Guides
These guides explain the formula behind a calculator, walk through one worked example, and show how to read the result without treating it as advice. They stay evergreen: we update a guide when the method changes, not when the news cycle does. Use the page to understand the model, then open the linked calculator and try your own figures.
House
How a mortgage payment is calculated
The principal-and-interest formula, what taxes and insurance do to the housing payment, and how to read the FinTools4U mortgage payment calculator.
3 min read
Read guide →How to read a mortgage amortization schedule
What each row of an amortization schedule means, why early payments are mostly interest, and how to compare two terms or rates.
3 min read
Read guide →How extra mortgage payments change the payoff
The fixed-rate payoff formula, what an extra principal payment does to interest and term, and how to read the FinTools4U mortgage payoff calculator.
9 min read
Read guide →When a mortgage refinance breaks even
How closing costs, a new rate, and a new term decide whether refinancing the balance you still owe actually saves money.
4 min read
Read guide →How a HELOC payment jumps after the interest-only period
The payment jump when the interest-only period ends, and the line left after the first mortgage.
8 min read
Read guide →Non-house debt
How a credit card payoff is calculated
Fixed minimum versus a higher payment, the interest gap, and how to read the FinTools4U credit card payoff calculator.
4 min read
Read guide →Debt snowball vs avalanche in plain English
How snowball and avalanche payoff work month by month, a worked example, and how to read the FinTools4U debt snowball calculator without picking a strategy for you.
4 min read
Read guide →Other guides
How e-commerce price and ROAS are calculated
The price that covers product, shipping, fees, returns, ads, and a margin, and the ROAS that price implies on the FinTools4U calculator.
8 min read
Read guide →How big an emergency fund should be
Months of essential spending, the gap still to save, and how to read the FinTools4U emergency fund calculator.
3 min read
Read guide →How a freelance hourly rate is calculated
Gross income from a net target, billable hours after time off and utilization, and how to read the FinTools4U freelance rate calculator.
7 min read
Read guide →How much home you can afford
Front-end and back-end ratios, the loan those ratios support, and how to read the FinTools4U home affordability calculator.
4 min read
Read guide →Inflation and real return
The Fisher equation, purchasing power, and how to read the FinTools4U inflation calculator without treating the rate as a forecast.
3 min read
Read guide →How rent versus buy is calculated
Net cost of owning and renting over a horizon you choose, what the optional rates change, and when the FinTools4U comparison is not enough.
9 min read
Read guide →How rental property ROI is calculated
Cash-on-cash, cap rate, and the rent-minus-mortgage cash flow the FinTools4U rental property calculator uses, with the default sketch worked in full.
7 min read
Read guide →How Roth versus a taxable account is calculated
Why the FinTools4U Roth and taxable balances match before tax, where the capital-gains gap comes from, and how to read the calculator.
7 min read
Read guide →How a savings goal is calculated
The future-value formula, the monthly deposit that hits a target, and how to read the FinTools4U savings goal calculator.
4 min read
Read guide →How house flip profit is calculated
The all-in house flip formula in plain English—purchase, rehab, holding, simple financing interest, and selling costs—with a worked example for the FinTools4U calculator.
7 min read
Read guide →Price versus SAC on a Brazilian home loan
How Tabela Price and SAC differ on payment, interest, and balance — and why this comparison is about Brazilian amortization, not a generic U.S. mortgage.
3 min read
Read guide →How compound interest actually works
The formula in plain English, a worked example, and how to read the FinTools4U compound interest calculator without treating the result as a forecast.
4 min read
Read guide →What Coast FIRE means in plain English
How the Coast FIRE number is calculated, a worked example, and how to read the FinTools4U Coast FIRE calculator without treating the projection as a plan.
4 min read
Read guide →From the guide to the calculator
Open the calculator hub and try the formulas with your own numbers.