Methodology

Calculator philosophy

FundlyCalc calculators provide educational estimates from inputs you enter. They are tools for exploring “what if” math under documented assumptions — not a bank, lender, broker, insurer, or advisor, and not a substitute for professional judgment.

Each engine is a pure TypeScript function: deterministic, side-effect free, and free of React, browser APIs, and network calls. Formulas do not live inside UI components. Invalid input is rejected rather than silently corrected.

Before a calculator ships, we define inputs, outputs, methodology, assumptions, edge cases, rounding, and invalid-input behavior, then verify with automated tests against independently calculated examples.

Formulas

We do not invent financial formulas for shipping tools. Each calculator’s main formulas and worked behavior are documented on that calculator page (typically under a “How this is calculated” section) and must match the implementation.

Shared building blocks — such as fixed-rate amortization helpers — are reused rather than reimplemented inconsistently. Percentage inputs entered as percents (for example 6.5%) are converted to decimals (0.065) before use in rate formulas.

Browse available tools on the calculators index. Comparison pages under /compare use the same engines with labeled illustrative inputs.

Assumptions

Many outputs depend on assumptions you choose or that a page explicitly labels (for example an illustrative savings rate, a maximum combined LTV, or a constant return rate). Assumptions are not market quotes, lender offers, or guarantees.

We aim to distinguish:

• Estimated — outcomes that depend on assumptions or future values
• Calculated — values derived mathematically from the inputs provided under the stated model
• Assumption — user-selected or product-defined modeling choices
• Market data — externally sourced rates or statistics (only when a documented source exists)

The MVP uses USD and US-oriented terminology. Pages do not automatically apply other countries’ tax, interest, or regulatory conventions.

Rounding

Intermediate calculations keep available precision. We do not round intermediate steps unless a calculator’s documented methodology requires it.

Unless a specific calculator states otherwise:

• Monetary display values are rounded to two decimal places
• Percentages are displayed according to the UI context
• Months are shown as whole months where appropriate
• Years use precision appropriate to the output

Presentation rounding happens after calculation. Floating-point arithmetic has inherent numerical limitations; engines document their monetary strategy where it matters for interpretation.

External data policy

Current mortgage rates, HELOC rates, insurance premiums, state-specific loan rates, tax rates, and similar figures are external facts — not calculator formulas. We do not fabricate rates or market statistics to fill a page.

If external financial data is introduced on the site, it must include source, date, geographic scope, applicable period, and an update approach. Structured datasets live under documented data files with provenance. Missing required data is treated as a gap — not something to invent.

MVP calculators generally run on user-entered assumptions rather than live market feeds. When a page uses illustrative inputs for a worked example, those inputs are labeled as illustrative.

Limitations

Educational models omit many real-world factors. Depending on the tool, limitations may include fees not modeled, taxes, insurance, credit decisions, variable rates, lender overlays, program rules, or behavioral factors.

Outputs can differ from lender quotes, statements, underwriting results, or professional analyses. Calculators do not provide qualification decisions, product recommendations, guaranteed savings, or guaranteed returns.

For how estimates should be interpreted legally and practically, read the financial disclaimer. Learn more about the project on about.