As a Canadian, I measure myself in feet and inches but I drive kilometres. At the grocery store, prices are often quoted per pound on the display but per kg on the receipt. What with our British colonial past and geographic and cultural American neighbour, we are a country that uses both the metric and imperial system on a daily basis.
As a scientist, the metric system makes sense. Metric is the SI unit, used internationally, making it a common language for scientists to communicate with. It is in base 10 units, which make conversions between units easy. This is important since in daily work in a lab for dealing with amounts of varying orders of magnitudes in a practical way. The relatively small increments also allow for a large degree of precision. I use the metric system so often in my daily life (largely because of work), that my weirdest work-related dream was of my colleague about to present a poster at a conference describing a new assay that worked preferentially with the imperial instead of metric system.
Given that the majority of the countries in the world has adopted the metric system officially, what is the utility of the Imperial system. Looking back at the origins of the units, we find some surprisingly useful features: a foot is approximately the length of a human foot. A mile (derived from the latin word for thousand) is approximately 1000 human paces. An inch is approximately the width of a human thumb (a good rule of thumb!), used widely in the cloth industry. We need not go further than the kitchen to realize the utility of a cup, teaspoons, and tablespoons, and a pound of meat is conveniently a nice portion size (maybe for dining with a friend these days, as we begin to realize the benefits of reducing meat consumption).
Although school math has always emphasized the correct answer, approximation is actually a more practical everyday skill and arguably, the more important problem-solving tool. Often, the thinking process and assumptions we make to arrive at a solution is more critical than the answer and knowing the exact answer is less relevant than knowing a ballpark figure. Therein lies the genius of Google’s famous interview questions like, “How many tennis balls fit inside a school bus?” and, “How much does the Empire State Building weigh?”
For everyday use, using parts of our bodies as tools for estimating measurement is just the more intuitive thing to do! Admittedly, there is a strong experiential factor in this – we are most used to what we learned growing up. So, for me, leaving a 1/4 inch seam when I’m at a sewing machine or pre-heating an oven to 400°F just works, although I measure furniture to the centimetre at IKEA and enjoy summery 25°C weather. By the way, I spell Canadian.