IRS http://www.irs.gov/publications/p17/ch04.html IRS.gov: Tax Withholding and Estimated Tax 110 percent of the previous year’s tax liability IRS http://www.irs.gov/publications/p17/ch04.html IRS: directorio de retención e impuesto estimado https://www.irs.gov/publications/p17/ch04.html 110 percent of the previous year’s tax liability “Safe Harbor”