Guides
Short, practical explainers — each one gives you the formula, a worked example, and a free tool that does the math instantly.
How to Calculate Percentages: Formulas & Examples
The simple formulas for percentages, percentage change and discounts.
BMI Explained: How to Calculate and Interpret It
What BMI is, how to calculate it, and what it can and cannot tell you.
Profit Margin vs Markup: What’s the Difference?
Two ways to express profit — and why confusing them costs money.
MPG vs L/100km: How to Compare Fuel Economy
Why mpg and L/100km move in opposite directions — and how to convert.
HEX, RGB and HSL: Understanding Color Codes
What HEX, RGB and HSL mean — and how to convert between them.
Number Bases Explained: Binary, Hex and Decimal
Why computers use binary and hex — and how to convert between bases.
How Compound Interest Works (with Examples)
Why your money grows faster over time — and the formula behind it.
How Monthly Loan & Mortgage Payments Work
The amortization math behind every car loan and mortgage.
How Many Calories Do I Need? BMR & TDEE Explained
The two numbers behind any calorie target.
Metric vs Imperial: A Quick Conversion Guide
Kilometres to miles, kilos to pounds, °C to °F — made simple.
What Is Base64 and When to Use It
Why binary data sometimes travels as text.
How to Calculate a Tip (and Split the Bill)
Fast mental math for 15%, 18% and 20% tips.
Small-Business Pricing: Margin, Markup, ROI & Break-Even
Price with confidence using four simple metrics.
Date Math: Days Between Dates, Age & Weekdays
The everyday date calculations, explained.
Bits & Bytes: Reading Data Sizes and Speeds
Why an “8 megabit” connection downloads about 1 MB/s.
Aspect Ratios Explained (16:9, 4:3, 21:9)
Keep images and video sharp — no stretching.
How to Create a Strong Password (and Why It Matters)
Length beats complexity — here’s how to do it right.
What Is a QR Code and How Do You Make One?
From a link to a scannable square in seconds.
How Sales Tax & VAT Work (with Examples)
Add it, remove it, or back it out of a total — explained.
How to Calculate a Discount and Sale Price
What “30% off” actually saves you, in seconds.
What Is JSON? A Beginner’s Guide
The data format behind almost every API, explained.
Word Count & Reading Time for Writers
Hit the limit, and know how long a read takes.
How to Read a JWT (JSON Web Token)
Decode the three dot-separated parts — safely.
Celsius to Fahrenheit: Formula & Conversion Guide
The °C → °F formula, worked examples and a handy reference table.
What Is a UUID (and When to Use One)?
Why UUIDs are practically unique — and when to reach for one.
What Is CAGR? Compound Annual Growth Rate Explained
The smoothed yearly growth rate — formula and worked example.
URL Encoding Explained (Percent-Encoding)
Why spaces become %20 — and when you need to encode a URL.
How to Calculate Hours Worked (Timesheets & Payroll)
Turn clock-in/clock-out times into total hours — breaks included.
What Grade Do I Need on the Final Exam?
The formula for the final-exam score you need to hit your target.
How to Calculate Your Running Pace
Turn distance and time into pace per km/mile — and predict finish times.
How to Simplify Fractions (Step by Step)
Reduce any fraction to lowest terms using the greatest common divisor.
What Is a Good Color Contrast Ratio? (WCAG)
The WCAG contrast thresholds that keep text readable for everyone.
How Much Water Should You Drink a Day?
The rules of thumb for daily water — and what changes your needs.
How to Find the Hypotenuse (Pythagorean Theorem)
a² + b² = c²: find the longest side of a right triangle.
How to Convert an Hourly Wage to an Annual Salary
Turn an hourly rate into yearly pay — and back again.
How to Read Roman Numerals (Chart & Rules)
The seven symbols, the subtractive rule, and how to read any numeral.
What Is the Golden Ratio (and How to Use It)?
Phi ≈ 1.618 — what it is, and how designers actually use it.
How to Calculate Electricity Costs (kWh)
Turn watts and hours into a real running cost per day, month and year.