Why “imperial” units still rule in daily life: they’re human-centered.
Here in America, we are often the target of ridicule by enlightened foreigners over our way of measuring things.
“It’s simply illogical! Twelve inches to the foot, three feet to the yard? And who can remember that eight fluid ounces make a cup, but sixteen ounce-ounces make a pound?” (I’m not doing the “foreign” accent right, but you get the idea).
After struggling mightily with a metric recipe last night, which called for hundreds of millilitres of one thing and tens of grams of another — sure, let me whip out my digital scale! — I realized: the metric system has an excellent API, but terrible UX.
Metric is great for comparing, not doing. I know at a glance that 150 meters is 15% of a kilometer (try doing that with feet and miles). However, when it comes to messy, imprecise human-scale interactions, the old imperial system shines.
Take cooking, for example. A cup is about a handful. An ounce is a solid splash. Tablespoons and teaspoons? Close enough to the ones that you use to sip your soup and stir your coffee. The imperial system wins because it reflects our natural mental model of measurement, with ones-units for amounts that one might actually use.
Metric, on the other hand, sacrifices this usable familiarity in exchange for logical consistency and effortless scalability. This results in units that, in isolation, fail to reflect the sorts of quantities we deal with every day. Have you ever seen a milliliter of anything? (Don’t answer that, street pharmacologists). Metric is a measurement system made by scientists, for scientists, with little consideration for how humans tend to interact with real-world quantities.
Building usable products starts with understanding your users’ mental models. You can’t meet and exceed users’ expectations if you don’t know what they are, or how they fit together. Metric surely works well for computers and Swiss people — I’ll stick with imperial in my kitchen.