What happens if I miss a quarterly estimated tax payment deadline?

Missing a quarterly deadline doesn't trigger a flat fine, but the IRS charges an underpayment penalty that works like interest, calculated on the shortfall from the day the payment was due until you pay it or file. The rate is tied to the federal short-term rate and has been meaningfully higher in recent years than it was in the low-rate era, so the cost has grown. The fix is to pay as soon as possible to stop the interest clock — don't wait for the next quarter. You can pay instantly through IRS Direct Pay or EFTPS. If you underpaid only because of an unusual event, you may reduce the penalty using the annualized method on Form 2210. Verify the current penalty rate on irs.gov, since the IRS resets it quarterly.