The third installment of estimated tax payments is due September 15, 2010.
There are multiple ways to calculate estimated taxes. The easiest way is to look at your total tax liability for 2009, subtract any withholding for 2010, and take any remainder and divide that into four equal installments. This method is designed to avoid the interest penalty the IRS charges for not paying sufficient amount of taxes throughout the year (the so-called penalty for underpayment of estimated tax). This penalty is basically an interest charge, and currently stands at 4%.
Another method I often use is to tally up current income and deductions, extrapolate those figures to represent a full year, and calculate the tax using the 2010 tax rates. This gives me and my clients a better picture of how much tax they expect to owe right now, and make payments accordingly.
The next payment deadlines are January 18, 2011 (the fourth quarterly installment) followed by the April 15th, 2011 payment deadline.
I prefer to use the EFTPS bill payment service operated by the Treasury Department for sending in my estimated payments. However, do be aware it takes several weeks to get properly registered and set up on this Web-based service. You can always mail in your 1040-ES payment vouchers to your local IRS payment processing center.
Detailed information about estimated taxes can be found in Publication 505, Tax Withholding and Estimated Tax, on the IRS Web site.