Guides
Longer explanations of the ideas behind our calculators, from where the metre came from to how a mortgage payment splits between interest and principal.
Compound Interest Worked Through: Savings, Starting Early, Fees and Debt
Interest that earns interest, shown with computed examples: how compounding frequency changes a balance, how accurate the Rule of 72 is, why ten early years of saving can beat thirty later ones, and how the same math works against a card balance.
Time Zone Differences: Why the Gap Changes and How to Schedule Around It
New York and Berlin are usually six hours apart, but not for three weeks every March. Dated examples show when gaps between cities shift and how to plan meetings and deadlines around them.
Energy Units Explained: Joules, kWh, Calories and BTU With Worked Examples
Every energy unit is a fixed number of joules. Once you know the handful of factors, a food label, a gas bill in therms and an air conditioner rated in BTU per hour all go on the same scale. Worked examples show the steps.
Exchange Rates, Spreads and Fees: What a Currency Conversion Really Costs
The rate on a currency quote hides most of its cost. Three worked examples show how to compare a bank credit, a card payment abroad and a money transfer against the ECB reference rate.
How Mortgage Payments Are Calculated: Worked Examples From Purchase to Refinance
Follow a $280,000 fixed-rate loan from its first payment to its last: how the payment is set, why early payments are mostly interest, when PMI can end, and how to judge a refinance or a rent-or-buy decision.
Kitchen Conversions: Cups, Grams, Spoons and Scaling Recipes
Volume-to-volume kitchen conversions are exact; volume-to-weight ones depend on the ingredient. Worked examples convert a metric cake, scale cornbread to 10 servings and halve a batch of cookies.
How to Calculate Your BMI, and What It Can't Tell You About Body Composition
BMI is weight divided by height squared. Worked examples in both unit systems, the WHO categories, and why two men with the same BMI can carry 22 lb or 56 lb of fat.
The Metric System: How SI Units Work, How to Convert, and Where They Came From
The SI is seven base units, a ladder of decimal prefixes and a handful of exact conversion factors. Here is how it fits together, how to convert to US units with worked examples, and how it grew out of cubits, the French Revolution and the 1875 Treaty of the Meter.