Self employed

This is the best explanation of quarterly estimates I've found. Estimated Taxes: How to Determine What to Pay and When and A Guide to Paying Quarterly Taxes  

• If your prior year Adjusted Gross Income was $150,000 or less, then you can avoid a penalty if you pay either 90 percent of this year's income tax liability or 100 percent of your income tax liability from last year (dividing what you paid last year into four quarterly payments). This rule helps if you have a big spike in income one year, say, because you sell an investment for a huge gain or win the lottery. If wage withholding for the year equals the amount of tax you owed in the previous year, then you wouldn't need to pay estimated taxes, no matter how much extra tax you owe on your windfall.
• If your prior year's Adjusted Gross Income was greater than $150,000, then you must pay either 90 percent of this year's income tax liability or 110 percent of last year's income tax liability.

You can always try TaxCaster and see what the estimates will look like too.