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Guides
Plain-language explanations of the math and ideas behind our tools. Each guide pairs with a calculator or converter so you can try the concept immediately.
How to Calculate Percentages (With Worked Examples)
Learn the three percentage calculations you'll actually use — percent of a number, what percent one number is of another, and percentage change — with step-by-step examples.
Read guideHow Compound Interest Works (And Why Starting Early Matters)
Compound interest is interest earned on interest. Learn the formula, see how small monthly contributions grow over decades, and understand why time matters more than amount.
Read guideUnderstanding Loan Payments: Principal, Interest, and Amortization
Learn how monthly loan payments are calculated, why early payments are mostly interest, and how extra payments can save you thousands over the life of a loan.
Read guideWhat Is BMI? What It Measures, Its Limits, and How to Read Your Number
BMI is a quick screening tool based on height and weight. Learn the formula, the official categories, and — just as important — what BMI does not tell you about your health.
Read guideWord Count Guide: How Long Should Your Essay, Blog Post, or Novel Be?
Standard word counts for essays, blog posts, cover letters, novels, and social posts — plus why word count matters and how to hit your target without padding.
Read guideWhat Makes a Strong Password? Length, Randomness, and Common Mistakes
A strong password is long, random, and unique. Learn why length beats complexity, how attackers actually crack passwords, and the habits that keep your accounts safe.
Read guideWhat Is JSON? A Beginner's Guide to the Web's Favorite Data Format
JSON is how apps and websites exchange data. Learn what it looks like, the six data types it supports, common syntax errors, and why formatting matters.
Read guideYouTube Titles That Get Clicks: Formulas, Length Limits, and Mistakes to Avoid
Your title is half your thumbnail's job. Learn the title formulas that consistently earn clicks, YouTube's length limits, and the line between curiosity and clickbait.
Read guideHow Much to Tip: Standard Percentages and the Mental Math to Calculate Them
Standard tipping percentages for restaurants, delivery, taxis and more — plus the mental math tricks to calculate any tip in seconds, and how to split a bill fairly.
Read guideHow to Calculate Your Exact Age in Years, Months, and Days
Why age math is trickier than it looks: borrowing months and days, leap years, and how to calculate the exact time between two dates by hand.
Read guideHow to Reach Your Savings Goal (And How Long It Takes)
A plain-language guide to saving for a target: how to work out months to goal, why the amount you've already saved matters, and realistic ways to get there faster.
Read guideHow Sales Tax Works: Calculating Tax, Totals, and Prices Before Tax
Understand sales tax step by step: how to add tax to a price, how to find the price before tax from a total, and how rates differ between countries and regions.
Read guideHow to Count the Days Between Two Dates
Learn how to count days between dates by hand and with a calculator — including counting conventions, converting to weeks and months, and where leap years fit in.
Read guideHow to Convert Units: Length, Weight, and Temperature Without Guesswork
A practical guide to unit conversion: how metric and imperial relate, the handful of conversions worth memorizing, and why temperature doesn't convert like everything else.
Read guideHow to Calculate a Discount (Percent Off, Stacked Deals, Real Savings)
Work out what any percentage-off deal actually costs you: the quick formula, stacked and bundle offers, and how to compare a flat saving against a percent one.
Read guideHow to Estimate Fuel Cost for a Trip
Turn distance, consumption and pump price into a fuel bill: the formula, how to find your car's real L/100km, and how to split a trip fairly.
Read guideHow to Calculate Profit Margin (and Why It Isn't Markup)
The difference between margin and markup, how to set a selling price for a target margin, and what a healthy margin looks like by business type.
Read guideHow to Calculate Your Break-Even Point
How many units you must sell to cover your costs: contribution per unit, fixed versus variable costs, and how sensitive the answer is to price.
Read guideHow to Calculate Running Pace (min/km) and Predict Race Times
Turn a distance and a time into minutes per kilometer, use that pace to predict a race finish, and understand why pace beats speed for runners.
Read guideHow to Add and Simplify Fractions: Same and Different Denominators
Add fractions with worked examples, simplify the answer, and convert it to a decimal. Learn why denominators cannot be added and how to handle negative fractions.
Read guideHow to Calculate Rectangle Area and Perimeter
Find a rectangle's area and perimeter, convert centimeters to meters correctly, and estimate rectangular floor space with clear worked examples.
Read guideHow to Calculate Box Volume and Convert Cubic Meters to Liters
Calculate the volume of a rectangular box or tank using length, width, and height. Convert meters and centimeters correctly and understand capacity in liters.
Read guideHow to Calculate Average Speed from Distance and Time
Find average speed in km/h and m/s, convert minutes to decimal hours, and avoid the mistake of averaging two speeds without accounting for time.
Read guideHow to Find GCD and LCM (and When to Use Each)
Find the greatest common divisor and least common multiple of two numbers. Use factors, prime factorization, and worked examples to simplify fractions and compare cycles.
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