Clear, beginner-friendly articles on percentages, interest, taxes and everyday math
Whether you're calculating a restaurant tip, figuring out how much you'll save in a sale, or tracking your investment growth — percentage calculations are one of the most practical math skills you can have. The good news? There are only three core formulas you need to know, and once you master them, almost every percentage problem becomes straightforward.
This guide breaks down all three formulas with real-world examples so you can apply them immediately — no advanced math required.
This is the formula you'll use most often. It answers questions like: "What is 20% of $85?" or "How much is 15% VAT on this item?"
This same formula works for tips (15% of your restaurant bill), tax calculations (8.5% sales tax on a purchase), or any situation where you need a portion of a whole number.
This formula converts any ratio or score into a percentage. It's perfect for grades, completion rates, and performance tracking.
Use this formula whenever you want to express a part-to-whole relationship as a percentage: completed tasks, survey responses, budget usage, and more.
This formula tells you how much something has grown or shrunk over time. It's essential for comparing prices, tracking investment returns, and understanding economic data.
A positive result means an increase; a negative result means a decrease. This formula is widely used in finance, economics, and business reporting.
Mistake 1 — Confusing percentage increase and absolute increase. A salary going from $50,000 to $53,000 is a $3,000 increase — but only a 6% raise. Always specify which one you mean.
Mistake 2 — Reversing a percentage change. If something increases by 50% and then decreases by 50%, you don't end up where you started. A $100 item that goes up 50% becomes $150 — then a 50% decrease brings it to $75, not $100.
Mistake 3 — Adding percentages directly. If your investment grew 10% this year and 10% last year, your total growth is not 20% — it's 21% (because the second 10% is calculated on a larger base).
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