I see estimated tax penalty questions constantly during tax season, so let me lay out the safe harbor rules that will keep you penalty-free. There are two ways to avoid the underpayment penalty under IRC Section 6654, and you only need to satisfy one of them.
The first safe harbor is the 100 percent prior year method. If your total tax payments for the current year, through withholding and estimated payments combined, equal at least 100 percent of your prior year total tax liability, you owe no penalty regardless of how much you owe this year. This increases to 110 percent if your prior year AGI exceeded 150,000 dollars. This is the simplest safe harbor for people with variable income like traders, freelancers, and business owners.
The second safe harbor is the 90 percent current year method. If your payments cover at least 90 percent of your current year tax liability, no penalty applies. This is harder to use because it requires accurately estimating your current year income, which is difficult for traders.
For people with capital gains, my practical recommendation is to use the annualized income installment method on Form 2210 Schedule AI. This allows you to pay estimated taxes based on when you actually earned the income during the year. So if you had a huge gain in Q4, you only need to make a large estimated payment for Q4 rather than spreading it across all four quarters. This can significantly reduce or eliminate penalties for people with lumpy income patterns.