Interest starts on the shortfall from the day it was due and grows daily. Pay it as soon as you can, not at the next deadline.
No letter arrives and no fine is issued. The IRS charges interest on the missed amount from its due date, and the charge grows day by day until the money comes in. The total is figured quarter by quarter when you file, on Form 2210.
So pay it now, not at the next deadline. Every day of waiting adds interest. You do not need to wait for a quarterly date to send money, and the interest stops the day your payment arrives.
Doubling your next installment does not undo the miss. The catch-up money is credited when you pay it, so the earlier gap keeps accruing interest until then.
Two things can reduce the interest. If you or your spouse has a W-2 paycheck, extra withholding for the rest of the year counts as paid evenly across all four quarters, which can cover an early miss in full. And if your income arrived unevenly this year, the annualized income method may show that the quarter you missed required less than the even-quarters math assumes.
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Sources: IRS underpayment penalty· IRS Publication 505