DanfioAbout Danfio
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Danfio is a small, independent tool site. It exists for one reason: most online tools make you hand your numbers — or worse, your files — to a server, and when one shows you a result it rarely says where that result came from. We do the opposite. The tools run on your device, your numbers and documents never leave it, and where a tool produces a number the formula and assumptions behind it are on the page. There is nothing to sign up for.
Who is behind Danfio
Sean Yu is an independent developer and longtime web development educator. He spent over 20 years teaching and researching web development in higher education, and he now builds Danfio — a collection of clear, verifiable tools for English-speaking users worldwide, with a primary focus on the U.S., covering money, files, text, and everyday tasks. It is one person, working without a company, a funding round or a content team whose job is to invent pages.
The project started from a plain annoyance: most online calculators ask for your numbers and sometimes your documents, send them to a server, and reply with an answer that never says where it came from. Building this site was a way to fix both of those problems in one place, and the tools kept growing until every financial, file and everyday task that kept coming back around had a page here.
Not every task that could have a page gets one. A tool earns its place here on two tests: it has to answer a question a U.S. reader actually asks, and the answer has to be one the reader can check — the formula, the inputs and the source of each assumption are on the page rather than hidden behind a result. Anything that cannot pass both is left out, however much traffic the keyword might carry.
The inbox, the codebase and the sources library are run by the same pair of hands, which is why a correction mailed to contact@danfio.com reaches the person who can actually change the tool. It is also why every page is checked by an automated suite before it ships — a project this size cannot be trusted by habit, so the checks are mechanical: formulas, examples, links and spelling all fail the build if they drift.
The mission
The mission is narrow: a free calculator that shows its work and never uploads your numbers. Free matters because the one thing a budgeting tool needs — the real figures a person would rather not type anywhere public — is exactly what a paywalled or signup-gated tool drives away. Advertising is the chosen way to pay the hosting bill precisely because it lets the tools stay free and account-free. What Danfio will not do to grow is also part of the mission: it will not sell data, hide a result behind an email form, or quietly change a formula to flatter a page.
How the tools are built
The site is a static site. Every tool — a calculator, a converter, a generator or a file utility — is a small program that ships with the page and runs locally in your browser. There is no server that receives your numbers or your files, which is why a mortgage figure, a PDF or an image never leaves your device. A single shared engine powers every loan, debt and payoff calculation, so two financial tools can never quietly disagree about the same formula.
How the numbers are checked
- Formulas are implemented once, in one engine, and covered by automated tests.
- The worked example on each page is generated by the tool itself at build time, so the printed answer and the interactive answer are always the same computation.
- Defaults reflect mainstream published averages, and the source of each assumption is listed on the page.
- Where a default or a rule comes from an official agency — HUD's Single Family Housing Handbook, an IRS publication, the FICO education pages or a state child-support schedule — the site keeps a dated snapshot of that source page in its sources library, and a scheduled job re-fetches those pages to catch a figure that has gone stale.
Data sources and how often they are updated
Figures on the site fall into two groups. Formulas — how a mortgage payment or a payoff schedule is computed — come from published, public mathematics and hold the same way everywhere. Assumptions — a default interest rate, a tax bracket, a child-support schedule, a credit-score description — come from named agencies and are fenced in with their source on the tool page. For HUD, IRS, FICO and state materials the site keeps a snapshot of the source page at the time it was read, so a number on any tool page can be traced back to the document it came from.
Update frequency is stated where it matters: every tool page carries a “Last reviewed” date that is only advanced when a person has read the page again, guides carry theirs in the header, and the scheduled re-check re-asks each agency URL so a changed figure is caught instead of going stale. Annual figures — tax brackets, contribution limits, child-support schedules — are tied to the year they describe, and the page says which year that is.
How the site is funded
Danfio is independently built and maintained. We support the operational costs of this site through non-intrusive display advertising, and this does not influence our commitment to neutral, accurate, and transparent calculation tools.
When advertising runs, it pays the hosting bill. That is the whole business model: we do not sell data, we do not require an account, and we do not hide a result behind an email form. If a page ever earns an affiliate commission, it is disclosed on that page.
How the content is maintained
The numbers on these pages do not keep themselves current, and this site tries not to pretend otherwise. Every guide carries a "Last reviewed" date that is only changed when a person has actually read the page again, and the sources each tool points at are re-checked on a schedule, so a link that an agency has moved or taken down gets caught rather than left to rot.
Corrections work the same way the rest of the site does: openly. If a figure is wrong or an assumption has gone stale, tell us at contact@danfio.com or use the contact page, and the page is corrected and its review date updated so the fix is traceable.
Not professional advice
These tools produce estimates, conversions and documents for planning. They are not financial, tax, legal, medical or insurance advice. Before acting on a number, check it against your own documents and, where the stakes are high, talk to a qualified professional. See the disclaimer and the methodology page.
Questions, corrections or a tool you would like to see? Write to contact@danfio.com or use the contact page.