Methodology & Accuracy
This page explains how MTGCalculator.net calculators are built, how their accuracy is verified, and what their limitations are. It exists so you can judge for yourself how much to trust a result — and so that "the calculator said so" never has to be taken on faith.
How MTGCalculator.net calculators are implemented
Every calculator's mathematics lives in small, pure functions written in TypeScript, separate from the page and interface code. Each function declares exactly what inputs it accepts, validates them (rejecting non-numeric, infinite, or out-of-range values), and returns either a typed result or an explicit, human-readable error. The interface layer only formats and displays what the engine returns — it never does its own arithmetic. Calculations run entirely in your browser; your inputs are never sent to a server.
Why calculation functions are tested
Separating the math from the interface makes it independently testable, and we use that: an automated test suite runs the engine against standard cases, decimals, negative numbers, zeros, division-by-zero attempts, very large and very small values, and floating-point corner cases before any change ships. Edge cases that once caused a bug become permanent tests, so a fixed problem cannot quietly return.
The worked examples printed on tool pages are computed by the same engine when the page is built. If the copy and the code ever disagreed, the site would fail to build rather than publish inconsistent numbers.
The amortization math we use
Payments use the standard amortized-loan (PMT) formula with monthly compounding, the convention behind US mortgages and most consumer loans. A fixed monthly payment covers the interest accrued on the balance that month, and the remainder reduces the principal; the schedule is computed one month at a time so extra payments and payoff dates are exact. Affordability uses the 28/36 debt-to-income guidelines, and property tax that scales with the home price is solved with a closed-form rearrangement rather than an approximation. The engine is validated against known reference figures — for example, a $200,000 loan at 6% over 30 years yields the industry-standard $1,199.10 payment and $231,676 of total interest.
How rounding is handled
- Internally, every calculation keeps the full precision of IEEE-754 double-precision arithmetic — nothing is rounded between steps, so month-by-month schedules accumulate accurately across hundreds of payments.
- On screen, money is rounded to the cent (or to whole dollars for large totals like lifetime interest), which is how lenders present figures. Percentages such as loan-to-value are shown to a small number of decimals.
The practical consequence: a displayed monthly payment of $1,199.10 is the rounded view
of a more precise internal value. Your lender's figure may differ by a few cents depending on their
rounding and day-count conventions — the difference is presentational, not an error.
Limitations to keep in mind
- The calculators model fixed-rate, fully amortizing loans. They do not model adjustable-rate mortgages, interest-only periods, balloon payments, or negative amortization.
- Property tax, insurance, PMI, and HOA are estimates from the rates and amounts you enter. Real figures vary by locality, reassessment, coverage, and lender, and PMI removal timing depends on your specific loan program.
- Affordability reflects general lender guidelines, not an underwriting decision. Actual approval depends on credit, employment history, the loan program, and lender-specific overlays.
- A correct calculation can still be the wrong calculation for your situation — choosing the right loan for a real-world decision is judgment we cannot automate.
For consequential financial decisions, verify results independently and consult a licensed mortgage professional. These tools are educational estimates, not financial advice.
Reporting a suspected issue
If a MTGCalculator.net result looks wrong, we want to know. Send the tool name, the exact inputs, the result you saw, and the result you expected via the contact page. Confirmed issues are fixed in the engine and locked in with a new automated test.