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 with a country-by-country table.
- 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, worked examples and a 2026 table of standard rates by country.
- 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, a healthy-weight-by-height table, 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.
- Metric ↔ Imperial: Exact Conversions and How to Use Them
The exact factors between metric and imperial units for length, weight, volume and temperature — with worked examples and why US and UK measures differ.
- MPG vs L/100 km: Understanding Fuel Economy
How miles per gallon and litres per 100 km relate, a conversion table, why US and UK mpg differ, and how to work out what a trip really costs.
- Margin vs Markup: The Difference That Costs Businesses Money
Margin and markup use the same two numbers but mean different things. How to calculate each, why they are not equal, and a quick reference table.
- How to Measure a Room and Work Out Flooring or Paint
Measure area the right way, convert between square metres and square feet, add a waste allowance, and turn area into a materials cost.
- How to Split a Bill Fairly (With Tip)
The right order of operations for splitting a restaurant bill among friends, how to handle the odd cent, and when to split evenly vs by item.
- Loan Payments Explained: APR, Term and Total Interest
What actually drives a monthly loan payment, why the interest rate and APR differ, and how the term quietly changes how much you pay in total.