Guides
Plain-English explainers for the everyday numbers our calculators handle — how the maths works and how to use it.
- How to Calculate a Tip (and How Much to Tip by Country)
Work out a tip in your head in seconds, decide whether to tip before or after tax, and see what's customary in the US, UK and Europe.
- How Compound Interest Works — and Why Starting Early Wins
Compound interest explained in plain language: interest on interest, the simple formula, why frequency and time matter more than the amount.
- How to Pay Off Credit Card Debt Faster
Why minimum payments trap you, how card interest is charged, and the snowball vs avalanche methods for clearing debt sooner.
- Understanding Your Mortgage Payment: PITI, APR and PMI
What actually makes up a monthly mortgage payment, plus the key terms — principal, interest, taxes, insurance, APR and PMI — every buyer should know.
- VAT Explained: How to Add and Remove It
What VAT is, how to add it to a net price and remove it from a gross price, with the simple formulas and worked examples.
- How to Convert an Hourly Wage to an Annual Salary
Turn an hourly rate into weekly, monthly and yearly pay — and back — with the simple formula, plus what gross vs net really means.
- What Is a Healthy BMI — and How Many Calories You Need
How to read your BMI, what the categories mean, the limits of the number, and how daily calorie needs are estimated.
- How to Calculate Percentages (the Easy Way)
Find a percentage of a number, a percentage increase or decrease, and what percent one number is of another — with quick mental tricks.