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."
You can and are supposed to report self-employment income on Schedule C (or C-EZ if eligible, which a programmer likely is) even when the payer isn't required to give you 1099-MISC (or 1099-K for a payment network now). From there, after deducting permitted expenses, it flows to 1040 (for income tax) and Schedule SE (for self-employment tax). See https://www.irs.gov/individuals/self-employed for some basics and lots of useful links. If this income is large enough your tax on it will be more than $1000, you may need to make quarterly estimated payments (OR if you also have a 'day job' have that employer increase your withholding) to avoid an underpayment penalty. But if this is the first year you have significant self-employment income (or other taxable but unwithheld income like realized capital gains) and your economic/tax situation is otherwise unchanged -- i.e. you have the same (or more) payroll income with the same (or more) withholding -- then there is a 'safe harbor': if your withholding plus estimated payments this year is too low to pay this year's tax but it is enough to pay last year's tax you escape the penalty. (You still need to pay the tax due, of course, so keep the funds available for that.) At the end of the first year when you prepare your return you will see how the numbers work out and can more easily do a good estimate for the following year(s). A single-member LLC or 'S' corp is usually disregarded for tax purposes, although you can elect otherwise, while a (traditional) 'C' corp is more complicated and AIUI out-of-scope for this Stack; see https://www.irs.gov/businesses/small-businesses-self-employed/business-structures for more. You may get some advice here, but you REALLY need to talk to a tax adviser who is familiar with the laws of both states, or possible two, one from each state.  There is a significant amount of money on the line here, not just tax but penalties and interest if you get this wrong.  Once you get it figured out, you probably will be able to take care of it yourself in the future, but make sure that it gets done right initially. Yes, you can do what you are contemplating doing, and it works quite well. Just don't get the university's payroll office too riled by going in each June, July, August and September to adjust your payroll withholding! Do it at the end of the summer when perhaps most of your contract income for the year has already been received and you have a fairly good estimate for what your tax bill will be for the coming year. Don't forget to include Social Security and Medicare taxes (both employee's share as well as employer's share) on your contract income in estimating the tax due. The nice thing about paying estimated taxes via payroll deduction is that all that tax money can be counted as having been paid in four equal and timely quarterly payments of estimated tax, regardless of when the money was actually withheld from your university paycheck. You could (if you wanted to, and had a fat salary from the university, heh heh) have all the tax due on your contract income withheld from just your last paycheck of the year! But whether you increase the withholding in August or in December, do remember to change it back after the last paycheck of the year has been received so that next year's withholding starts out at a more mellow pace. First - get a professional tax consultation with a NY-licensed CPA or EA. At what point do I need to worry about collecting sales taxes for the city and state of New York? Generally, from the beginning. See here for more information on NYS sales tax.  At what point do I need to worry about record-keeping to report the income on my own taxes? From the beginning. Even before that, since you need the records to calculate the costs of production and expenses. I suggest starting recording everything, as soon as possible. What sort of business structures should I research if I want to   formalize this as less of a hobby and more of a business? You don't have to have a business structure, you can do it as a sole proprietor. If you're doing it for-profit - I suggest treating it as a business, and reporting it on your taxes as a business (Schedule C), so that you could deduct the initial losses. But the tax authorities don't like business that keep losing money, so if you're not expecting any profit in the next 3-4 years - keep it reported as a hobby (Misc income). Talk to a licensed tax professional about the differences in tax treatment and reporting. You will still be taxed on your income, and will still be liable for sales tax, whether you treat it as a hobby or as a business. Official business (for-profit activity) will require additional licenses and fees, hobby (not-for-profit activity) might not. Check with the local authorities (city/county/State). A tax liability account is a common thing. In my own books I track US-based social insurance (Medicare and Social Security) using such an account.  At the time I pay an employee, a tax liability is incurred, increasing my tax liability account; at the same time, on the other end of the double-entry, I increase a tax expense account. Notably, though, the US IRS does not necessarily require that the tax is paid at the time it is incurred.  In my case I incur a liability twice a month, but I only have to pay the taxes quarterly.  So, between the time of incurring and the time of remitting/paying, the amount is held in the tax liability account.  