Guides

• Q4 Estimated Tax: What to Pay by January 15, 2027 — How to work out the fourth 2026 estimated tax payment: catch up to the safe harbor, count what you paid, and the February 1, 2027 file-and-pay option.
• The Estimated Tax Safe Harbor: 90%, 100% and the 110% Rule — When you avoid the underpayment penalty: 90% of this year's tax, 100% of last year's, or 110% if last year's AGI was over $150,000 — and the $1,000 exception.
• The Annualized Income Installment Method, Explained — How Form 2210 Schedule AI lowers required installments when income is uneven: the four periods, the 4 / 2.4 / 1.5 / 1 factors and the 22.5% to 90% percentages.

Tools

• Estimated tax calculator for the self-employed
• Form 2210 Schedule AI, worked out from your transactions

Comparisons

• A lighter alternative to Keeper for quarterly taxes