October 31: reserve, independent offers and equal-budget housing
This release adds three software-assisted calculation studies, an interactive reserve planner, a two-offer comparison and an optional equal-budget investment layer for renting versus buying. Automated checks are not an independent professional financial review or a study of real users. No such review or user-test results are claimed.
Emergency reserve specification
E is the sum of seven non-overlapping monthly essential-cost categories. Target = E × runway + immediate shock allowance. Funding gap = max(0, target − current cash); surplus is reported separately. A monthly projection uses constant APY converted to a monthly effective rate, with deposits at month-end. Goal timing checks complete months up to 1,200; a positive unfunded gap without sufficient growth/deposits is “not reached,” not a numeric infinity. Required deposits use the ordinary-annuity formula for the selected whole-month deadline. Taxes, fees, withdrawals, changing expenses and changing rates are omitted.
Two independent car offers
Each offer has its own price, tax amount, financed fees, upfront fees, down payment, trade allowance, old-loan payoff, rebate, contract rate and term. Borrowed balance = price + tax + financed fees + payoff − down − trade − rebate. Total cash paid = scheduled loan payments + cash down + unfinanced fees. The value of the traded asset, insurance, operating costs and resale are excluded. APR is not calculated; compare the lender’s disclosure separately.
Equal-budget housing accounting
- The renter begins with the down payment plus purchase closing costs invested. The buyer uses that same cash to buy.
- Mortgage principal amortizes monthly. Rent and entered ownership overhead grow at yearly boundaries. PMI applies for exactly the entered number of months. Renter insurance is flat.
- Whichever housing path costs less each month invests that positive cash difference at month-end. Both portfolios use (1 + entered net annual return)^(1/12) − 1. Existing balances grow before deposits.
- At each year-end, buyer wealth = home value − sale cost − loan balance + buyer portfolio. Renter wealth = renter portfolio. Negative sale equity is retained. Common household assets and expenses are omitted.
- At 0% investment growth, buyer wealth minus renter wealth must equal rent costs minus net buying costs. This reconciliation is tested across multiple horizons and appreciation assumptions.
The return is a hypothetical net-of-fees-and-taxes input, not tax-account modeling or a promised yield. Cash-cost and wealth results are separate. Utilities differences, renovations, moving costs, tax deductions, volatility and return sequence are not modeled. First favorable year means the first positive tested year-end, not a permanent or precise monthly break-even point.
Reproducible release checks
| Case | Expected output |
|---|---|
| Reserve: $3,800 essentials, 6 months, $2,500 shock, $5,000 cash, $500 monthly at 0% | $25,300 target; $20,300 gap; 41 deposits; $563.89/month for a 36-month deadline. |
| $30,000 vehicle, $3,000 down, $500 financed fees, 6%, 48 months | $27,500 borrowed; $645.84 monthly; $34,000.24 loan payments plus down. |
| Housing article, year five, 4% net investment growth | Renter portfolio $102,488.25; buyer equity $99,277.79; renting ahead $3,210.45. |
Download full public study inputs and expected outputs (JSON). Read the pure calculation source. These are public editorial examples, never submitted user information.
Voluntary browser handoff
The home-buying sequence transfers only numeric inputs and compatible derived totals after a user chooses a Carry button. The next page requires a separate Use action. Data stays in same-tab sessionStorage, expires after one hour and can be cleared immediately. Required housing costs exclude optional extra principal; users must add utilities and maintenance themselves. A negative post-purchase cash balance is warned about rather than transferred as negative emergency savings.
November 1: closing cash and moving cash are separate
The cash-to-close worksheet now reports the modeled settlement cash separately from the moving/setup budget. Its default is $51,500 due at closing plus $3,500 moving/setup, for an unchanged $55,000 overall budget and $10,000 reserve. Excess entered closing credits are not treated as cash refunds and cannot offset moving expenses. The optional handoff uses the same accounting. Tests cover the default, excess credits, a remaining-cash shortfall and blank fields. This is not a calculation of regulatory credit limits or final settlement figures.
November 3: compound-interest lab and reverse auto loans
The compound-interest comparison applies the same numeric annual rate as APY and as a nominal rate compounded monthly, with both start- and end-of-month deposits. This produces four scenarios, not four equivalent quotes. The contribution term alone receives the extra month of growth for month-start deposits. At 0%, every case equals starting balance plus deposits. No taxes, fees or variable returns are modeled.
The reverse auto tool inverts the monthly annuity formula, then solves for price after entered tax, fees, down payment, trade allowance, payoff and rebate. Tax applies to the full vehicle price, excluding financed fees; local rules may differ. Nonpositive vehicle prices are rejected. It is not an underwriting or household-affordability model.
Automated checks compare compound formulas with an independent month-by-month cash-flow simulation; reverse results are fed into the forward loan formula. Checks include 0%, tiny rates, fractional-period rejection, negative trade equity and impossible purchases. Public examples: $5,000 plus $250/month for 10 years at 0% equals $35,000 in all four compound cases; $500/month for 60 months at 0%, $3,000 down and no other costs implies a $30,000 loan and $33,000 price. Source: readable calculation models. These software checks are not independent professional financial review or assistive-technology user testing.
Release record
| Specification owner | Bojan Marinkovic, Production Technician and digital publisher |
|---|---|
| Model release | October 31, 2026; original baseline August 16, with page-specific corrections recorded below. |
| 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, CSV and a separate two-offer comparison with independent prices, fees, rates and terms. 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 | Cash before housing is take-home income − debt − non-housing spending − protected savings − chosen buffer. The displayed housing ceiling cannot be negative; the surplus or shortfall retains any existing deficit. | Personal housing ceiling, complete surplus or shortfall after proposed all-in housing, and share of take-home pay. |
| Cash to close | Closing cash = max(0, down payment + estimated fees + prepaids − documented deposits/credits); moving/setup is added separately. Excess credits do not fund moving. | Cash still needed, estimated closing-fee line, and reserve remaining from current cash after deposits already paid. |
| Emergency reserve | Essential monthly costs × chosen runway + immediate shock allowance. | Funding gap, surplus, modeled months to target and required monthly saving for a chosen deadline. |
| Rent versus buy | Growing rent cash flow compared with purchase cash, mortgage payments, ownership overhead, remaining balance, sale value, and selling cost. | Net modeled cash costs, optional symmetric investment portfolios, yearly wealth outcomes and three home-appreciation cases. PMI duration, renter insurance and ownership-cost growth are explicit inputs. |
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
| Cash before housing | take-home pay − required debt − non-housing spending − protected savings − selected buffer; this can be negative |
|---|---|
| Personal housing ceiling | max(0, cash before housing) |
| Surplus or shortfall | cash before housing − proposed all-in housing cost; use the raw amount, not the zero-limited housing ceiling |
| Cash still needed | max(0, full down payment + price × closing-fee rate + prepaids − documented deposits/credits) + separate move/setup cash |
| 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.
For example, if non-housing commitments already exceed take-home pay by $500, the housing ceiling is $0. Adding a proposed $800 housing payment produces a $1,300 shortfall, not an $800 shortfall. Zero-limiting the ceiling must never erase a deficit from the cash-flow comparison.
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 |
|---|---|
| October 28, 2026 | Corrected the written affordability equations to distinguish raw cash before housing from the zero-limited ceiling; the underlying deficit calculation is unchanged. Extended complete-number validation to all seven calculators. Added an explicit stale-result warning, disabled exports until recalculation, and local copy/reset controls so inputs stay attached to their results. Visual updates alone do not change article review dates. |
| 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.