CP2100 notice on your desk? Two dates and one number.

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

CP2100 / CP2100A notice date Is this the first or second B-notice for these payees? Payees listed on the notice payees Payees who already returned valid proof payees Reportable payments per payee, next 12 months USD Backup withholding rate %

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

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.

Open full tool → getreadystack.com