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.
The 50/30/20 rule, zero-based budgeting, how to track expenses, and why most budgets fail in the first 90 days.
How savings grow — the savings formula, compound vs simple interest, the three types of savings, and the power of automation.
The compound interest formula, the Rule of 72, real returns by asset class, and why starting early beats saving more.
The amortization formula, APR vs interest rate, fixed vs variable, origination fees, and how to compare loan offers.
Standard vs graduated vs IDR repayment, the amortization formula, SAVE plan, PSLF forgiveness, and your fastest payoff path.
The compound formula, employer match math, Roth vs Traditional, contribution limits, and how to model your retirement nest egg.
The avalanche vs snowball methods, amortization math, how to stack debts, and the real cost of minimum payments.
The breakeven formula, closing costs, rate-and-term vs cash-out, and how to decide if refinancing saves you money.
The 5-year rule, total cost of ownership, opportunity cost, closing costs, and why the math rarely matches the emotion.
The compound growth formula, Roth vs Traditional tax math, contribution limits, income phase-outs, and backdoor Roth.
The present value and future value formulas, ordinary vs annuity-due, and how retirement income streams are priced.
How bond pricing works — coupon yield, current yield, YTM, and why bond prices move opposite to interest rates.
How certificates of deposit compound, the early-withdrawal penalty, and whether a CD beats a savings account.
How FHA loans work — the 3.5% down payment, upfront and annual MIP, and who actually qualifies.
How much equity you can borrow, the 85% CLTV rule, HELOC vs home-equity loan, and the payoff risk.
How net present value and internal rate of return rank projects — discounting cash flows and reading what the % means.
How expense ratios and loads compound against you, the SEC yield, and why a 1% fee costs a fortune over 30 years.
How payroll withholding works — federal and state income tax, FICA, pre-tax deductions, and your real take-home.
How VA loans work — no down payment, no PMI, the funding fee, and the entitlement you can reuse.
The future value formula, lump-sum vs annuity growth, and how a one-time deposit compounds over decades.
How basal metabolic rate is estimated — the Mifflin-St Jeor and Harris-Benedict formulas and what "resting" really means.
How total daily energy expenditure is built from BMR and activity, and using it to set a deficit or surplus.
How a due date is estimated from LMP and ultrasound, trimester milestones, and the dating-window rules.
How 1RM is estimated from submaximal reps — the Epley and Brzycki formulas and why you shouldn't max out weekly.
How the Pythagorean theorem finds the hypotenuse, where it comes from, and the real-world distances it solves.
How the quadratic formula finds roots, the discriminant that predicts real vs complex answers, and worked examples.
Population vs sample standard deviation, the formulas, and how to read what the number tells you about a dataset.
The length × width formula, L-shaped and multi-room spaces, and converting square feet to square meters.
What a British Thermal Unit measures, the 20-BTU-per-sq-ft rule, and why oversizing a unit wastes money.
How watts become kilowatt-hours, the cost formula for any device, and where phantom load hides.