The W-9 looks simple, but the consequences of getting it wrong are big — anywhere from 24% backup withholding on every payment to identity theft. Here are the mistakes I see most often, and how to avoid each one.

1. Putting your business name on Line 1 (when you should put your personal name)

Most common mistake for single-member LLC owners. Line 1 is for the name on your tax return — for SMLLCs, that's your personal legal name. The LLC name goes on Line 2.

2. Checking the wrong tax classification box

Single-member LLCs should check "Individual / sole proprietor / single-member LLC" — NOT the LLC box. The LLC box is only for multi-member LLCs or LLCs with corporate tax elections.

3. Putting both your SSN and EIN on the form

Use only one. Putting both creates IRS matching errors and may trigger backup withholding.

4. Using a TIN that doesn't match the name on Line 1

If Line 1 says "Jane Smith" and the TIN is the LLC's EIN, the IRS computer will fail to match. The name and TIN must correspond.

5. Forgetting to sign Part II

Without your signature, the W-9 is incomplete and legally insufficient. The payer is required to start backup withholding.

6. Crossing out item 2 of the certification when you shouldn't

Item 2 says you're NOT subject to backup withholding. Only cross this out if the IRS has actually sent you a notice saying you ARE subject to backup withholding. Crossing it out unnecessarily triggers withholding.

7. Using an outdated address

If you've moved, the 1099 may be sent to your old address. Update your W-9 with all clients when you move.

8. Not updating the W-9 when business structure changes

Switched from sole prop to LLC? Got an EIN? Made an S-corp election? Send a fresh W-9 to all clients. Mismatched information triggers backup withholding.

9. Submitting a W-9 when you should be using a W-4

If your relationship with the company is actually employment (they control your hours, provide tools, you work in their office), you should be a W-2 employee, not a 1099 contractor. Misclassification has consequences.

10. Submitting a W-9 when you're a foreign person

W-9 is for U.S. persons only. If you're not a U.S. citizen, green card holder, or resident alien, you should be filing a W-8BEN (or other W-8 variant) instead.

11. Sending the W-9 by unsecured email

A W-9 contains your SSN or EIN — sensitive data. Use encrypted email, secure file upload portals, or mail. Never send it via plain text email or insecure messaging.

12. Filling out a W-9 for a phishing scam

Verify any W-9 request before completing. Scammers send fake W-9 requests to steal SSNs. Confirm the request is legitimate by calling the company directly using a phone number from their official website (not from the email).

The double-check before you submit

Before sending a completed W-9, run through this quick mental checklist:

Does my Line 1 name match what's on my tax return?
Did I put my business name on Line 2 (only if different from Line 1)?
Did I check exactly one tax classification box on Line 3?
Did I leave Line 4 blank (unless I have a specific exemption code)?
Is my address current?
Is my TIN exactly correct? (No transposed digits.)
Did I use only ONE — either SSN or EIN, not both?
Did I sign and date Part II?
Am I sending it through a secure channel?

Two minutes of double-checking saves months of dealing with backup withholding and corrected forms.

Common W-9 Mistakes