The base amount. Section 22(c)(2) sets the initial amount at $5,000 for a single filer, or a joint return on which only one spouse qualifies; $7,500 for a joint return on which both spouses qualify; and $3,750 for a married person filing separately. For a qualified individual under 65, the initial amount cannot exceed their disability income for the year, which the statute defines as amounts included in income as wages, or payments instead of wages, for the time the person is absent from work because of permanent and total disability.
Two reductions. Section 22(c)(3) subtracts tax-free retirement and disability benefits paid under Title II of the Social Security Act, the Railroad Retirement Act or a law administered by the Department of Veterans Affairs (with a carve-out for the military-injury benefits described in section 104(a)(4)), plus similar amounts excluded from income under any law outside the Internal Revenue Code. Section 22(d) then subtracts one-half of adjusted gross income above $7,500 for a single filer, $10,000 for a joint return and $5,000 for a married person filing separately. Whatever is left is multiplied by 15 percent. Solving those rules for zero gives the limits the Schedule R instructions print in their Table 1: a single filer, for example, generally cannot take the credit with adjusted gross income of $17,500 or more, or with $5,000 or more of those nontaxable benefits.
Married couples must usually file jointly. Section 22(e)(1) allows the credit to a married person only on a joint return, unless the spouses lived apart at all times during the year. A separate filer who lived with their spouse at any point loses it. The Schedule R instructions also allow a married person who qualifies to file as head of household to claim it, even if the spouse lived in the home during the first six months of the year.
The under-65 route has three conditions. The person must have retired on permanent and total disability, must have taxable disability income for the year, and must not have reached the employer's mandatory retirement age on January 1. Permanent and total disability means being unable to engage in any substantial gainful activity because of a condition that has lasted or is expected to last at least a year or to result in death. A physician completes the statement in the Schedule R instructions, which the taxpayer keeps rather than files, and a Department of Veterans Affairs certification on VA Form 21-0172 can be used instead.
Why the fixed amounts matter. The amounts in section 22 date from the 1980s and have never been indexed, while Social Security benefits and the standard deduction are adjusted for inflation. For a single filer, nontaxable Social Security of $5,000 a year, which is about $417 a month, eliminates the credit by itself. And at the income levels where the credit survives, the standard deduction can leave no income tax at all for a nonrefundable credit to offset. The IRS's own worked example in the Schedule R instructions ends with a credit of $8.