Backup Withholding Explained: What Payers Need to Know

reading time: 17 minute(s)

If you pay contractors, vendors, or investors, this is your legal obligation — not just the recipient’s problem.

Picture this: You run a growing marketing agency. You’ve just wrapped a big project and sent a $15,000 payment to a freelance designer you’ve worked with for two years. 

A few months later, you receive an IRS notice, a formal notification that the name and TIN on the 1099 you filed don’t match IRS records. The clock starts ticking. You now have 15 business days to act, or you’re legally required to start withholding a certain amount from future payments to that person.

This is backup withholding, and it lives on your side of the ledger.

What is backup withholding, really?

Backup withholding is a mandatory 24% federal income tax withholding on certain reportable payments, taken from the payment before it reaches the recipient and remitted directly to the IRS 

Payers generally don’t withhold taxes from payments reported on Forms 1099 and W-2G, as it’s assumed the recipient will self-report. Backup withholding is the IRS’s mechanism to step in when that assumption can’t be made. 

Think of yourself as a temporary tax collector. You hold the money, send 24% to the IRS, and pass the remainder to the payee. 

Which payments are subject to backup withholding?

Not every payment triggers this rule, but a wide range of common business payments do. Backup withholding can apply to most kinds of payments reported on Form 1099, including:

• Contractor and freelance payments (Form 1099-NEC) — the most common for small and mid-size businesses
• Interest income — paid by banks, credit unions, and businesses to account holders or lenders
• Dividends — from investments, reported on Form 1099-DIV
• Royalties — from intellectual property licensing
• Brokerage proceeds — investment and securities transactions (Form 1099-B)
• Certain gambling winnings (Form W-2G)
• Payments reported on Forms 1099-MISC, OID, K, and PAT

When are you required to withhold?

This is where most payers get tripped up. There are four distinct triggers, each requiring a different action.

1. Missing or obviously incorrect TIN 

If a payee hasn’t returned Form W-9 before payment, or their TIN isn’t exactly nine digits, withholding starts immediately — from the very first payment.

2. IRS notifies you of a TIN mismatch — The “B” Notice Program

The IRS sends a CP2100 or CP2100A notice to payers who file certain information returns with incorrect TINs to begin backup withholding.

The IRS mails these notices twice a year — in September/October and again in the following April. When one lands in your mailbox, here is exactly what you must do:

Step 1 — Compare the notice to your records (immediately). Some mismatches are simple data entry errors that you can resolve internally without contacting the payee.

Step 2 — Send the appropriate “B” Notice within 15 business days. Send a First B Notice (with a blank W-9) to the corresponding payee within 15 business days. If the same payee appears on a CP2100 again within that same three-year window, send a Second B Notice — this time without a W-9. The payee must verify their TIN directly with the IRS or SSA, via a Social Security card or IRS Letter 147C. 

Step 3 — Start withholding if there’s no response. If the payee doesn’t respond, you must begin backup withholding from any future payments no later than 30 business days after you received the CP2100 or CP2100A notice. 

Real-world example: Your agency pays a copywriter $8,000 annually. In October, a CP2100A arrived, listing her as a TIN mismatch. You send the First B Notice with a blank W-9 on day 10. She returns a corrected W-9 on day 25. You must stop backup withholding no later than 30 calendar days after you receive her corrected TIN. If she never responds, you begin withholding 24% of every future payment until she does.

3. Underreported Income — IRS-Directed Withholding

The IRS may also notify you to begin withholding on interest or dividend payments because a payee has underreported that income on their tax return. The IRS will do this only after mailing the payee four notices over at least a 120-day period. In this case, the IRS instructs you directly — you don’t have discretion.

4. Payee fails to certify on Form W-9

When a payee completes a W-9, they certify under penalties of perjury that they are not subject to backup withholding. If they check the box indicating that they are subject to backup withholding — or if that certification is missing, you must begin withholding immediately.

Payer’s four core obligations

Once withholding is triggered, your responsibilities are clear:

1. Withhold 24% from the gross reportable payment — if you owe $5,000, the payee receives $3,800 and the IRS gets $1,200.
2. Deposit via EFTPS on your assigned schedule — monthly if your prior-year Form 945 liability was $50,000 or less, semiweekly if it exceeded that.
3. File Form 945 by January 31 of the following year, reporting all backup withholding collected.
4. Reflect the withheld amount in Box 4 of the payee’s 1099, so they can claim credit when they file.

The mistakes that could cost payers

Paying before collecting a W-9. The most preventable trigger. Make W-9 collection a hard gate before any first payment goes out.

Skipping TIN Matching. The IRS’s TIN Matching program lets you verify name and TIN combinations before filing 1099s — payers who use it receive significantly fewer CP2100 notices. 

Missing the 15-business-day B Notice window. Once a CP2100 arrives, the clock runs. Late notices don’t eliminate your withholding obligation — they add a compliance gap on top of it.

Assuming a third-party payroll provider covers you. You remain legally responsible for ensuring deposits and returns are filed correctly, even if you’ve outsourced to a third party.

The bottom line

Backup withholding is entirely preventable in most cases. A consistent pre-payment W-9 process, TIN matching before the 1099 season, and prompt B Notice responses will handle the vast majority of situations before they become costly.

When a CP2100 lands on your desk, treat it as the legal trigger it is — not a routine mail item. Know your deadlines, send the right notice, deposit on time, and file Form 945 accurately. Done right, it’s a manageable part of your compliance calendar. Done wrong, it becomes a liability that compounds with every payment you make.