Who has to pay
• Self-employed and 1099 earners • S-corp / partnership owners taking distributions • Landlords and investors • Anyone with significant non-wage income
Safe-harbor rule
You avoid the underpayment penalty if your total payments equal: • 90% of this year's tax, OR • 100% of last year's tax (110% if last year's AGI was over $150K). This is the easiest target — divide last year's total tax by 4 and pay that each quarter.
How to pay
Use IRS Direct Pay or EFTPS. Pick 'Estimated Tax' and the right tax year. Save the confirmation — you'll need it at filing time.