Backup withholding means holding back 24% of a payee’s reportable payments and sending it to the IRS, and it kicks in mainly when a payee’s name/TIN doesn’t match or they never gave you a correct TIN. After a mismatch, the IRS sends you a CP2100 notice, which starts the B-notice clock: you have 15 business days to send the payee a notice. A first B-notice asks for a corrected W-9; a second B-notice (same payee, mismatched twice in three years) requires IRS or SSA validation instead. Cure it in time and withholding stops; miss it and 24% comes off the top.
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What Backup Withholding Is
Most 1099 payments are reported but not withheld — the contractor gets the full amount and settles up at tax time. Backup withholding is the exception. It forces you to hold back a slice of the payment and remit it to the IRS on the payee’s behalf, as a backstop against unreported income.
The rate is a flat 24%. It applies to the reportable payment — the same categories that land on the backup-withholding-eligible 1099s: nonemployee compensation, interest, dividends, rents, royalties, broker proceeds, and certain others. When it’s on, you calculate 24% of the payment, pay the payee the rest, and deposit the withheld amount with the IRS like any other withholding.
It isn’t a penalty on you, and it isn’t extra tax on the payee — they get credit for it on their return. But it’s a real obligation with deposit rules and reporting attached, so you want to trigger it only when you actually have to.
What Actually Triggers Backup Withholding
There are a handful of triggers, but two account for most real-world cases: a payee who never gave you a correct TIN, and a payee whose name/TIN the IRS says doesn’t match.
| Trigger | How it happens |
|---|---|
| Missing TIN | The payee didn’t give you a TIN, or gave one in the wrong format |
| Incorrect TIN (C-notice / CP2100) | The IRS notifies you the name/TIN combination doesn’t match |
| Payee failed to certify | The payee didn’t sign the W-9 certification when required |
| IRS notified underreporting | The IRS tells you the payee underreported interest or dividends |
The mismatch trigger is the one this guide follows, because it’s the one tied to CP2100 notices and B-notices. Notice that a clean W-9 collected up front — with the payee’s exact legal name and TIN — defuses most of these before they can fire.
The CP2100 Notice Starts the Clock
You don’t decide on your own that a TIN is wrong. The IRS tells you, through a CP2100 or CP2100A notice, usually sent in the fall and again in the following spring. The notice lists the returns you filed with name/TIN combinations that didn’t match IRS records.
Once that notice lands, you’re on a schedule. You have 15 business days from the date you receive the CP2100 (or the date of the notice, if later) to send the affected payees a B-notice and, where required, begin backup withholding. This is where filers get tripped up — the clock is short and it’s measured in business days.
The window to react to a CP2100 is tight. Between pulling the payee list, matching it to your records, printing the correct B-notice, and mailing it, 15 business days disappears fast. Build the response as a repeatable process, not a scramble.
First B-Notice vs. Second B-Notice
The word “B-notice” covers two different letters, and the difference is what you ask the payee to send back.
A first B-notice goes out the first time a payee’s TIN is flagged. You send the IRS-prescribed letter along with a fresh Form W-9, and you ask the payee to certify their correct name and TIN. Their signed W-9 cures it. The first B-notice is printed on paper and must be mailed — and the envelope has to be marked to alert the payee that it’s an IRS matter.
A second B-notice is different. If the same payee is flagged again within a three-year window, a plain W-9 no longer cures it — the payee has to obtain validation directly from the source. For an SSN, that means a Social Security card or a letter from the SSA; for an EIN, a Letter 147C from the IRS. You’re telling the payee: don’t just re-send your number, prove it with the agency.
| First B-notice | Second B-notice | |
|---|---|---|
| When | First mismatch for this payee | Second mismatch within 3 years |
| What the payee sends | Signed Form W-9 | SSA validation (SSN) or IRS Letter 147C (EIN) |
| A plain W-9 cures it? | Yes | No |
Working the B-Notice, Step by Step
Here’s the sequence to run each time a CP2100 arrives. Do it the same way every year and it stops being stressful.
Compare the notice to your records
For each name/TIN on the CP2100, check what you actually filed against your W-9 on file. If you made a keying error, correct your records — no B-notice needed for a payee whose W-9 was right all along.
Day 1Determine first or second
Check whether this payee was flagged before within three years. That decides whether you send a first B-notice (W-9) or a second (agency validation).
Day 1–2Mail the B-notice
Send the correct IRS-prescribed B-notice, with a W-9 for a first notice, within 15 business days. Mark the outer envelope as required so the payee treats it as important.
By day 15Start backup withholding if needed
If the payee doesn’t cure it in time, begin withholding 24% on reportable payments. Withholding generally starts no later than 30 business days after you receive the CP2100.
~Day 30Stop when cured
When the payee returns a valid W-9 (first notice) or agency validation (second notice), stop backup withholding on future payments. Keep the documentation.
On cureStep 1 matters more than it looks. If the mismatch came from a data-entry error on your side — you transposed two digits — the fix is to correct your records and file a correction, not to hassle the payee with a B-notice. Sending B-notices for your own mistakes annoys good vendors and wastes the 15-day window.
Reporting the Amounts You Withheld
Backup withholding doesn’t just get deposited — it gets reported in two places. On the payee’s 1099, the amount you held goes in the federal income tax withheld box (Box 4 on most of the affected forms), so the payee can claim the credit. And the total you withheld across all payees flows onto Form 945, the annual return for non-payroll withholding.
Deposits follow the same federal deposit schedule rules as other withheld taxes, so if you’re holding meaningful amounts you may have deposit deadlines during the year, not just at filing. When you file the payee’s 1099 through IRIS, the withheld amount rides along in Box 4 as part of the record.
Filing 1099s with backup withholding?
e1099f carries the Box 4 withholding into your IRIS XML and validates the record before it transmits.
The Best B-Notice Is the One You Never Get
Every step above is triggered by a mismatch that reached the IRS. Kill the mismatch before it files and the whole chain never starts.
Match before you file
Name/TIN problems flagged pre-transmission mean fewer CP2100 notices in the fall.
Box 4 handled
Backup withholding carries into the IRIS record so the payee gets proper credit.
No 45-day wait
FIRE-format in, IRIS XML out — file the same day, no new TCC, before the deadline slips.
Clean name/TIN data in means no B-notice scramble in the fall.
Frequently Asked Questions
What is the backup withholding rate?
What triggers backup withholding?
What’s the difference between a first and second B-notice?
How long do I have to respond to a CP2100?
How does a payee stop backup withholding?
Where do I report backup withholding?
Can I avoid B-notices entirely?
Not tax advice. This guide is general information for filers and may not reflect the latest IRS rules; thresholds and amounts change. Confirm current-year requirements with the IRS or a tax professional before you file.