What you actually have to pay
You are not required to predict this year's income perfectly. The rules give you a safe harbor: pay a set amount based on last year's return — a number you already know — and the penalty cannot touch you, no matter how much this year grows.
The 110% cliff
Cross $150,000 of prior-year AGI and your safe harbor rises from 100% to 110% of last year's tax. Most people who cross that line for the first time — after a good year, a practice sale, a liquidity event — keep paying the old number and get a penalty for it. If last year was unusually large, the 90%-of-current-year route is often far cheaper, which is exactly what the calculator above compares.
What people ask
Is the safe harbor really a guarantee?
For the federal underpayment penalty, yes — meet it and the penalty does not apply, even if you end up owing a great deal more in April. It protects you from the penalty, not from the bill. You still owe the balance by the April deadline.
Does withholding count toward it?
Yes, and it has a useful quirk: withholding is treated as paid evenly across the year regardless of when it actually happened. That means a large withholding late in the year can repair an earlier shortfall in a way a late estimated payment cannot. It is a genuinely useful lever if you catch the problem in Q4.
What if my income is lumpy?
If a large share of your income arrives in one part of the year — a sale, a bonus, a distribution — the annualized income installment method can reduce or eliminate penalties on the earlier quarters. It is more work, and it is frequently worth it. This is a conversation, not a calculator.
Do states work the same way?
Broadly, but not identically. Illinois and most states run their own estimated-payment rules and their own safe harbours. If you file in several states, the federal number above is the start of the answer rather than the whole of it.