19 original, reviewed guides
Make sense of the number behind the number.
Practical explanations for the moments when a calculator gives you an answer and you want to understand what it means.
How to calculate percentages: the simple guide
A clear, practical explanation of the percentage calculations people use most.
BasicsWhat percentage is one number of another?
Use the part ÷ whole × 100 formula confidently, with examples and checks.
Work & dataPercentage difference vs. percentage change
Choose the right comparison when values have—or do not have—a clear starting point.
Work & dataPercentage increase vs. decrease: why the math feels uneven
Understand base values, percentage points, and why equal changes do not cancel out.
BasicsPercentage shortcuts for 10%, 15%, 20%, and 25%
Fast mental-math methods for common percentages, with checks you can do anywhere.
BasicsCommon percentage mistakes and how to avoid them
Spot reversed bases, added discounts, and other errors before they affect a decision.
ShoppingHow to compare sale prices without getting fooled
A calmer way to compare discounts, stacked offers, and the price you actually pay.
ShoppingHow to calculate a discount in your head
Estimate common sale prices quickly using 10%, fractions, and a simple subtraction.
ShoppingHow stacked discounts really work
Combine two or more discounts correctly instead of simply adding their percentages.
ShoppingHow to find the original price before a discount
Reverse a sale price using the percentage paid, with clear examples.
EverydayA practical guide to tip percentages in the United States
Understand common tip calculations while leaving room for service and local custom.
EverydayHow to split a restaurant bill fairly
Choose an equal, itemized, or hybrid split and include tax and tip clearly.
Work & dataProfit margin vs. markup: what is the difference?
Understand the two common profit percentages and avoid using one in place of the other.
Work & dataHow to calculate profit margin step by step
Turn revenue and cost into a percentage, then interpret the result carefully.
Work & dataHow to calculate percentage error
Compare a measured value with an accepted value and report the size of the error.
EverydayHow to calculate progress toward a goal
Turn completed work, savings, or distance into a clear progress percentage.
EverydayHow to calculate a test score percentage
Convert points earned into a percentage and understand weighted scores.
EverydayHow to calculate attendance percentage
Measure attendance from sessions attended and sessions held, with common edge cases.
Work & dataHow to reverse a percentage increase
Find the value before a known increase without making the common subtraction mistake.