convert in your head · without a calculator
Built as a memo rather than a table: the thing worth hanging it up for reads across the room, with the full reference in smaller type below it.
Three sizes in one archive — from a sheet on the fridge to a poster on the wall.
These shortcuts stay within a few per cent — enough to judge a price tag or a road sign.
It’s the one temperature where Celsius and Fahrenheit agree: −40 °C equals −40 °F.
The scales have different steps and different zeros, so their lines must cross somewhere. They cross at −40.
Why the numbers can be trusted. Every countable number here is computed from data rather than typed in, and the sheet must pass 3 independent checks before it is allowed to build — how many items there are, the order they come in, and the claim in the highlighted box itself. If one fails, there is no file.
