Release record
| Specification owner | Bojan Marinkovic, Production Technician and digital publisher |
|---|---|
| Model date | August 16, 2026 |
| Execution | Calculations run in the visitor's browser. Calculator input values are not submitted to a MyCalcVault account or calculation server. |
| Numeric handling | Models retain JavaScript numeric precision during calculation and round only the displayed currency and percentage values. |
| Correction owner | Bojan reproduces submitted cases and is responsible for page-specific formula, wording, source, and accessibility corrections. |
Exact scope of the published models
| Tool | Core model | Outputs beyond the primary total |
|---|---|---|
| Mortgage | Fixed-rate amortization on price minus down payment; entered tax, insurance, HOA, and PMI are added to the monthly estimate; optional extra principal uses an accelerated schedule. | All-in outflow, payoff time, interest saved, rate/term cases, annual amortization, and CSV. |
| Auto loan | Price + estimated tax + financed fees + old-loan payoff − cash down − trade allowance − rebate, followed by amortization at the contract interest rate. | Trade-equity note, term comparison, extra-principal payoff, visible schedule, and CSV. Fees-inclusive APR is a separate comparison measure. |
| Savings | APY-equivalent monthly growth with beginning- or end-of-month deposits. | Required contribution for a goal, estimated goal time, yearly deposits and interest, and CSV. |
| Investment | Constant gross annual return minus entered annual fee, converted to an equivalent monthly rate; monthly contributions may grow annually. | Fee drag, inflation-adjusted value, lower/base/higher return cases, yearly schedule, and CSV. |
| Affordability | Take-home income − debt − non-housing spending − protected savings − chosen buffer. | Personal housing ceiling, gap to proposed all-in housing, and share of take-home pay. |
| Cash to close | Full down payment + estimated closing fees + prepaids + move/setup cash − paid deposits and documented closing credits. | Cash still needed, estimated closing-fee line, and reserve remaining from current cash after deposits already paid. |
| Rent versus buy | Growing rent cash flow compared with purchase cash, mortgage payments, ownership overhead, remaining balance, sale value, and selling cost. | Net modeled costs and three home-appreciation cases over the selected stay. |
Public reproducible test cases
These cases provide concrete expected values that a reader or developer can reproduce. The first four are deterministic regression checks for the shared loan and growth engines. The three housing rows are default page-contract checks repeated when those worksheets change.
Download the machine-readable regression fixture (JSON) for versioned inputs and unrounded expected outputs used in educational verification and software testing.
| Model check | Inputs | Expected result before or after display rounding |
|---|---|---|
| Fixed-payment loan | $300,000 principal; 6% annual rate; 360 payments | $1,798.651575 monthly before display rounding |
| Zero-rate boundary | $1,200 principal; 0% rate; 12 payments | $100.00 monthly |
| APY growth | $4,000 initial; $250 at month-end; 4.3% APY; 120 months | $43,331.761662 before display rounding; $34,000 deposited |
| No-fee investment baseline | $10,000 initial; $400 monthly; 7% gross return; 0% fee/inflation/contribution growth; 240 months | $241,711.395250 before display rounding; $106,000 contributed |
| Affordability default | $8,800 take-home; $450 debt; $3,550 spending; $1,300 savings; $600 buffer; $2,900 proposed housing | $2,900 ceiling; $0 gap; 33.0% housing share |
| Cash-to-close default | $400,000 price; $40,000 down; 3% closing estimate; $4,500 prepaids; $3,500 move/setup; $5,000 paid deposit credited at closing; $65,000 current cash after paying that deposit | $55,000 still needed; $12,000 closing estimate; $10,000 remaining |
| Rent-buy default | $2,400 rent; 3% rent growth; $400,000 price; $40,000 down; 6.4%, 30 years; five-year stay; displayed ownership assumptions | $152,903 rent cost; $140,831 net buy cost in the entered appreciation case |
Loan calculation specification
M = P × [r(1 + r)n] / [(1 + r)n - 1]
P is principal, r is the annual loan rate expressed as a decimal and divided by 12, and n is the scheduled number of monthly payments. At 0%, M = P / n.
The mortgage principal is purchase price minus down payment. Entered property tax and insurance are divided by 12; HOA and PMI are already monthly. Those costs are added to the scheduled payment but do not alter mortgage amortization. Extra principal reduces balance after the scheduled principal for each modeled month and stops when the balance reaches zero.
The auto worksheet first constructs an estimated financed balance. Its sales-tax estimate applies the entered tax rate to vehicle price; actual taxable bases differ by location. Trade allowance and old-loan payoff stay on separate lines so positive or negative equity remains visible. The buyer's order and loan contract replace that estimate when available.
The auto payment uses the contract's fixed annual interest rate. APR also includes certain borrowing fees, as explained in the CFPB auto-loan key terms; it must not be substituted automatically for the interest rate when financed fees are already in the modeled balance. The tool does not calculate a regulatory APR or reconcile prepaid finance charges with the disclosure's amount financed.
