Comment by sam_goody

While this is nice, it is not inherently related to an A4 paper.

You could have memorized the length of a cheapo Bic pen if that is common in your area; Or a Parker or a Monte-Blanc if you carry one of those.

All recent iPhones (regular models since `03) have a width of 71.5mm. Remember that, and as long as someone near you has an iPhone, you are good to go using it as a ruler. (And people will definitely be, um, impressed).

We have in my kitchen several brands of small forks, all are 19.2mm (just checked. The large forks have a range of sizes). Next time I need to measure something I could just request a fork...