Methodology
This page documents the formulas behind the calculators and the source of the default rate data. All calculations run client-side in your browser using the same open functions.
Rate data
Default interest rates come from Federal Reserve Economic Data (FRED), St. Louis Fed, retrieved automatically on a recurring schedule. The current snapshot was updated August 28, 2026:
- 30-year fixed mortgage: 6.35% (FRED series
MORTGAGE30US, as of August 28, 2026) - 15-year fixed mortgage: 5.58% (FRED series
MORTGAGE15US, as of August 28, 2026) - Federal funds effective rate: 4.33% (FRED series
FEDFUNDS, as of July 1, 2026) - 48-month new-car loan: 7.82% (FRED series
TERMCBAUTO48NS, as of May 1, 2026) - Bank prime loan rate: 7.5% (FRED series
DPRIME, as of August 27, 2026)
These are national averages. Your actual rate depends on credit score, loan-to-value, loan type, points, and lender. Always override the default with a real quote when you have one.
Core formulas
Amortized loan payment
The fixed monthly payment is P = L · r / (1 − (1 + r)^−n), where L is
the loan amount, r is the monthly rate (annual rate ÷ 12), and n is
the number of payments. A 0% rate falls back to L / n.
Amortization schedule
Each month: interest = balance · r; principal = payment − interest (plus any extra you enter);
balance = balance − principal. The schedule ends when the balance reaches zero, which is earlier
than n when extra principal is applied.
Break-even points
Refinance and discount-point break-evens divide the up-front cost by the monthly payment reduction. If the new payment isn't lower, there is no break-even.
Debt snowball / avalanche
A month-by-month simulation: accrue interest on each debt, pay every minimum, then direct all remaining budget to the target debt (smallest balance for snowball, highest APR for avalanche). Freed-up minimums roll into the budget as debts are cleared.
Real-estate metrics
- NOI = (annual rent + other income) − vacancy allowance − annual operating expenses. Excludes debt service, depreciation, and capital expenditure.
- Cap rate = NOI ÷ price.
- Cash-on-cash = annual pre-tax cash flow ÷ (down payment + closing costs + rehab).
- 70% rule MAO = ARV × rule% − repair costs − wholesale fee.
- BRRRR refinance loan = ARV × refinance LTV; cash left in deal = all-in cost − refinance loan (floored at zero).
Rent vs. buy
A simplified total-cost model over your holding period. Buying accrues mortgage interest, property tax, insurance, maintenance, HOA, closing and selling costs, and the opportunity cost of the down payment, offset by principal paydown and appreciation (equity at sale). Renting accrues rent grown annually. The model does not attempt to estimate income-tax effects, which vary widely by filer.
Rounding
Intermediate math is done at full precision; only displayed values are rounded. Headline dollar figures show whole dollars; payments and schedules show cents.
Changelog
Material formula changes and corrections will be listed here with dates.