Quick overview

When the IRS flags a math error or mismatch it will send a notice describing the proposed change (examples include CP11 for simple math corrections and CP2000 for third‑party reporting mismatches). If you agree, you can accept the change and pay any tax due. If you disagree, you must respond by the deadline with supporting documents (typically 30 days). See IRS guidance on notices and what to do next (https://www.irs.gov/newsroom/what-to-do-when-the-irs-sends-you-a-notice).

How the IRS finds math errors

  • Automated systems re‑calculate totals and cross‑check income and withholding against third‑party filings (W‑2s, 1099s).
  • Missing or mismatched information often triggers a proposed adjustment rather than a full audit.
  • The IRS will explain the specific issue, proposed change, and any resulting balance due or refund reduction.

Your three main options

  1. Review and accept — If the IRS correction is accurate, follow the notice’s payment instructions or adjust your return accordingly. Accepting moves the process along and avoids further notices.
  2. Dispute the change — If you disagree, respond in writing by the deadline with copies of the documents that prove your original figures (pay stubs, 1099s, bank records). Many math‑adjustment notices give about 30 days to reply; check your notice for exact timing and instructions (see CP2000 information: https://www.irs.gov/individuals/cp2000-notice-of-proposed-changes).
  3. Request collection relief or set up payment — If you owe and cannot pay in full, contact the IRS to request an installment agreement or temporary delay. Interest and penalties usually continue to accrue until the balance is paid.

Practical step‑by‑step checklist (what I do with clients)

  • Read the notice fully and note the deadline. Don’t ignore it.
  • Pull the tax return and the supporting forms (W‑2s, 1099s, brokerage statements).
  • If you agree, sign and return any required form or pay the amount due.
  • If you disagree, prepare a concise written reply and attach clear copies of documents that resolve the discrepancy. Keep originals.
  • Mail via tracked delivery or follow the IRS notice’s secure upload instructions, and retain proof of submission.

Typical timelines and consequences

  • Response window: usually about 30 days from the notice date — but verify your specific notice. If you miss the deadline, the IRS may assess the proposed change and send a bill.
  • Interest: accrues on any unpaid tax from the original due date of the return until paid.
  • Penalties: may apply depending on the reason and amount of underpayment.

When to escalate or get professional help

  • If the notice proposes a large increase, if you can’t obtain supporting documents, or if the IRS rejects your initial reply, call a tax professional. In my practice, early engagement prevents escalation and reduces the chance of penalties and collection actions.
  • If you receive a formal Notice of Deficiency (90‑day letter) or other higher‑level notice, you may have additional legal options (for example, petitioning Tax Court). For help appealing changes see our guide: How to Appeal a Notice That Proposes Additional Tax.

Common mistakes to avoid

  • Waiting to respond — silence often results in assessment and extra interest.
  • Sending originals instead of copies — always send copies and retain originals.
  • Assuming a math adjustment equals an audit — most math adjustments are narrow, automated corrections.

Documentation to keep

  • Pay stubs, W‑2s, 1099s, brokerage statements, cancelled checks, and contemporaneous receipts. Keep tax records at least three years; for issues with substantial omissions keep them longer (IRS guidance: https://www.irs.gov).

Helpful internal resources

Bottom line

A math adjustment is usually fixable if you act promptly, provide clear documentation, and follow the notice instructions. If you can’t resolve the issue yourself, consult a tax professional right away to avoid added interest, penalties, or collection actions.

Disclaimer: This article is educational and not individualized tax advice. For advice tailored to your situation, consult a licensed tax professional or the IRS (https://www.irs.gov) or the Taxpayer Advocate (https://www.irs.gov/taxpayer-advocacy/problem-with-irs).