Methodology
Every number on FinanceTools.io comes from a documented method, and every documented method is tested before it ships. This page explains how we validate formulas and keep the interface, the content and the math consistent.
Single source of truth
For each calculation, one definition drives everything: the tool's interface, the formula shown on the page, the worked example, the frequently asked questions and this methodology. Because they all come from the same source, the interface never shows a number that contradicts the published content — and when the method changes, every place that references it changes together.
Formula validation
Every deterministic formula is checked with a Node-based test harness before it is released. Each test suite covers:
- Normal cases — realistic inputs within the intended range.
- Zero — inputs and results equal to zero.
- Boundaries — values at the edges of the supported ranges.
- Negative inputs — where a negative value is meaningful it is validated; where it is not, it is rejected or clamped with a clear message.
- Very large values — to confirm the calculation stays stable.
- Decimals — precision and rounding behave as documented.
- Input errors — invalid values produce clear errors instead of silent nonsense.
- Division by zero — guarded so the tool never returns an undefined result.
- Golden tests — known results from independent sources. For example, compound interest: 1,000 at 10% for 1 year with annual compounding must equal 1,100.
Review cadence
Formulas are re-reviewed on release. Every tool page shows a "Method reviewed <date>" line in its methodology section, so you can always see when the method was last checked. If a formula is changed, the review date is updated and the change is recorded in the changelog.
Approach
FinanceTools.io produces educational estimates, not financial advice. Fractional and parameterized models state their assumptions and limitations so you can judge what an estimate depends on. We do not fabricate data: when a live source is unavailable, a tool shows "unavailable" rather than inventing a value. Referral or affiliate status never changes a calculation, a ranking or a displayed result.