Why you collect a W-9 before the first payment, what it tells you, how a TIN mismatch triggers 24% backup withholding, the single-member LLC gotcha, and a vendor cleanup workflow.
A W-9 is one page, takes a contractor five minutes to complete, and costs you nothing — and it's the document that stands between you and a 24% withholding obligation, mismatched IRS notices, and a scramble every January. The rule worth tattooing on your accounts-payable process: collect the W-9 before you send the first payment. A contractor waiting on a check fills it out same-day. A contractor who's been paid in full may never answer your email again.
The W-9 exists to feed your 1099 filings — it tells you whether a vendor needs a form at all and gives you the name and taxpayer identification number to put on it. If you're reconstructing a year of vendor payments after falling behind on the books, W-9 collection is usually the slowest step, which is why our catch-up bookkeeping playbook puts it early in the sequence rather than last.
Here's what the form actually captures, what happens when the information is wrong, and the once-a-year cleanup that keeps the whole system honest.
The W-9 captures three things you can't file accurate 1099s without:
The W-9 also protects your deductions in a quieter way. In an audit, a contractor payment backed by a W-9, an invoice, and a 1099 is an easy deduction to defend. A $12,000 cash-app payment to someone whose legal name you never collected is an invitation for questions. Which vendors need the resulting 1099-NEC — thresholds, deadlines, corporate exemptions — is covered in our 1099-NEC filing guide.
Backup withholding is how the IRS makes information reporting self-enforcing. Two situations trigger it:
Note who carries the obligation in both cases: you, the payer. The 24% comes out of the contractor's check, but you're the one who must implement the withholding, deposit it with the IRS on schedule, and report it. If you were supposed to withhold and didn't, the IRS can hold your business liable for the amounts you failed to withhold. That's the same uncomfortable position employers face with payroll withholding — the money was never yours, but the liability is.
Concretely: a contractor invoices you $5,000 and has ignored three W-9 requests. You pay $3,800 and deposit $1,200 with the IRS. The contractor gets credit for the $1,200 when they file their own return — backup withholding is a prepayment of their tax, not a penalty — but in practice, watching 24% disappear from a check produces a completed W-9 within the hour.
The most common W-9 error we see: a single-member LLC owner writes the LLC's name on line 1 and the LLC's EIN in the TIN box. That combination fails IRS matching, because a single-member LLC is a disregarded entity — the IRS matches against the owner, not the LLC.
The correct fill: the owner's name on line 1, the LLC's name on line 2 as the business name, and the owner's SSN (or the owner's own EIN as a sole proprietor) as the TIN. When one of your vendors is a single-member LLC and their W-9 shows only the LLC name and EIN, send it back — that's a future B-notice you can prevent in thirty seconds. And if you operate as a single-member LLC yourself, fill your own W-9s the same way; our guide to keeping business and personal finances separate covers the related discipline of keeping the LLC's identity clean in your banking, too.
W-9s never get sent to the IRS. You keep them. That surprises a lot of owners — the form's entire job is to sit in your records supporting the 1099s you file and proving you did your due diligence if a B-notice or audit arrives.
A sane storage system:
Once a year — ideally October or November, before 1099 season — run this loop:
The whole system in one sentence: no W-9, no payment — and every January becomes a report instead of a manhunt.
If your vendor list lives in a spreadsheet nobody maintains, this workflow is painful once and easy forever after it's built into your monthly bookkeeping — new vendor onboarding should include the W-9 the same way it includes payment details.
The W-9 is leverage that expires: demand it before the first check and compliance is automatic; skip it and you inherit a 24% withholding obligation and a January of unanswered emails. Build it into vendor onboarding, fix single-member LLC forms on sight, and run the fall cleanup. Taxagon's CPAs and EAs handle vendor compliance and 1099 filings for business clients every season — if you'd rather not chase paperwork, reach out.
This article is general information, not tax advice for your specific situation. Tax law changes; figures are for the years stated.
Talk it through with a licensed US tax professional — we'll tell you honestly whether it applies to your situation.
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