The IRS requires self-employed individuals to pay estimated taxes in four quarterly installments (1040-ES). Many freelancers calculate the full annual projected liability, divide by 4, and pay equal amounts each quarter. This is the mistake.
The US tax system is pay-as-you-go—and the penalty is assessed on each quarter individually based on income earned in that specific quarter. If Q1 was your highest revenue month and you underpaid in Q1, the penalty applies even if Q2–Q4 are overpaid.
The IRS allows the Annualized Installment Method (Form 2210, Schedule AI), which calculates each quarter's required payment based on actual income for that period. A freelancer earning $60,000 in Q1 and $10,000 in Q2–Q4 would massively overpay if using the standard 25% approach. With AIM, Q1's payment correctly reflects the high-revenue period, and Q2–Q4 adjust downward—eliminating the hidden penalty.
Run your quarterly scenarios through savvy.metaphysicflow.com to verify your Q1–Q4 allocation before the IRS makes the adjustment for you.