Calculator Methodology

A calculator is useful only when its assumptions are visible. This page explains the shared methods behind ToolGrym calculators. Each individual tool page then documents the formula, inputs, worked example, and limits specific to that calculation.

Inputs are assumptions, not predictions

ToolGrym separates values you know—such as a loan balance or monthly contribution—from values you must estimate, such as a future return, inflation rate, home appreciation, or rent growth. The result is a scenario calculated from those inputs, not a promise that markets, lenders, or tax rules will behave that way.

Timing and compounding

Loan tools normally convert an annual interest rate to a monthly rate and model payments at the end of each month. Investment tools state whether they use monthly or annual compounding and whether contributions occur monthly. Comparison tools place both choices on the same timeline so that cash used in one path remains available to the other path as an opportunity cost.

Percentages are entered as ordinary percentages: 6.5 means 6.5%, not 0.065. Currency results are displayed to the nearest dollar or cent for readability, while calculations retain full JavaScript numeric precision until the result is formatted.

What automated tests verify

Core formulas live in pure functions separate from the interface. Known-value tests pin those functions to reference examples, test zero-rate and boundary cases, and guard against regressions when a tool changes. The full test suite and registry validation run before the production build. Tests can establish that code follows the documented model; they cannot make uncertain inputs certain or replace the terms of a real financial product.

How to use a result responsibly

1. Start with values you can verify from statements, disclosures, or official documents.
2. Run a conservative, expected, and optimistic case for uncertain assumptions.
3. Notice which input changes the decision and whether a small change reverses it.
4. Compare the estimate with a lender, plan document, tax professional, or other qualified source before acting.

Privacy and saved scenarios

Calculations run in your browser. Inputs are not sent with analytics events. If you explicitly save a named scenario, its inputs are stored only in your browser's local storage so you can reopen it on that device. Copying a result link puts the chosen inputs in that URL; anyone you share the link with can read them.

Corrections and model changes

When a formula or assumption changes, we add or update a regression test, revise the explanatory content, and update the page's review date. Report a possible error to [email protected]. For sourcing and review rules, read the editorial policy; for the limits of educational estimates, read the financial disclaimer.