The U.S. tax system is pay-as-you-go. W-2 employees pay every paycheck through withholding. Self-employed people must pay
quarterly estimated taxes using Form 1040-ES. If you owe $1,000+ in taxes and don't pay quarterly, the IRS charges a penalty.
Who needs to pay quarterly: Generally, if you expect to owe $1,000+ in tax for the year after subtracting withholding and credits. If your side gig income is under $5,000 and you have a W-2 job covering most of your tax, you might not need to — but it's safer to pay anyway.
How to pay: Go to
irs.gov/payments and use IRS Direct Pay (free, linked to your bank account) or pay by card. You can also mail a check with Form 1040-ES.