We really should have good full metric decades ago. Old ***** like me would still be "thinking" in standard units but we would have a couple of generations thinking metric by now.
Everybody seems to get all up in arms about conversion. But no one converts liters to gallons or ounces when they get a bottle of pop. They have seen a 2 liter bottle for 50 years in the store and know how much it is even if they can't make the conversion.
And yes, it has been that long. The 2 liter bottle was introduced in 1970, believe it or not.
I have a few simple conversion tricks in my head if I have to use them.
I know that 82 degrees F is 28 degrees C (just flip the digits around), 32 F is 0 C (freezing), 212 F is 100 C (boiling), and -40 F is -40 C (handy in Minnesota). Those help to give an idea when I hear or see a C temp.
I also know what a 2 liter bottle looks like and that a fifth of whiskey is about 750 mL and a gallon is about 3.5 L.
I also know that 50 miles is 80 km. Or that 5k is 3.1 miles or 10k is 6.2 miles from running races. Also from running I know that 400m is about 440 yards and so on. I also know that a meter is just a few inches longer than a yard so if I hear that a shark is 3 meters I know it is about 10 feet. If I hear that it is 6 meters, I know to get out of the water.