What this is
Plain Finance publishes free calculators for the money decisions people actually agonise over: whether to overpay a mortgage, how long a debt will take to clear, what a savings rate compounds into. No sign-up, no email capture, no "get your personalised report" funnel.
It exists because most financial calculators are frustrating in the same three ways. They hide their assumptions, so you cannot tell whether the answer applies to you. They produce a single number with no working, so you cannot check it. And they are frequently a lead-generation form wearing a calculator costume, where the real product is your phone number. This site is an attempt at the opposite.
How the calculators are built and checked
Every calculator here follows the same discipline, and it is worth explaining because it is the main thing that separates a trustworthy tool from a plausible-looking one.
- The math is separated from the interface. Each calculator's financial logic lives in its own module of pure functions with no connection to the page around it. That sounds like a technical detail, but it is what makes the next point possible.
- That math is unit tested. Every module ships with an automated test suite covering the ordinary path and the awkward edges: zero-interest loans, a lump sum larger than the remaining balance, a payment that never covers the interest, the final payment that has to be capped so you do not overpay.
- Results are cross-checked against known figures. The mortgage calculator is verified against published amortization tables and against exact, unrounded math. Where the two differ, the page says why.
- Money is rounded the way a lender rounds it. Amounts are rounded to the cent each month, and the final payment absorbs the rounding residual — which is what your servicer actually does. Skipping this makes a schedule drift several dollars away from your real statement over thirty years.
- Assumptions are stated, not buried. Each calculator says what it does not model. The mortgage payoff tool covers principal and interest only — not taxes, insurance, PMI or fees — and says so on the page rather than in a footnote.
You do not have to take any of that on faith. The site is open source: you can read the code and the tests on GitHub, including the exact worked example published on the mortgage page.
Who runs this site
Plain Finance is published by Luas Goods, an independent publisher. It is not owned by, funded by, or affiliated with any lender, broker, bank, mortgage servicer or other financial services company, and no such company has any input into what appears here.
We are not licensed financial advisers, and we do not present ourselves as any. What we are good at is getting the arithmetic right and showing our working — which is why the section above spends its time on how the calculators are tested rather than on credentials. Judge the tools on whether they match your lender's numbers, and read the what this site is not section below before acting on anything here.
You can reach us at luasgoods@gmail.com — for corrections, questions about a calculation, or anything else about the site.
How this site makes money
Advertising, and nothing else. The site displays ads served by Google AdSense. That is what pays for the calculators to be free, and it is the only revenue source: there are no affiliate links, no referral fees from lenders, and no paid placements.
This matters more than it might seem. A calculator funded by lender referrals has a quiet incentive to nudge you toward a product. This one does not — nobody pays us based on what you decide, so the tools have no reason to steer you. If that ever changes, it will be disclosed here and on the affected pages.
Ads are kept out of the way: never above a calculator, never between your inputs and your results, and never styled to look like part of the tool.
What this site is not
Nothing here is financial advice. These are calculators and general explanations, not a recommendation about your situation. They cannot see your income, your other debts, your tax position, your job security or your plans, and all of those change the right answer.
Use these tools to understand the shape of a decision and to ask your lender or a qualified financial professional better questions. Confirm any figure that matters with your servicer before acting on it — their payoff quote is authoritative, ours is an estimate.
Corrections
If you think a number here is wrong, please say so — luasgoods@gmail.com. Financial math has genuinely debatable edges — how interest accrues in a partial month, whether a year is 360 or 365 days, how a servicer applies an overpayment — and reasonable implementations differ.
Reports of an error are taken seriously and checked against the test suite. If a calculator is wrong, it gets fixed, and the fix is visible in the public commit history. If it is a difference of assumption rather than an error, the assumption gets documented on the page so the next person does not have to ask.
Privacy, briefly
The numbers you type never leave your browser — there is no server to send them to. The site sets no cookies of its own and runs no analytics. The full detail, including what our advertising provider and web host do collect, is in the privacy policy.