Counting Days Between Dates: Leap Years, Business Days and Off-by-One Errors
Why two date calculators can disagree by a day, when to count inclusively, and how month-end arithmetic behaves.
Calculator guides and explainers
The reasoning behind the numbers: how the formulas work, where they come from, and where they stop being true.
Metric and Imperial: The Conversions People Get Wrong
Exact factors for the conversions that matter, which are defined rather than measured, and the one that needs an offset as well as a factor.
Exchange Rates: Mid-Market, Spread and What Your Bank Charges
What the mid-market rate is, how a 'no fee' transfer charges you 3%, and how to work out what a conversion actually cost.
Hourly or Salaried? How to Compare Two Offers Properly
A $30 hourly contract and a $62,000 salary are not the same offer. Here is the arithmetic that makes them comparable.
Why a Cup of Flour Is Not a Cup of Sugar: Baking by Weight
A US cup is a fixed volume. What it weighs depends entirely on what fills it — and in baking, that difference is the whole recipe.
Price per Square Foot, Unit Price and Other Comparisons That Mislead
Reducing two things to one number makes them comparable and makes them look more comparable than they are.
What an Electric Car Actually Costs to Run
Cost per mile, the three prices of electricity, and where the savings are smaller than the headlines suggest.
Do You Really Need 10,000 Steps? What the Evidence Says
The number came from a 1965 pedometer advert. The research since then points somewhere slightly different.