Plain-language guides on how the numbers behind your daily life actually work — mortgages, loans, investments, and more. Each guide pairs with a free calculator so you can put the theory into practice.
The amortization formula, the three levers that move your payment, PMI and escrow, and how extra payments save you tens of thousands over the life of a loan.
How compound interest grows your money exponentially — the formula, the Rule of 72, and why starting early beats saving more.
The amortization formula behind every installment loan, what moves your monthly payment, total interest, and how early payoff saves money.
The amortization math for car loans, APR vs. interest rate, the long-term trap, and how depreciation creates negative equity.
How daily-compounding interest and minimum payments trap you, plus the avalanche vs. snowball payoff strategies.
How savings compound to a nest egg, the 4% safe withdrawal rate, inflation erosion, and sequence-of-returns risk.
Progressive marginal brackets, the standard deduction, credits vs. deductions, and how take-home pay is derived.
Convert between hourly and annual salary, account for overtime, and bridge the gap between gross and take-home pay.
Standard tip percentages, how to split a bill fairly, and tipping etiquette around the world.
How to add tax to a subtotal, extract tax from a total, and why US state rates vary widely.
The BMI formula, healthy weight ranges, and why BMI can mislead athletes and older adults.
How daily calorie needs are calculated — BMR via Mifflin-St Jeor, activity multipliers, and building a sustainable deficit.
The US Navy tape-measure formula, healthy ranges by age and sex, and why body fat beats BMI for fitness tracking.
The five percentage calculations — X% of Y, % of a total, % change, increase, decrease — with worked examples and common pitfalls.
How a scientific calculator evaluates expressions — PEMDAS, trig and log functions, degrees vs. radians, and scientific notation.
How age is calculated in years, months, and days — leap years, calendar quirks, and why simple subtraction isn't enough.
Date arithmetic — adding/subtracting days, counting days between two dates, and handling business days.
The factor-label method, common conversion factors, and why temperature conversions need a formula not a multiplier.
The 4.0 scale, credit-hour weighting, weighted vs. unweighted GPA, and converting letter grades.
What makes a password strong — entropy, character-set size, dictionary attacks, and actual crack times.