At the time that I remit payment to the IRS, the transaction will decrease both my checking account and also, on the other end of the double-entry, my liability account. To answer your question in short, use an expense account for your other-side-account. "H.R. basically consults Publication 15 (this is the link to 2015) to determine how much to hold, based on filing status, exemptions, and pay amount. What's described here is a form of estimation, or, in other words, H.R. withholds what would be your actual taxes, dividing across the number of paychecks you receive. Assuming your gross pay and exemptions do not change, this usually results in a zero-sum for taxes owed (you will receive nothing, and owe nothing). As you can see from the charts, the year is basically broken down into equal tax units that reflect how much you would owe if you worked at that bracket all year. This estimation works best when you have steady hours from check to check. In other words, your taxes are based on the estimate of what you'd make if you earned that much all year, scaled down to the time frame (e.g. 1/52 if you are paid weekly, or 1/26 if you paid biweekly). They do not go ""up"" near the end of the year, because they're estimated in advance. You don't move up a tax bracket, but are instead taxed at a particular bracket every paycheck. There's also other forms of estimation mentioned there, but basically follow the same scheme. Note that all estimation forms are just that-- estimates. It's best to use a calculator and compare your current taxes whenever a significant change occurs-- a raise, a new child, getting married or divorced, etc. You'll want to be able to alter your exemptions so that enough taxes are coming out. That's also the reason for the ""withhold extra"" box, so that you can avoid owing. For example, if you're making $44 a week for the first 26 weeks, and then you make $764 a week for the second 26 weeks of the year, you'll end up with an actual tax liability of $2,576.6, but end up paying only $2,345.20. You would owe $231.40. Of course, the actual math is a lot more complicated if you're an employee paid by the minute, for example, or you have a child, go to college, etc. Paychecks that vary wildly, like $10,000 one week and $2,000 the next tend to have the hardest-to-predict estimates (e.g. jobs with big commission payouts). You should avoid living check-to-check with jobs that pay this way, because you'll probably end up owing taxes. Conversely, if you've done your estimates right and you're paid salary or exactly the same number of hours every week, you'll find that the taxes are much easier to predict and you can usually easily create a refund situation simply by having the correct exemptions on your check. So, in summation, if your check falls in the 25% category (which is, of course, 25% above the tax bracket break point), you're already paying the correct amount, and no further drop in your check would be expected." You may be able to find the answers to your question on the IRS web site: http://www.irs.gov/businesses/small/article/0,,id=98263,00.html Specifically, using this form to estimate taxes for salary: http://www.irs.gov/pub/irs-pdf/f1120w.pdf and this form to estimate taxes for dividends: http://www.irs.gov/pub/irs-pdf/f1040es.pdf Follow the instructions on the W-4. It says exactly how you are supposed to calculate the number of allowances. You shouldn't have to figure out how to get the right number. Just follow the instructions. The only part at all complex is if you have large deductions. In that case you're supposed to subtract a standard amount from your actual deductions -- for 2017, $12,700 if married filing jointly -- divide by $4,050, and then add the result to the number of allowances. In general, following the instructions on the W-4 should result in slightly more tax being withheld from your paycheck than you actually owe, so that you get a modest refund next April 15. In the long run it doesn't matter if you have too much withheld, as you'll get it all back eventually anyway. I suppose the withholding could be so high that it doesn't leave you enough to live on while waiting for your refund, but that shouldn't normally be the case. If you pay too little, you could be subject to penalties and interest, so you really want to avoid that. 75k is short of the 'highly compensated' category.  Most US citizens in that pay range would consider paying someone to do their taxes as an unnecessary expense.  Tax shelters usually don't come into play for this level of income.  However, there are certain things which provide deductions. Some things that make it better to pay someone:   Use the free online tax forms to sandbox your returns.  If all you're concerned about is ensuring you pay your taxes correctly, this is the most cost efficient route. If you want to minimize your tax burden, consult with a CPA.  Be sure to get one who is familiar with resident aliens from your country and the relevant tax treaties. The estimate you're looking at may be the withholding, of which you may be eligible for a refund for some part of that withholding.   Tax treaties likely make sure that you get credit on each side for the money paid in the other.  For example, as a US citizen, if I go to Europe and work and pay taxes there, I can deduct the taxes paid in Europe from my tax burden in the US.  If I've already paid more to the EU than I would have paid on the same amount earned in the US, then my tax burden in the US is zero.  By the same token, if I have not paid up to my US burden, then I owe the balance to the US.  