CPA here specializing in self-employment tax. The estimated tax penalty under IRC Section 6654 is one of the most common and most avoidable penalties that 1099 workers face. Let me break down the two safe harbor methods.
The first safe harbor is the 100% of prior year method: if your total estimated tax payments for the current year equal or exceed 100% of your prior year total tax liability, you owe no penalty regardless of your current year income. If your prior year AGI exceeded $150,000, the threshold increases to 110% of prior year tax.
Practical tip: if you also have a W-2 job in addition to 1099 income, the easiest approach is to increase your W-2 withholding using Form W-4 rather than making separate estimated payments. W-2 withholding is treated as paid evenly throughout the year regardless of when it is actually withheld, which means you can avoid per-quarter underpayment penalties that would apply to estimated tax payments.