Offer your thoughts or opinion on the input financial query or topic using your financial background.

How to estimate federal and state taxes likely to be due on my side income?
Most states that have income tax base their taxes on the income reported on your federal return, with some state-specific adjustments.  So answering your last question first:  Yes, if it matters for federal, it will matter for state (in most cases). For estimating the tax liability, I would not use the effective rate but rather use the rate for your highest tax bracket and apply that to your estimated hobby income, assuming that you primary job income won't be wildly higher or lower than last year. As @keshlam noted in a comment, this income is coming on top of whatever else you earn, so it will be taxed at your top rate. Finally, I'd check again whether this is really "hobby" income or if it is "self-employment" income.  Self-employment income will be subject to self-employment tax, which comes on top of the regular income tax.