How We Calculate

Where the formulas come from

From the body that actually sets the rule, read directly rather than through somebody else's summary. These are the sources the live calculators rest on today:

• Consumer Financial Protection Bureau for how mortgages and loans work in practice: when private mortgage insurance can be removed, what an escrow account is, how a rate differs from an APR, and what a prepayment penalty is.
• myFICO for how credit enquiries are treated when you shop several lenders.
• State revenue departments for sales tax on a car. California's Department of Tax and Fee Administration and Washington's Department of Revenue set out opposite treatments of a trade-in, which is why the auto loan calculator has a toggle rather than a rule.
• Centers for Disease Control and Prevention for the adult BMI categories and for how BMI is handled under 20.
• World Health Organization and the National Heart, Lung, and Blood Institute at the NIH for what BMI does and does not measure, and for waist circumference thresholds.
• Peer-reviewed literature where a measure has no official home: BMI Prime is cited to its 2006 paper through PubMed, and the kg/m³ form of the Ponderal Index to a 2017 JAMA Pediatrics study.

The payment and amortization maths itself is the standard formula, the one lenders and finance courses use. We do not invent methods. Where a field has more than one accepted approach, the page names the one it uses, because two reasonable methods give different answers and you should know which you are looking at.

How it is verified

The calculation for every tool lives in its own module, away from the interface, so it can be tested directly. At the time of writing that is 404 automated tests across the formulas and the components, plus 96 browser tests that drive real Chrome. They run on every change and check four things.

• A published worked example. At least one figure that can be checked by hand against the source, so a silent change in the formula fails loudly.
• The awkward edge. The input that breaks the maths, such as a 0% interest rate, which makes the standard payment equation divide by zero.
• Internal consistency. Totals have to reconcile. Every payment of principal on a loan must add back up to the amount borrowed, to the cent, and the yearly rows must sum to the same figure as the monthly ones.
• Round-tripped exports. The browser tests download the PDF, spreadsheet, CSV and image, then open the files and read them back. Code that runs without error is not evidence that the file it wrote is right; a PDF once passed its checks while every money value in it was missing, because the check only looked at the first four bytes.

Precision, and why a lender's number can differ

Money is handled with decimal.js rather than ordinary floating point, because floating point cannot represent 0.1 exactly and the error compounds across three hundred and sixty payments. Each currency is rounded to its own minor unit, so yen round to whole yen and dollars to cents, and the schedule is summed from the rounded rows rather than from a running total. Otherwise twelve rows each half a cent out leave the year six cents short of its own total.

The last payment absorbs whatever rounding is left over, which is what lenders do too. Expect a lender's figure to differ from ours by a few cents on the final row. Larger gaps are not rounding: they come from fees, points, insurance, or a different day-count convention, and those are worth asking your lender about.

The assumptions, and where each one is stated

A calculator cannot avoid assuming things. It can avoid hiding them.

• PMI stops at 80%. The mortgage calculator charges private mortgage insurance until the balance falls to 80% of the home price, then stops. That is the CFPB's cancellation point, and the mortgage page says so beside the figure. Your servicer may apply it differently.
• Trade-ins are taxed by state. Most states tax the price after a trade-in is deducted; some tax the full price. The auto loan calculator carries a toggle, on by default, and the page names the states on each side with the revenue department that says so.
• Currency is display only. Changing the currency changes the symbol, the grouping and the number of decimals. It never converts. There are no exchange rates anywhere on this site, and 25,000 stays 25,000 whichever symbol sits in front of it.

What a calculator cannot know

It knows the numbers you typed. That is the whole of it. It does not know your credit score, the fees your county charges, what your lender adds at closing, or anything about your medical history. Those are exactly the things that move a real quote away from an estimate, which is why every finance and health page carries a disclaimer and why the answer here is a starting point for a conversation rather than the end of one.

What "Reviewed by" means

The line at the foot of a calculator page means one thing: on that date, Usman Ali read the page end to end, checked every number in it against the calculator's own output, and opened every source linked from it to confirm it still says what the page claims. It is not a medical, legal or accounting review. The line is there so you know who to hold responsible for the numbers, not to suggest a qualification behind them.

When a calculator is published

Only when its maths is tested, its page explains the method, its exports have been opened and read, and it works on a phone as well as a desktop. Anything still being built is not linked anywhere, so every link on this site leads to a finished tool.

Who maintains this

HarborsCalculators is built and maintained by Usman Ali. If you think a result is wrong, the contact page explains what to send, and corrections are handled as described in the editorial policy.