But this is way better than paying taxes to your home country and to the host country where you earned the money. "So after a great deal of clarification, it appears that your question is how to adjust your withholding such that you'll have neither a refund, or a balance due, when you do your 2016 taxes next year.  First, a little terminology. The more you have withheld, the more money will be taken out of your check to cover your estimated tax liability. Confusingly, the more allowances you select on your W-4, the less money you will have withheld (more allowances means more dependents/deductions/other reasons why you will owe less tax). When you go to file your 2015 tax return next year, you'll figure out exactly how much you owe. If you had too little tax withheld, you'll have to pay the difference. If you had too much tax withheld, you'll get a refund back.  Given your situation, simply following the instructions on the W-4 should work pretty well. If you want to be more precise, you can use the IRS Withholding Calculator to figure the number of allowances and submit a new W-4 to your employer.  It's a little hard to tell whether ""paying this much/year in taxes seem steep?"" because you've lumped all the taxes together in one big bucket. Does the $543.61 in taxes per paycheck include Social Security (OASDI) and Medicare taxes? Whatever you do, it's not going to be an exact science. Come tax time, you'll figure out exactly what you owe and either pay the balance or get a refund back. As long as you're relatively close, that's fine. You can always adjust your withholding again next year after you've done your taxes." It would be quite the trick for (a) the government to run all year and get all its revenue in April when taxes are due and (b) for people to actually save the right amount to be able to cut that check each year. W2 employers withhold the estimated federal and state taxes along with the payroll (social security) tax from each paycheck. Since the employer doesn't know how many kids you have, or how much mortgage interest, etc you will take deductions for, you can submit a W4 form to adjust withholdings.   The annual Form 1040 in April is to reconcile exact numbers, some people get a refund of some of what they paid in, others owe some money.  If one is self-employed, they are required to pay quarterly estimated taxes. And they, too, reconcile exact numbers in April. Consult with a CPA in your local community, since determining how your income is reported is dependent on how it was earned, and how much it was and other factors.  The details are personal, should not be shared on the internet, and don't affect the way we can answer this question.   A CPA might be able to help you with some other advice.  Things like how to structure your expenses to minimize tax burden, while staying within the rules. "Do you have other income that you are not considering?  Interest and dividends would be an example, but there are all sorts of options.   Also with your witholding is it set up such that your employers have any idea of your tax bracket ultimately based on your combined incomes?  Usually what they do is take out money assuming you will be in the tax bracket of any given paycheck spread out over the course of a year.  For example, for federal I had an option to select (in an online form that fills out my W4 for me) ""married: withold at higher single rate"" and did to try and cover this fact.  Eventually I may end up having to calculate my own witholding to fix a too-low problem like yours." The amount earned is taxable. It needs to shown as income from other sources. Although the last date for paying Advance tax is over [15 March], there is still time to pay Self-Assessment tax till 15 June. If the tax amount due is less than 10,000/- there is no penalty. If the tax is more than Rs 10,000/- there is penalty at the rate of 1% per month from March, and if the amount of tax exceeds 40% of the total tax, there will be additional 1% interest from December.  The tax can be paid online via your Banks website or using the Income Tax website at https://onlineservices.tin.egov-nsdl.com/etaxnew/tdsnontds.jsp The form to be used is 280. You can use the Income tax website to calculate and file your tax returns at https://incometaxindiaefiling.gov.in/ or use the services of a CA. Edit: If the income is less than expenses, you need not pay tax. Maintain proper records [receipts] of income and expenses, if possible use a different Bank account so that they remain different from your main account.  The tax to be paid depending on your income slab. The additional income needs to added to you salary. The tax and slabs will be as per this. There is no distinction on this amount. Its treated as normal income. All Tax for the given year has to be paid in advance. i.e. for Tax year 2013-14, 30% of total tax by 15-Sept, Additional 30% [total 60%] by 15-Dec and Balance by 15-Mar. Read Page 3 and page 10 of http://incometaxindia.gov.in/Archive/Taxation_Of_Salaried_Employees_18062012.pdf "You can simply use the previous year's tax liability as your basis for payments. Take the amount of tax you owed the previous year, divide by four, and use that amount for your estimated payments. As long as you're paying 100% of what you owed last year, you won't have any penalty. Except if your AGI is above a certain limit ($150k for married filing jointly in 2011), then you have to pay 110%. See IRS Pub 505 for details (general rule, special rule, under ""Higher Income Taxpayers""). (H/T to @Dilip Sarwate for pointing out the 110% exception in a comment below.)"