At tax filing time, if the amount you owe is 10% more than your total tax liability, only then is an underpayment penalty assessed. I always send in 20% of my gross business earnings each quarter. That practically guarantees me a refund every year and I'm never assessed a penalty for anything. For me, 20% easily keeps me within 10% of my total tax liability every year. Some years I get a refund, some years I owe. But if I owe, it's always less than 10% of my total tax liability.
