What happens if you filed your tax return on time (by April 18 this year) but did not include your tax payment?

If you realize the mistake, send in your payment to lessen penalties and interest. Otherwise, expect a letter from the IRS, and be sure to read it carefully and immediately — and, of course, review it with your accountant right away, as each tax situation is unique to the individual. The letter will ask you to send in a check for the taxes owed plus penalties and interest for failure to pay taxes. (Because you filed your return, you won’t be charged penalties for failure to file.)

Expect to receive the letter, called an initial tax assessment notice (CP14), in early-to-mid-June if you filed on time. “By law (Section 6303 of the Internal Revenue Code), [the IRS] must notify you of a tax assessment within 60 days,” an IRS spokesperson told me. Read more about CP14 at tinyurl.com/3ezx6cj4.

The penalty for failure to pay is 0.5% of the unpaid taxes for each month or part of a month that the tax remains unpaid. The penalty “won’t exceed 25% of your unpaid taxes,” quoting the IRS Failure to Pay Penalty webpage ( tinyurl.com/mtbec4c2).

Interest will also be assessed. The current quarterly interest rate (April through June) for underpayment of taxes is 7% ( tinyurl.com/mr49bmj4), and the rate is compounded daily. Importantly, if you receive a notice, “you will not be charged interest on the amount shown if you pay the amount owed in full on or before the ‘pay by’ date,” according to the IRS Interest webpage ( tinyurl.com/m58hbew4).

It is possible to have penalties waived in certain...