Learn the math behind the tools
Plain-English explainers for the questions people search for — how compound interest works, the BMI formula, how sales tax is calculated, and more. Each guide links to the free tools you can use right away.
How Compound Interest Works
A plain-English guide to compound interest: the formula, compounding frequency, and why it grows money faster than simple interest.
HealthThe BMI Formula Explained
What BMI is, the exact metric and imperial formulas, the category ranges, and the known limits of BMI as a health measure.
FinanceHow Sales Tax Works
How US sales tax is calculated, why the rate varies by state and locality, and how to add tax to a price (or back it out).
FinanceMortgage Payoff: How Extra Payments Save You Money
Why even small extra principal payments cut years off a mortgage, how amortization front-loads interest, and how to estimate your savings.
MathHow to Calculate Percentages
The three percentage operations people actually need — finding a percent of a number, percent change, and the percentage one value is of another.
FinanceHow Currency Conversion Works
A plain-English guide to exchange rates: what a rate is, how to convert an amount, cross-rates through USD, and why the mid-market rate is not what you pay.
FinanceUnit Price Explained: How to Find the Best Value
Why the cheapest box is rarely the best value, how unit price (price per gram, ml, or item) is calculated, and how to compare products in different units.
ProductivityThe Pomodoro Technique Explained
How the Pomodoro time-management method works: 25-minute focus blocks, short breaks, a long break after 4 cycles, and the three rules that make it stick.
HealthSleep Cycles Explained: Why 7.5 Hours Can Beat 8
Why sleep runs in ~90-minute cycles, why waking at a cycle end feels better than mid-cycle, and how to work backward from a wake-up time to pick a bedtime.
FinanceFuel Cost Calculation: How to Estimate the Cost of a Trip
The simple fuel-cost formula (distance / efficiency × price), why mpg divides instead of multiplies, unit conversions, and multi-stop trips.
TimeHow a Date Countdown Works
A plain-English guide to date countdowns: subtracting two dates, breaking raw days into weeks and calendar months, and the overnight/timezone and leap-year details.
FinanceHow to Split a Bill (With Tax and Tip)
A plain-English guide to splitting a shared bill: the equal-split formula, whether to tip on the pre-tax subtotal or the taxed total, weighted splits, and cash round-up.
FinanceTime Card Calculation: How to Total Work Hours
A plain-English guide to totaling a time card: shift duration from clock-in/out times, overnight wraps, unpaid breaks, and weekly overtime.
HealthBlood Alcohol Content (BAC) Explained
A plain-English guide to BAC estimation: grams of ethanol from a drink, the Widmark formula over body water, and metabolism that lowers BAC over time.
FinanceCompound Interest Comparison: How to Compare Savings Plans
A plain-English guide to comparing compound-interest scenarios: the shared formula, what to vary, ordinary annuity vs annuity due, and effective APY.
FinanceHow Payroll Tax Works: From Gross Pay to Net Paycheck
A plain-English guide to what comes out of a paycheck: federal income tax brackets, FICA, state tax, pre-tax vs post-tax deductions, and how take-home pay is computed.
FinanceThe 4% Rule Explained: A Safe Retirement Withdrawal Rate
A plain-English guide to the 4% rule: where it comes from, why 4% is considered safe, how inflation adjustments work, and when it breaks.
FinanceExchange Rates Explained: What Moves a Currency's Value
A plain-English guide to exchange rates: floating vs fixed rates, what makes a currency rise or fall, real vs nominal rates, and purchasing power parity.
MathNumber Base Systems Explained: Binary, Hex, and Beyond
A plain-English guide to number bases: why we use base 10, how binary and hex work, positional notation, and why computers love base 2 and 16.
MathRandom Sampling Explained: Shuffles, Picks, and Fair Draws
A plain-English guide to random sampling: why randomness matters, the Fisher-Yates shuffle, sampling with and without replacement, and avoiding bias.