Every spring I talk to at least one owner who paid their entire tax bill on time and still got a bill from the IRS for interest.
They are not wrong to be annoyed. The rule is not about how much you paid. It is about when you paid it.
Employees never see this. Money comes out of every paycheck, automatically, and the IRS treats withholding as if it had been paid evenly across the year. Nobody hands an employee a form in March and says good luck. Owners are on their own, and the IRS expects four payments: April 15, June 15, September 15, and January 15 of the following year.
The test is quarterly, not annual
Miss a quarter and interest runs on that shortfall from the day after the due date until the day you cover it. The rate is the federal short-term rate plus three points, compounded daily. Right now that is 7%. It is not a penalty you can get waived for being forgetful, and first-time abatement does not apply to it. For most people the underpayment charge is simply interest with a different name printed on the notice.
Here is the part that surprises owners. Say you needed $8,000 in by April and you sent $3,000. It does not matter that your June and September payments were generous. Interest ran on that $5,000 from April 16 until you caught up, even though the year's total was paid in full by January.
Four equal payments are simply the wrong shape for a business that earns most of its money in six months. I watched a Tucson landscaper get nicked for this three years running while his annual bill was completely covered.
The two harbors
There are two safe harbors, and hitting either one makes the underpayment charge disappear entirely.
The first is 100% of last year's tax. That number is sitting on line 24 of the return you already filed. It is fixed, it is knowable in January, and you can divide it by four, automate the payments, and stop thinking about it. If your adjusted gross income last year was over $150,000, the target is 110% instead. That is the version to use in a growth year, because you are allowed to owe the extra tax in April. You are not allowed to owe it early.
The second is 90% of this year's tax. Harder to hit, since you are guessing at a number you will not know until December. It is the right call when the year is down and the prior-year harbor would mean handing the IRS more than you need to.
And if you expect to owe less than $1,000 after withholding, there is nothing to do at all.
Withholding is the better lever
This is the piece owners underuse. Withholding counts as paid evenly across the year no matter when it actually comes out of a paycheck. An S-corp owner who realizes in November that the year went better than expected can raise W-2 withholding on the December payroll and have the whole amount credited as though it had been paid in April. An estimated payment sent that same week does not get that treatment. It counts on the day it arrives. That one difference has saved clients more money than any investment I could have recommended to them.
What to actually do
Pick a harbor in the spring. Write the four dates down. Automate the vouchers so nobody has to remember them. Then, in January, before that last payment goes out, look at where the year really landed and adjust one way or the other. Most years that final look is worth a few hundred dollars of interest. Some years it is worth several thousand.
The short version: estimated taxes reward the owner who decides in April over the owner who decides in April of the following year. If you have received the same interest notice twice, that is not bad luck. It is a system you have not set up yet.