How the calculator works
The failure-to-file penalty is 5 percent of the unpaid tax for each month or part of a month the return is late, capped at 25 percent, reduced by the failure-to-pay penalty in months where both apply. If the return is more than 60 days late the penalty is at least the smaller of $525 (returns due in 2026; the 2027 figure is published in the form instructions) or 100 percent of the unpaid tax. The failure-to-pay penalty is 0.5 percent per month or part of a month, capped at 25 percent. Interest is the quarterly federal underpayment rate compounded daily on the unpaid tax and on penalties as they accrue; this estimate applies it to the tax only, so the real figure is slightly higher.
With a valid extension the failure-to-file penalty starts from the extended due date instead. The failure-to-pay penalty and interest are unchanged, which is the whole point of the comparison box.
This is an estimate for planning. The IRS computes the exact amount on the notice.