Quick answer
SSDI pays monthly benefits to people who have a disability that stops or limits their ability to work and who have enough work history.[1] The Social Security Act defines disability as the inability to engage in substantial gainful activity because of an impairment that has lasted or is expected to last at least 12 months or result in death.[2] Benefits usually begin after a five-month waiting period.[3]
The legal definition of disability
Social Security Act § 223(d)(1)(A)
inability to engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment which can be expected to result in death or which has lasted or can be expected to last for a continuous period of not less than 12 months
Source: [2]
The Act adds that the impairment must be so severe that the person cannot do their previous work and cannot, considering age, education and work experience, do any other kind of substantial work that exists in the national economy.[2]
Who is eligible
SSA states that SSDI eligibility requires a disability or blindness and “enough work history,” and that the monthly payment is based on that work history.[1] SSA also notes that benefits may include Medicare.[1]
Substantial gainful activity
SSA uses a monthly earnings level to decide whether someone is engaging in substantial gainful activity (SGA). For 2026, that amount is $1,690 per month for people who are not blind and $2,830 per month for people who are blind.[4]
The five-step evaluation
SSA uses a five-step sequential process for adult disability claims under both SSDI and SSI.[5][6]
- Is the person working above the SGA level? If yes, they are not disabled.[6]
- Does the person have a severe impairment that meets the duration requirement?[6]
- Does the impairment meet or equal a listing in SSA’s Listing of Impairments? If yes, the person is disabled.[6] The listing for TBI is 11.18. See Social Security’s TBI listing.
- Considering the person’s residual functional capacity, can they still do past relevant work?[6] SSA says past relevant work typically includes work done in the 5 years before the decision.[7]
- Considering residual functional capacity, age, education and work experience, can the person adjust to other work? If not, they are disabled.[6]
How head injuries fit
Many people with a brain injury do not meet the TBI listing but may still be found disabled at steps 4 and 5 based on their residual functional capacity, which SSA assesses when a condition affects the ability to work on a regular basis but is not as severe as a listed impairment.[7] For brain injuries, SSA generally needs evidence from at least 3 months after the injury to evaluate the claim and may defer the decision.[8] Evidence of cognitive limitations, such as neuropsychological testing, can be relevant.
Who decides
Initial medical decisions are made by a state Disability Determination Services (DDS) agency. SSA explains that “an adjudicative team consisting of a medical or psychological consultant and a disability examiner usually makes the determination.”[3]
Waiting periods and Medicare
SSA states that disability benefits for workers “usually cannot begin for 5 months after the established onset of the disability,” so benefits begin in the sixth full month.[3] The Social Security Act describes this as “five consecutive calendar months.”[2] Medicare generally covers people who have been receiving disability benefits for at least 24 months.[3]
Frequently asked questions
What is the difference between SSDI and SSI?
SSDI depends on work history.[1] SSI is for people with little or no income and resources.[9] The definition of disability for adults is the same for both.[3]
Does a mild TBI qualify?
There is no category of injury that automatically qualifies. SSA evaluates whether the impairment prevents substantial gainful activity for at least 12 months.[2]
How much can I earn and still qualify in 2026?
SSA’s 2026 SGA amount is $1,690 per month for non-blind individuals.[4]
When does Medicare start?
Generally after 24 months of receiving disability benefits.[3]
This page provides general information about the law from the sources listed below. It is not legal advice, and reading it does not create an attorney-client relationship. Laws vary by state and change over time. See our legal disclaimer.
Sources
- Social Security Administration. “Disability Benefits.” https://www.ssa.gov/disability
- Social Security Act § 223, “Disability Insurance Benefit Payments.” https://www.ssa.gov/OP_Home/ssact/title02/0223.htm
- Social Security Administration. “Disability Evaluation Under Social Security, Part I: General Information.” https://www.ssa.gov/disability/professionals/bluebook/general-info.htm
- Social Security Administration. “Substantial Gainful Activity.” https://www.ssa.gov/oact/cola/sga.html
- Social Security Administration, POMS DI 22001.001, “Sequential Evaluation of Title II and Title XVI Adult Disability Claims.” https://secure.ssa.gov/apps10/poms.nsf/lnx/0422001001
- 20 CFR § 416.920, “Evaluation of disability of adults, in general.” https://www.ssa.gov/OP_Home/cfr20/416/416-0920.htm
- Social Security Administration. “How We Decide If You Are Disabled (Step 4 and Step 5).” https://www.ssa.gov/disability/step4and5.htm
- Social Security Administration. “11.00 Neurological Disorders – Adult,” Listing of Impairments. https://www.ssa.gov/disability/professionals/bluebook/11.00-Neurological-Adult.htm
- Social Security Administration. “Supplemental Security Income (SSI).” https://www.ssa.gov/ssi
Sources last checked: October 11, 2026
