Reference code 9021.
Reference Code for all math error conditions
Math error corrected
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What it means.
The IRS manual's wording:
Reference Code for all math error conditions
What to do.
The IRS changed the return for a math error. A notice (often CP11 or CP12) explains the new amount.
Questions people ask
What does reference code 9021 mean?
The IRS changed an amount on the return. The IRS manual's own wording is quoted on this page.
What should I do about code 9021?
The IRS changed the return for a math error. A notice (often CP11 or CP12) explains the new amount.
Sources
Checked against these sources on October 30, 2026.