Freelancers, founders, and investors who underpay get hit with a quiet penalty every year. The safe-harbor rules make avoiding it simple — if you know them.
The two safe harbors
Pay either (a) 90% of the current year's tax, or (b) 100% of last year's tax (110% if AGI > $150k). Hit either and the IRS waives the underpayment penalty.
For most freelancers, option (b) is easier: divide last year's total tax by four and pay equal installments.
Due dates
April 15, June 15, September 15, and January 15 of the following year. Mark these as recurring calendar events — the IRS doesn't send reminders.
Annualized income method
If your income is lumpy (e.g. a Q4-heavy year), Form 2210 Schedule AI lets you match payments to when the income was actually earned, avoiding penalties from uneven cash flow.