Percentage Calculation Guides
Concepts that are confusing with a calculator alone, explained with real numbers. You can open the calculator from each guide and follow along.
01
3 Basic Percentage Calculations — Formulas and Everyday Examples
B% of A, A is what percent of B, and percent change. Most percentage problems come down to these three formulas, explained with tips, test scores and pay raises.
02
Percent Change and Its Traps — Does a 50% Drop Then a 50% Rise Break Even?
The percent change formula, why successive changes multiply, why a 50% fall followed by a 50% rise does not break even, and how to handle zero or negative starting values.
03
The Stacked Discount Illusion — Why 20% + 10% Off Is Not 30%
When discounts are stacked, the actual discount is smaller than their sum. How to calculate it by multiplying, the order of application, and why order matters when a fixed coupon is mixed with a percentage discount.
04
Percent (%) vs Percentage Point (%p) — What's the Difference?
An interest rate going from 3% to 4% is a 1 percentage point rise and also a 33.3% rise. How the two expressions differ and how to avoid confusing them in news and reports.
05
VAT Calculation Basics — Net Amount, Tax, Gross and the Reverse Formula
What VAT-exclusive and VAT-inclusive mean, getting the gross from the net amount, the ÷1.1 formula for working back to the net amount, rounding the tax amount, and common mistakes — with South Korea's 10% VAT as the example.
06
Margin vs Markup — Telling Them Apart with Cost, Price and Profit
The difference between margin (profit ÷ price) and markup (profit ÷ cost), the formula for setting a price from a target margin, a conversion table and common mistakes.
07
Reverse Percentages — Original Price from a Sale Price, Pre-Raise Pay from Post-Raise Pay
What was the original price of an item that costs $80,000 after 20% off? Why adding the percentage back to a changed value is wrong, and the correct division formula, with examples.
08
Rounding, Truncating, Rounding Up and the 0.1 + 0.2 Problem — All About Rounding in Calculators
The difference between rounding, truncating (rounding down) and rounding up, rounding money to whole units, banker's rounding, why 0.1 + 0.2 becomes 0.30000000000000004 on a computer, and how this calculator handles it.