You work full-time and then have a side job providing administrative support and am paid by an individual $5500 per month. You have to estimate your income for the year and the amount of taxes withheld by an employer. TurboTax has a series of calculators that you can use to calculate your tax liability. See link
https://turbotax.intuit.com/tax-tools/
In order to avoid underpayment penalties you should file estimated taxes and pay the amount of estimate due on a quarterly basis, the payments are due April 15th. June 15th, September 15th and January 15 of the following year. You can also avoid an underpayment penalty if you pay an amount equal to 100% of the prior year tax (instead of the amount actually due). If the adjusted gross income is over 150,000 (or 75,000 if married filing separate), then the estimate has to be 110% of the prior year tax. The balance of the tax is due on April 15th. ( Note you compute the estimated tax due, then you deduct any withholding payments and calculate the balance that is required to be paid by estimated taxes.)
