Editorial Policy & Calculator Methodology

Who writes this site

FirePlanIO is written and maintained by the FirePlanIO Editorial Team — a small, independent group of writers with backgrounds in personal finance journalism, spreadsheet modelling, and software engineering. We are not brokers, lenders, insurers, or registered investment advisers, and we do not sell financial products. Every article is written in-house in English; we do not publish machine-translated, syndicated, or scraped content.

How an article is produced

1. Question first. We start from a concrete reader question ("what does one extra mortgage payment a year actually save?") rather than a keyword.
2. Primary sources. Rates, amortization rules, withdrawal-rate research, historical index returns, and subscription list prices come from primary or official sources, not from other blogs.
3. Worked example. Every claim that involves money is demonstrated with a full calculation you can re-run in the matching calculator on this site.
4. Second read. A second editor checks the arithmetic, the assumptions stated, and that nothing reads as personalized advice.
5. Review cycle. Guides are reviewed at least annually, and immediately whenever a rule of thumb, tax threshold, or product price we cite changes. The review date is shown on each article.

Calculator methodology

All five tools run entirely in your browser in JavaScript. Nothing you type is sent to a server, stored, or shared. The models are deliberately transparent:

• Compound interest — future value of a lump sum plus periodic contributions, compounded at the frequency you select: FV = P(1 + r/n)^nt + PMT × [((1 + r/n)^nt − 1) ÷ (r/n)]. Returns are nominal and constant; real markets are not.
• Savings goal — solves the same equation for the contribution needed to reach a target by a chosen date, given a starting balance and an assumed return.
• Mortgage / loan payoff — standard monthly amortization, interest charged on the outstanding balance each period, with extra principal applied after the scheduled payment. Escrow, taxes, insurance, prepayment penalties, and fees are excluded.
• FIRE retirement — target portfolio = annual spending ÷ withdrawal rate (4% by default, editable), with the accumulation path projected from your savings rate and assumed real return. It does not model sequence-of-returns risk, taxes, or pensions.
• Subscription tracker — normalizes every recurring charge to a monthly and annual figure, then prices the long-run opportunity cost of that spending at an assumed investment return.

Limits you should assume

Every output is an estimate produced by a simplified model. Real outcomes are affected by taxes, inflation, fees, market volatility, lender-specific rules, and your own behaviour. Past returns never guarantee future results. FirePlanIO does not provide personalized financial, investment, legal, or tax advice; consult a certified professional before acting on any figure produced here.

Advertising and independence

FirePlanIO is free to use and is funded by display advertising. Advertisers have no input into what we publish, no advance sight of articles, and no influence over the calculators. We do not accept paid guest posts, paid links, or sponsored placements inside editorial content. Where an ad appears it is visually separated from the article and labelled by the ad network.

Corrections

If you find an error in a formula, a figure, or a source, email contact@fireplanio.com or use the contact page. We correct confirmed mistakes promptly and update the review date on the affected page.