B-Notice Clock: CP2100 Backup Withholding by ReadyStack

Two statutory dates and one dollar figure, straight off the IRS notice on your desk.


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• 15-business-day B-notice mail-by date — Counts 15 business days from the CP2100 / CP2100A notice date and prints the calendar date the payee copy must be in the mail.
• 30-business-day withholding start date — Counts 30 business days from the same notice date and prints the date backup withholding must begin on every payee who has not cured.
• Payer liability in dollars — Multiplies the payees who have not cured by their annual reportable payments and the backup withholding rate, so the number on screen is the payer's own money.
• 24% / 28% rate switch — The rate is a field, not a guess: 24 percent is what the IRS backup withholding C program page states today, 28 percent is the pre-sunset figure still quoted in older guidance and by chatbots.
• First vs second notice cure rule — A first B-notice is cured by a signed Form W-9; a second B-notice is not, and needs an SSA card copy or IRS Letter 147C. The tool names which one applies.
• Source — Timing rules from IRS Publication 1281 (CP2100/CP2100A B-notice procedures); rate from the IRS Backup Withholding C Program page, checked 2026-09-22, which states 24 percent.

| You type | |
|---|---|
| CP2100 / CP2100A notice date | default 2026-09-14 |
| Is this the first or second B-notice for these payees? | default first |
| Payees listed on the notice | default 18 |
| Payees who already returned valid proof | default 6 |
| Reportable payments per payee, next 12 months | default 12500 |
| Backup withholding rate | default 24 |

| It answers | |
|---|---|
| Mail the B-notice by | |
| Start backup withholding by | |
| Payees still uncured | payees |
| Payer liability if you do not withhold | USD |
| What cures it | |

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