conditions, except for ALS (Lou Gehrig's disease) and end-stage renal disease.
For the differences between SSDI and SSI — including the medical-vocational allowance, work credits, and family maximums — see our SSDI vs SSI comparison.