Savings and investment growth specification
i = (1 + annual rate)1/12 - 1
The savings rate is entered as APY. The investment model first calculates net annual return as gross return minus the entered annual fee, then converts that net assumption to a monthly equivalent.
For an end-of-month savings deposit, the existing balance grows first and the deposit is added afterward. A beginning-of-month deposit is added first and receives that month's modeled growth. The required-contribution result solves the ordinary or annuity-due future-value equation for the monthly contribution. Goal timing is found by advancing the same monthly model until the goal is reached, capped at 1,200 months.
The investment model applies any annual contribution-growth assumption at twelve-month boundaries. Its inflation-adjusted result divides the modeled future balance by (1 + inflation)years. The return path remains smooth and constant; it does not simulate volatility, sequence risk, taxes, withdrawals, or asset allocation changes.
Housing worksheet equations
| Affordability ceiling | take-home pay − required debt − non-housing spending − protected savings − selected buffer |
|---|---|
| Affordability gap | cash-flow ceiling − proposed all-in housing cost |
| Cash still needed | full down payment + (price × closing-fee rate) + prepaids + move/setup cash − paid deposits and verified closing credits |
| Cash remaining | current purchase cash after any deposits already paid − modeled cash still needed |
| Rent cost | sum of twelve monthly rent payments for each selected year, with the entered annual increase applied between years |
| Net buy cost | down payment + purchase closing cost + mortgage payments + entered recurring ownership costs − estimated net sale proceeds |
| Net sale proceeds | projected home value − entered selling cost − remaining mortgage balance |
Validation, rounding, and exports
- Inputs are converted to numbers and impossible combinations are rejected before a result is shown.
- Calculations retain available numeric precision through the complete monthly model.
- Monthly loan payments display cents; most long-horizon summaries display whole dollars; percentages display one decimal place.
- Amortization and milestone CSV files are generated from the unrounded schedule rows, not from values copied back out of the formatted page.
- Changing a result label or adding a field requires the visible explanation, model table, test case, and accessibility labels to remain consistent.
Boundaries that remain part of every result
Use these calculators to compare controlled scenarios: one rate versus another, a shorter term versus a longer term, or one recurring contribution versus another. Do not use a simplified output as a lender quote, account statement, tax calculation, or prediction of market performance.
The purpose of the methodology page is not to claim precision where precision is impossible. It is to show the assumptions clearly enough that a reader can decide whether the estimate is good enough for the question they are asking.
A result is suitable for an early planning comparison only when the excluded items are either irrelevant to that comparison or added separately. For a commitment or regulated disclosure, use figures from the relevant lender, financial institution, account provider, or qualified professional.
Correction protocol and change record
Bojan Marinkovic is the named person responsible for reproducing calculation reports. Send the page URL, every input, the result shown, the expected result, browser/device details if relevant, and the source or independent calculation used for comparison to contact@mycalcvault.com.
| Date | Change |
|---|---|
| September 6, 2026 | Corrected the affordability result to retain an existing non-housing deficit. The rent-versus-buy worksheet now rejects partial years so both paths cover the same period. Both worksheets require complete numeric inputs and hide stale results after an error. Clarified the auto-loan contract interest rate versus fees-inclusive APR, the current-cash basis for paid closing deposits, and the selectable savings contribution timing. Regression fixture version 2 records the new deficit and validation cases. |
| August 16, 2026 | Documented expanded mortgage and auto models, goal-based savings, fee/inflation investment scenarios, three interactive housing worksheets, public test cases, and page-specific correction ownership. |
| July 12, 2026 | Published the original fixed-loan, APY-consistent savings, and annual-effective investment formula notes. |
Primary technical and disclosure references
Primary U.S. reference for lender-provided mortgage terms, projected payments, and estimated cash to close.
CFPB: compare auto-loan offersOfficial guidance for comparing amount financed, APR, term, monthly payment, and total cost rather than payment alone.
CFPB Regulation DD: APY definitionDefines annual percentage yield as reflecting interest and compounding over a 365-day period.
Investor.gov: compound-interest calculatorOfficial comparison point for principal, recurring contributions, time, rate, and compounding.
Investor.gov: how fees affect a portfolioPrimary investor-education material explaining that ongoing fees reduce investment returns.
U.S. Bureau of Labor Statistics: CPI inflation calculatorOfficial context for interpreting changes in purchasing power; the site model uses the user's own constant inflation assumption.
Rate and timing notes
Both are treated as effective annual rates. Each calculator uses (1 + annual rate)^(1/12) - 1 so twelve modeled months compound to the entered annual rate.
No. Calculations retain the available numeric precision during the model and round only the values shown on the page.
Savings deposits follow the selected timing: beginning-of-month deposits are added before that month's growth, while end-of-month deposits are added afterward. Investment contributions are always added at month-end, after growth.