The Rule of 72: How Fast Does Money Double?
Divide 72 by the yearly percentage rate and you have a quick estimate of how many years it takes to double. Here is why it works and when it breaks down.
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Plain-English guides to the numbers in everyday life: how a calculation works, where people go wrong, and which tool to reach for.
Running Pace Explained: Per Km, Per Mile and Speed
Pace is time per distance, speed is distance per time, and they are two views of the same number. Here is how to calculate and convert them by hand.
Scale Factors: How Lengths, Areas and Volumes Change
A scale factor multiplies every length by the same number, but areas grow by its square and volumes by its cube. Worked examples for maps, models and pans.
How to Write Numbers in Scientific Notation, Step by Step
Scientific notation writes any number as a value between 1 and 10 times a power of ten. Here is how to convert, calculate and read calculator output.
Shoe Sizes Around the World: Why Charts Disagree
A size 9 is not a size 9 everywhere. Learn how the main shoe size systems are built, why converting between them is approximate, and why foot length is the best starting point.
A Simple Sales Forecast: Projecting Revenue From Past Growth
You do not need special software to project next quarter's sales. Three simple methods, one data set and an honest look at what each assumes.
Simple vs Compound Interest: How the Gap Grows
Simple interest grows in a straight line and compound interest curves upward. A side-by-side example shows how small the gap is at first and how large it becomes.
Sleep Cycles and Bedtimes: Working Out When to Sleep
Count back from your wake-up time in blocks of about 90 minutes to find a sensible bedtime. Here is the arithmetic, a quick-reference table and what the method cannot promise.
Sleep Debt: How to Calculate It and Pay It Back
Sleep debt is simple bookkeeping: hours needed minus hours slept. Here is how to total it for a week and spread the repayment sensibly.
Solving Proportions: Cross-Multiplication, Direct and Inverse
A proportion says two ratios are equal. Learn to solve for the missing number, spot when a relationship is not proportional, and handle inverse cases.
How to Split Shared Trip Costs Fairly Among Friends
When friends pay for different things on a trip, someone ends up out of pocket. Here is how to total the costs, find each balance and settle up with the fewest payments.
Square Feet to Cubic Yards: Ordering Mulch, Gravel and Topsoil
Landscaping materials are measured in cubic yards but areas are measured in square feet. One formula links them, and one short table covers most jobs.