Writing about calculators
Nine write-ups on arithmetic that looks simple and is not: why 20% off then 10% off is not 30% off, how VAT is worked out backwards from a total, what a loan schedule hides in its first year, and what BMI does not measure.
What BMI Actually Measures, and What It Misses
One division, two measurements, and a number that gets read as a verdict on a body it never looked at.
Read CalculatorsMetric to Imperial: The Conversions Worth Knowing
Six exact numbers cover nearly every metric-to-imperial conversion. The trouble is the units that share a name and not a size.
Read CalculatorsHow to Calculate VAT: Adding It and Stripping It Out
Adding VAT is one multiplication. Removing it is where people reach for the wrong arithmetic and end up with an invoice that is wrong on every line.
Read CalculatorsHow to Calculate Percentages Without Getting Lost
Four formulas cover almost every percentage question you will ever have. The mistakes come from picking the wrong base, not from the arithmetic.
Read CalculatorsHow to Calculate a Discount in Your Head
Multiply by what is left, not by what comes off. The mental version, plus the two cases where the obvious arithmetic gives the wrong answer.
Read CalculatorsHow Much Should You Tip? Country by Country
The percentage changes at every border, and in half the world the real question is whether a service charge is already on the bill.
Read CalculatorsHow Loan Payments Are Calculated (and Where They Go)
One formula sets the payment on every fixed-rate loan. The interesting part is what happens to that payment once it leaves your account.
Read CalculatorsHow Compound Interest Actually Works
Interest earning interest, shown with real numbers — plus the rate your bank advertises and the one you actually get.
Read CalculatorsCelsius to Fahrenheit: The Formula and the Shortcut
The exact formula, a shortcut that gets it right in your head every time, the rough one that drifts, and the two conversions people still get wrong.
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