Backup withholding is the IRS's fallback mechanism for making sure tax gets collected when its normal information-reporting system breaks down somewhere — and it can catch freelancers, contractors, and investors off guard when a payment suddenly arrives smaller than expected. * Unlike your regular paycheck withholding, which is calculated based on your specific income and filing status, backup withholding is a flat 24% taken directly off the top of a payment, regardless of what your actual tax situation looks like. * The most common trigger is simple paperwork trouble: if you don't provide the payer with a correct taxpayer identification number — your Social Security number or EIN — for a 1099 or similar reportable payment, the payer is legally required to start withholding 24% of what they pay you and send it directly to the IRS. * The other main trigger involves a track record issue rather than a paperwork error: if you've previously failed to report or under-reported interest or dividend income on your tax return, or failed to certify that you're not subject to backup withholding, the IRS can direct payers to start withholding on future payments to you. * Backup withholding isn't permanent once it starts — it stops once you fix whatever triggered it: providing your correct TIN to the payer, filing any missing tax returns, or paying what you owed on previously underreported income. Once resolved, the money already withheld isn't lost either — it shows up as a payment already made when you file your return, similar to regular paycheck withholding.