Articles & guides
Learn the math behind the calculators — practical guides on money, health, numbers and measurement, written in plain language.
Heart Rate Zones: The Math That Runs Your Workouts
Max heart rate, training zones, and the 220 minus your age rule. Here is the arithmetic behind the number on your wrist.
Auto Loan Math: What an Actual Car Loan Costs
The monthly payment is easy to find. The total cost of the loan is another story. Here is the math that sits between the sticker price and the key.
Ratios in Real Life: From Recipes to Price Tags
A ratio says how two quantities relate, and it shows up in cooking, mixing, and shopping. Here is the math for reading and scaling them.
Roofing & Flooring Math: From Footprint to Purchase
Roofs are priced in squares, floors in square feet. Here is the math that turns your ground measurements into the right amount to buy.
How Much Water Do You Really Need?
The eight glasses rule is a guess wearing a badge. Here is the actual math of daily water: the targets, the shortcuts, and when to drink more.
VAT & Sales Tax: The Math Behind the Receipt
VAT and sales tax do the same job in different ways. Here is the math behind both, and how to read the tax line on any receipt.
Tip & Split Calculator Guide: No More Math at the Table
The fast formulas for tipping and splitting bills — percentage tips that keep the math easy, fair party splits, and the rounding etiquette groups live by.
Calorie Math: BMR, TDEE, and How to Use Them
Understand the energy math behind Calorie — basal metabolic rate, total daily expenditure, and the deficit or surplus that moves your weight.
Construction Material Estimates: Concrete, Gravel & More
Estimate concrete, gravel and other materials by converting project dimensions into cubic volume — with the waste-factor math every DIY build needs.
Debt Payoff Math: Avalanche vs Snowball Strategy
Two proven ways to order your debt payments — avalanche saves the most interest, snowball builds the most momentum. Here's the math behind both, and how to pick.
Unit Conversions Made Simple: A Practical Guide
From meters to miles to megabytes: learn the one strategy that makes any unit conversion easy, plus the traps that catch everyone.
Percentage Calculations: A Practical Guide
Master the three core percentage problems — percent of a number, percent change, and part-to-whole — with formulas and everyday examples.
Retirement Planning Basics: How Much You Need
A step-by-step retirement planning framework: estimate your goal, harness compound growth, and build a savings plan you'll actually stick to.
The Compound Interest Guide: How Wealth Compounds
Compound interest is the eighth wonder of the world — here's the math, the magic of time, and practical strategies for savers and investors.
BMI Explained: What It Measures and Its Limits
A practical guide to Body Mass Index — how the math works, what the categories mean, and where BMI falls short.
How Mortgage Interest Works: Rates and Amortization
Understand exactly how mortgage interest is calculated, why early payments are mostly interest, and how rates and terms shape what you pay.