About Harbors Calculators
Harbors Calculators is a set of free calculators that show their working. You get the answer, the formula behind it, the assumptions it rests on, and the full schedule you can take away as a PDF, an image, a spreadsheet or a CSV.
The problem with most calculators
Type your numbers in, press the button, and a figure appears. That figure is the end of the conversation. You cannot see which formula produced it, what it assumed about your taxes or your insurance, or what happens in year seven. If it disagrees with the number your lender gave you, there is no way to find out why.
We do the opposite. Every calculator here states its formula in plain terms, lists what it assumed, and shows the whole schedule payment by payment. A mortgage does not just give you $2,194.79 a month; it shows you the $357,125.12 of interest underneath it, the month the balance crosses zero, and every row in between. Download it and check it against whatever your bank sends you.
Who runs it
One person: Usman Ali, a web developer. He writes the formulas, the tests, the explanations and the code, and he is the one accountable when something turns out to be wrong.
Every formula is taken from a primary official source rather than from experience, each one carries unit tests, and the downloads are verified by opening the generated files and reading them back. The methodology sets out exactly how that works, including the parts our numbers cannot cover.
What is live today
4 calculators are finished and live. Nothing half-built is linked anywhere, so every link you find leads to a working tool with its own explanation, worked example and sources.
The list grows slowly on purpose. A calculator ships when its maths is tested and its page is written, not when the inputs render.
How it stays free
Advertising pays for it. Ads sit in fixed slots that are marked as ads and kept away from the results, and they have no say in what gets built or what any calculator returns. Your figures are worked out in your browser and never sent anywhere.
What this site will not do
It will not give you personal financial, tax or medical advice. A calculator knows the numbers you typed and nothing else: not your credit file, not your local fees, not your medical history. Use a result to work out what to ask, then take it to the lender, accountant or clinician who can see the rest of the picture.