You are able to view the Social Security lump-sum calculation amongst the paperwork with your income tax return.
You should find an Earlier Year Lump-Sum Social Security Worksheet for the prior year(s) and a Lump-Sum Social Security Worksheet for the calculation.
The taxable amount 2024 Social Security benefit is based upon your 2024 Federal income tax return information. Perhaps, the characteristics of your 2024 Federal income tax return computed that 85% of the 2024 lump-sum amount should be taxable.
And, perhaps, the characteristics of your 2025 Federal income tax return computed that 85% of the 2025 Social Security benefit should be taxable.
Then, the sum of the two amounts were reported on line 6a of the 1040 income tax return.
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