How do I calculate my quarterly estimated tax payment if my income is unpredictable?

The simplest protection is the federal safe harbor: pay at least 100% of last year's total tax (110% if your prior-year adjusted gross income exceeded $150,000) in even quarterly installments, and the IRS won't charge an underpayment penalty even if you end up owing more. If your income swings a lot, you can instead use the annualized-income installment method on Form 2210, which lets you pay based on what you've actually earned each quarter rather than assuming even income. That helps someone with a big Q4 bonus or a year-end capital gain avoid front-loading payments. Set aside a percentage of each payment you receive into a separate account. The IRS updates thresholds and rules yearly, so confirm current figures on irs.gov before you calculate.