From my colleague superuser hbl3973:
Our
income tax system is a "pay as you go" system. This means that as you
earn income, you should be paying an appropriate amount of income tax
along with it. For those with just one job and no other significant
income, payroll withholding handles this automatically. However, if
you, for example, work two jobs, often not enough tax is withheld. This
is because tax rate percentages increase with increasing income, the
so-called tax brackets. Each employer only withholds at the lower rate
for the one job they see, leading to a shortfall.
For 2018, if
your payroll withholding totaled less that 80% of the calculated
federal tax (after subtracting various credits), then you are considered
significantly underpaid and Form 2210 kicks in to calculate extra
penalty interest for the periods of time during which you were
significantly underpaid. This penalty underpayment is calculated using a
target of 90% of the federal tax. (I know, why 90% when 80% is the
threshold? That's because the legal threshold has always been 90% but
the new tax law changes caused so many problems that the IRS cut it
first to 85% and then further to 80% just for 2018.)
I strongly
suspect your Form 2210 is there purely to invoke the 80% exemption from
penalty interest. Just look to see if the 80% box was checked. If so, you do not need to do anything.
