What is the Charitable contribution base floor under OBBBA?

The charitable contribution base floor is a new 0.5 percent threshold added to IRC Section 170(b)(1)(I) by OBBBA Section 70425. For taxable years beginning after December 31, 2025, itemized charitable contributions are allowed only to the extent aggregate contributions exceed 0.5 percent of the taxpayer's contribution base for the taxable year. Contribution base generally refers to adjusted gross income computed without regard to any net operating loss carryback. The floor applies in addition to the existing Section 170 percentage limitations and ordering rules, and amounts disallowed by the floor may increase charitable carry-forward amounts under the applicable carryover rules. Existing IRC Section 170 substantiation requirements continue to apply. Taxpayers coordinating charitable giving with capital gain or portfolio planning can use Tax loss harvesting planning to model both decisions together before year-end.