When you click on one of the Pay Now options on the IRS website, you will come to a page that asks you to identify the type of payment you're making. One of the choices will be "estimated tax." That's what you choose.
There is no precise tool for calculating the amount to pay in estimates, because everyone's situation is different. 20-25% of your anticipated 1099 income each quarter is sufficient for most people, but it is a "guesstimate". Once you get the hang of making quarterly estimated payments, you'll be able to adjust the percentage you pay according to your own needs.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
