Taxes on irregular income
This is really a non-issue, if you work a regular job: Your employer withholds taxes. It's also not much of an issue if you're self-employed and your income is reasonably steady through the course of the year: You estimate your annual tax bill and send in quarterly estimated tax payments. If your income is unpredictable, though, it is an issue. Happily, the IRS does not demand that you be able to predict the future.
I'm in this situation myself. Although I've got a few months experience as a full-time writer, that's not enough for me to produce an accurate estimate of what my income is going to be this year. That makes it really tough to file my quarterly tax payments (or even to know whether or not I need to).
The current rules were put in place to deal with people who tried to game the system by holding onto their money until the last minute (thereby keeping for themselves interest that the IRS figures belongs to Uncle Sam). If your earnings are level though the course of the year (and you don't have money withheld by your employer), then you have to make substantially equal quarterly payments. If you try to back-end load those payments, you owe interest for the amount that each quarterly payment was short of being its equal share.
It's easy enough to come up with an estimate of the investment income--it'll be about the same as last year's. (It's also pretty low, as the biggest chunks of investments are in IRAs.)
In a sense, it's easy to estimate the business income as well--I just averaged the last three months for which I have final data and then multiplied by 12 to get an annual value. The problem is, what if the actual results are wildly different from my estimate? If you underpay your estimated taxes, you have to pay interest on the underpayment.
Documenting irregular income
The IRS does make it clear that you didn't have to make quarterly tax payments on income until you receive it. But it also automatically calculates the interest that you owe, if your quarterly payments come up short.
After doing some research, I've found the place where you can document the uneven earning stream: It's Schedule AI of form 2210. (That happens to be the same form that you'd file to figure the penalty that you owe for underpayment.)
It's not something to undertake lightly: Schedule AI looks like a nightmare version of an ordinary tax form. There is, however, a weird kind of logic behind it. Basically, there are four columns for the four quarterly tax payment periods. For each quarter, you put in how much money you made that quarter (and some other stuff about your deductions and exemptions). You can then calculate how much tax you need to pay for that quarter. An additional source of confusion comes from the fact that you have to "catch up" to the current quarter if you receive more in later quarters than in earlier, but at least you don't have to pay taxes on money that you haven't gotten, nor do you have to pay interest for being late, as long as you make the payment for the quarter in which you received the money.
The IRS document with all this info is Publication 505 Tax Withholding and Estimated Tax. Check out the example for Schedule AI of form 2210.
If you expect your income to be relatively stable through the course of the year, you don't need to fiddle around with any of this, and you can just go with Form 1040-ES Estimated Tax for Individuals.
This being tax law, there are (of course) special rules if you make most of your money from farming or fishing.
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