At worst you’ll pay a penalty that is based on the interest on the amount that you should have paid over the time you didn’t pay it. If you make payments to catch up, you’ll avoid future penalties, so you’ll only have a few months of interest-based penalties, so a very small percent that you’re underpaid. Another possibility is that if you have a regular job, that you have more withheld from your pay there. The IRS does not have any idea when they receive the withholding that’s eventually credited it to you when they receive your employer’s W-3 and W-2 packet that says how to assign all the withholding that the company has paid in and reconciled on their 941s throughout the year. They assume that it’s paid in evenly throughout the year, because they can’t know otherwise. If you have income that varies during the year with more at the end, you can adjust the numbers on the penalty form (2210 I think) that show your actual income by quarter as well as the actual withholding by quarter that might be more in your favor.
If you pay via estimated payments, the IRS knows when you paid them, and will credit it to your account as of the date of the payment made (they don’t care when they receive it, only when you send it in). When you file your return, there is a calculation of how much you should have paid in every quarter based on your total income, but you can claim that your income was earned unevenly throughout the year, and that can lower the penalty by lowering the amount that you were required to pay in for the first three quarters, assuming you paid in enough overall.
There are two ways that might be used to determine the amount that you need to have paid in as of each estimated payment date. You only need to pay whichever is the least. These are:
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Based on your previous tax return. You need to pay in at least 100% of your previously year’s total tax, or at least 110% of your previous year’s tax if your income was over $150,000 (or $75,000 if married filing separately, but NOT single).
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Based on your current year’s tax due. You need to have paid in 90% of your current year’s tax, or if you don’t pay that much in tax, $1,000 less than the amount you owe.
In terms of what you need to have paid in each quarter, the amount needed to be paid in is generally evenly per quarter. However, this interacts a bit strangely if your income varies throughout the year, especially because the estimated payment dates are not evenly spaced. So the way that you calculate whether you owe a penalty is based on your income and deductions as of certain dates, and your income is annualized as of those dates, which are not evenly spaced through the year. So for the June 15 deadline, you take the information for the year through May, and multiply it by 12/5 (rather than 2) since only 5 months have elapsed. But based on the shortfall for the year that you calculate, you’ll need to have paid in 50% of that shortfall (well, 45%, because you only need to pay in 90% total, but I generally suggest clients pay in ignoring the 90% current year rule as a safety measure for forgetting about some things) between your first and second quarter estimate. Similarly, for the September 15 deadline, you determine your income through August and multiply it by 12/8 = 3/2 = 1.5 (rather than by 12/9 = 4/3) and then pay in an amount so that you’ll have paid in 75% (or 67.5% if applying the 90% rule) of your projected shortfall between the first three quarterly payment.
The “shortfall” mentioned above is the amount that withholding you’ll have for the quarter is less than the required payments as mentioned earlier. As mentioned, the amount of withholding you’ll be credited will generally be 25% of your total withholding per estimated payment due date, despite the dates not being evenly spaced throughout the year, though you can claim withholding was actually withheld earlier in the year on the 2210 form.