How interest is calculated
Out of the two charges you can face, the interest is the more straightforward to calculate. The IRS interest rate is determined by the Federal short-term rate plus 3%. Since the Federal short-term interest rate has been close to 0% for some time now, the interest rate charged on late tax payments is 3% as of this writing (October 2015).
Keep in mind that interest rates are widely predicted to start increasing in the not-too-distant future, so this can (and likely will) change over time.
Interest is computed on a daily basis, so each day you are late paying your taxes, you'll owe 0.0082% of the balance.
So, if you owe the IRS $1,000 and you're 90 days late, first calculate your daily interest charge, which would be about $0.082. Then, multiply it by 90 days to arrive at the total interest charge of $7.40.
