How these calculators are built
Penalties are unforgiving arithmetic on dates. A month that begins the day after a due date, a rate that applies only to its own quarter, a minimum that depends on the year the return was due. Get one of those wrong and the answer is wrong by hundreds of dollars. So every rule these tools use is a dated record: jurisdiction, return type, tax year, the dates it is effective, the rate, the method, the minimum and maximum, the exception, how the form rounds, the official document, its revision and the day a person last checked the figure against it. The results page prints the records your calculation used.
The rules are checked two ways before they reach a page. A separate file holds every figure typed again from the source document, and a build gate fails if the two disagree. A test suite then reproduces the worked examples the IRS and IDOR publish: the Internal Revenue Manual's $5,000 return filed three months late, the Form 2220 instructions' 56-day installment, IDOR's Penalty Peterson example on Form IL-2210, and stops the release if any of them differ.
What they will not do
They do not assume relief. Reasonable cause, first-time abatement, disaster postponements, waivers on Form 2210. Each is listed on the results page as something you can request, and none is applied unless the rule requires a fact you entered. They do not apply a rate the government has not published yet: a payoff date in a quarter with no announced rate stops the computation and says so. And they do not compute the annualized income methods; those are accepted as a choice and marked for professional review.
Privacy
There is no field for a name, Social Security number, employer identification number or account number, and nothing you enter leaves your browser or is kept after you close the page. Print, save as PDF, download the schedule as CSV or copy the summary. All of it works from the page in memory.