Guides and explainers
Short, practical guides written to explain the ideas behind our calculators. Each guide links to the tools it uses, so you can follow the examples with your own numbers.
How to Calculate Percentages: 3 Methods and the Mistakes to Avoid
Learn the three kinds of percentage problem, quick mental shortcuts for each, and the errors that make discounts and increases come out wrong.
Understanding Compound Interest: Why Time Matters More Than Amount
How compound interest works, the rule of 72, why compounding frequency changes results and how it differs from simple interest.
How Loan Payments Work: Principal, Interest and the Cost of Time
See how a fixed-rate loan is repaid, why early payments are mostly interest and how the loan term changes the total cost.
VAT and Sales Tax Explained: Adding, Removing and Avoiding Errors
How VAT and sales tax differ, how to add or remove tax from a price correctly and how to avoid the most common invoicing mistakes.
Markup vs Margin: The Difference and Why It Affects Your Pricing
Markup and profit margin use the same profit but different bases. Learn how to convert between them and find your break-even point.
BMI, Calories and Water Intake: What the Estimates Really Tell You
Understand what BMI, the Mifflin-St Jeor calorie estimate and daily water formulas measure, and where their limits lie.
Unit Conversion Mistakes to Avoid: Gallons, Gigabytes, mpg and More
The most common unit conversion errors, from US and imperial gallons to decimal and binary storage units, and how to avoid them.
Working with Dates: Day Counts, Working Days and Unix Timestamps
How to count days between dates, calculate working days, handle leap years and read Unix timestamps without mistakes.
Password Safety Basics: Length, Uniqueness and Password Managers
What makes a password strong, why length beats complexity, why you should never reuse passwords and how to store them safely.
CAGR and Inflation Explained: Growth, Prices and Real Returns
What compound annual growth rate means, how inflation erodes purchasing power and why a real return is lower than the headline number.
Grades, Fractions and Proportions: Everyday School Maths Made Clear
How weighted grades work, how to simplify fractions and how to solve direct and inverse rule of three problems.