I suggest a minimum of 30% - you're subject to income tax rates and 15. 3% for self employment taxes. It may or may not be too much (depends on if you have any W-2 income, spouse with W-2 income, your income tax rate, etc), but that's usually my ballpark suggestion. Worst case, you have extra saved. If you file quarterlies, it's sort of a moving target. If you underestimate your income with your first quarter, and then project it higher later, you have to catch up the extra taxes that you missed the first quarter, etc.