Here's my pitch for encouraging the adoption of metric units in holdout places like the US: acknowledge that the foot is a convenient goldilocks unit for describing "human-scale" objects i.e. those that you are likely to find yourself using your hands to work with, and advocate for the widespread use of an SI-based unit that is neither the centimeter (itself being too small for this purpose) nor the meter (being too large).

The decimeter (i.e. 10cm; ⅒ of a meter) has been hiding in plain sight and fits the bill adequately. It is, however, awkward to communicate. It's about the same width as the width across an average adult's hand, so let's rebrand the decimeter as the "metric hand", or just "hand" in informal contexts.

Conveniently, there are approximately 3 metric hands in 1 foot. The error, 1.6mm per hand, is negligible, and in instances where accuracy and/or precision is at a premium you can rest safe knowing that it really does accumulate at 1.6mm per hand exactly, and not some intractable quantity like the one that comes up with approximating 1 kg as 2.2 lbs.

Previously: <>

The video that this one is alluding to (or paying tribute to?) is called "Powers of Ten" from 1977. If that does not sound familiar, I recommend you watch it as well. It's a gem.

There is one cool trick I use with metric paper: since A0 is a square meter, and comprises of 16 A4 sheets, when you see paper with weight per square meter, you need to divide by 16 to get the weight of a single sheet of paper.

In practice regular paper has ~80g/m^2, meaning 5 grams per sheet, meaning a ream of 500 sheets has exactly 2.5 kg.

The standard was proposed by mathematician Lazare Carnot.

In addition to A series, there is also B series (and C series). They all have the same aspect ratio √2

B0 is 1 metre wide, while A0 is 1 meter long. The area of B series sheets is the geometric mean of successive A series sheets. Many books are printed in size B5. When you fold A4 it fits neatly between the pages.

Letter written on A4 paper fits inside a C4 envelope, and both A4 paper and C4 envelope fit inside a B4 envelope.

It's the ratio which is important here. The type of unit (metric or not) doesn't really matter.
It's silly, but I wish the HD aspect ratio had been the golden ratio. There's something very satisfying about it, even if its properties would be of little practical use. (Though I wouldn't have minded the extra vertical space!)