People who are self-employed are required to send in quarterly estimated tax payments.  But if your income varies from month to month or year to year, it's tough to determine the amount of taxes to pay. You don't want to send in so much that your daily expenses suffer or so little that you get a shocking tax bill at tax return time, not to mention underpayment penalties.

If you follow one or more of these tips for calculating your quarterly estimated tax payments, you'll minimize your chances of incurring penalties or breaking your budget.

Base Your Payments on Last Year's Earnings

You can avoid paying a penalty by paying at least the same amount as you did last year if you were self-employed then as well.

Key Takeaways

  • Open separate checking and credit card accounts. They're an easy reference for your business expenses and income.
  • File your quarterly payments and, if anything, overpay a little. You'll get it back.
  • The first year, get advice from a self-employed friend, an accountant, or the IRS helpline. You'll soon learn the ropes.

You can find the total taxes you paid on last year's tax return. Just divide last year's taxes into four equal payments and send each of them in by the IRS's quarterly due dates: April 15th, June 15th, September 15th, and January 15th.

You will still be liable for differences in taxes between this year and the last, but you won't pay an underpayment penalty.

For example, let's say you paid $4,000 in taxes last year. You send in four equal payments of $1,000 this year. You calculate your taxes at the end of this year at $5,500. You can send the IRS a check for the $1,500 difference without paying a penalty.

You should note that if you weren't self-employed in the prior year, you won't incur a penalty for not making any payments before tax return time. You can choose how much to send in throughout the year, but beware that you could have a large tax bill at the end of the year if you don't send in enough.

Get Advice the First Year

During the first year of self-employment, talk to a friend who's self-employed or hire an accountant to handle your taxes for you.

Either way, you can figure out how much to pay, determine what expenses you can deduct, and get comfortable with the process.

Use Separate Accounts for Business Expenses

It will simplify your life and make it easier to estimate your quarterly taxes owed. Open a separate bank account and a credit card account reserved for business expenses only.

These provide records of your business expenses for easy reference come tax time. Much easier than wading through a pile of paper receipts when you're doing your taxes. Much easier to check back through if a question about expenses comes up.

Keep a Running Tally of Your Income

You could get sticker shock next April if you don't keep a running tally of your income and pay your estimated tax accordingly.

Calculate your income at the end of each quarter and use this as a basis for whether you should increase or lower your quarterly payments.

If you use that dedicated checking account for your business income deposits, a running tally will take a couple of minutes to check in your online bank records.

Use the IRS's 1040ES Worksheet

If your income varies widely from year to year, the best way to estimate your quarterly payments is to use the 1040ES worksheet from the IRS. The worksheet guides you through calculating your expected tax liability and takes into account certain deductions you may qualify to claim.

The penalty rate for underpayment is the federal short-term funds rate plus 3%.

The worksheet can be filled out every quarter, or at least every quarter that represents an unusual change in your income. If your business is going gangbusters in the second quarter, you'll need to adjust your estimated tax payment to avoid a big surprise next April.

Always Overestimate, At Least a Little

Tax penalties can be pricey, depending on how much you underestimated your taxes due.

Interest is charged on the amount you underpay from the day your quarterly payment is due until the day it's paid. The interest rate is the federal short-term rate plus 3% and is announced quarterly. So, if you underpay for the first quarter of a tax year, you could owe a different amount than if you underpaid for the third quarter.

If you're unsure of the exact amount to pay each quarter, slightly overestimate your taxes. You won't lose any money. You'll get it back as a refund.

Put the IRS Tax Help Line on Speed Dial

The IRS free helpline is your best source for answering any question you may have: The number is 800-829-1040.

Unless you hire someone to do your taxes for you, you are your own accounting department. Unless you actually are an accountant, you may well have some questions, at least during the first year of self-employment.