How are Social Security disability benefits calculated?

Mathematically speaking, Social Security Disability Insurance (SSDI) is calculated in the same way as Social Security retirement benefits. Both are based on your record of “covered earnings” — work income on which you paid Social Security taxes.

The Social Security Administration (SSA) starts by figuring your average monthly income across your working life, adjusted for historical wage growth. Then, it then plugs that figure into a formula to determine your primary insurance amount (PIA), also known as your full retirement benefit.

The PIA formula is progressive — weighted to provide proportionally higher benefits to lower earners — and it’s the same whether you’re claiming retirement or disability benefits. What differs is how much income data is used to determine your full benefit and when you can collect it.

For retirees, the SSA uses the 35 highest-earning years to calculate the monthly average income and PIA. (Only yearly earnings up to an annually adjusted cap are counted. In 2026, the cap is $184,500.) You become eligible to claim that full amount at full retirement age, which is 66 and 10 months for people born in 1959 and 67 for those born in 1960 or later. Benefits are reduced if you claim earlier, by as much as 30 percent if you start taking them at the minimum age of 62.

Because a worker may become unable to work due to an injury or illness before reaching retirement age, Social Security uses a different time frame to determine the primary insurance amount for SSDI claims. The number of years of income used to calculate the benefit depends on the age at which your disability began.

Exactly how much of your earnings history is included depends on arcane Social Security terms like “elapsed years” and “computation years,” but basically, here’s how it works.

• The SSA tallies the number of years from the year you turned 22 to the year before you became disabled​.
• It throws out between one and five years (the longer you’ve been working, the more “dropout years”).​
• The resulting number is how many of your highest-earning years will go into the PIA calculation.