A lot of people think that it is too complex to convert US measurements into metric measurements, or vice versa, to even come up with an approximation. However, that isn’t strictly true. There are a number of tricks that can give a person a good general idea, regardless of which system they are used to. Here are some examples:
Miles and kilometers and speed
A kilometer is about 6/10 of a mile, so if a person says that they traveled 100 km, they’ve gone about 60 miles. Since speed is measured in distance over time, and time doesn’t change between measurement systems, speed can be approximated in the same way. Someone traveling at 100 kph is moving at about 60 mph.
Meters and Yards
A meter is a bit longer than a yard. While it isn’t exact (a yard is three feet while a meter is 39 inches; 3 feet 7 inches), it is easy enough to have an approximation by substituting one for the other. A hundred yards is a bit less than a hundred meters, but it is close enough that a person has a general idea. A yard equals three feet, too, so if a person wants to figure out how tall a mountain is when the altitude is given in feet, for instance, just divide by 3. That will be close to how tall the mountain is in meters. For example, a mountain that is 15,000 feet tall is close to 5,000 meters tall. (Actually, 15,000 ft is 4,572 meters, but it still gives an excellent idea.)
Inches and centimeters
This one is required learning in electronics in the US in college: There is 2.54 cm per inch. This makes it pretty easy to convert in your head to get a reasonable idea, either direction you are converting. Three inches would be around seven and a have centimeters. Four centimeters is about 10 inches.
Liquid measures
A liter is a little more than a quart and a gallon is 4 quarts. There are also 2 pints to a quart and 2 cups to a pint, in other words, 4 cups per quart. This gives you an easy way to get an approximate conversion, either way, for liquid measures. There would be a bit more than 4 cups in a liter. Gasoline in the US is sold by the gallon, not by the liter.
Temperature
This is the measurement that is complex. Fahrenheit is far more precise than either Celcius or Centigrade, but it isn’t based on the metric. At ocean level, water boils at 100 C or 212 F. Water freezes at 0 C or 32 F. Obviously, the conversion is more complex and people usually can’t figure out an approximation in their heads, unless they are a whiz at math calculations.
What you might not know is that the US measurement system isn’t American. It was actually invented by the British, though it was originally more arbitrary than it is now. The US considered moving to a metric system, but the idea was rejected. The exception is that science in the US does use the metric system. Have you ever wondered what the biggest obstacle was in regard to switching to metric in the US?
It was done primarily due to cost. You see, all literature that had measurements would have to be reprinted. That isn’t bad enough. The US has a far more extensive and longer road system than any other nation on earth. All of the road signs would have to be changed and road signs aren’t inexpensive. The cost of replacing the road signs would be phenomenal.
As of 2008, there were over 2,734,000 miles of paved roads in the US and over an additional 1,300,000 miles of unpaved public roads. Every mile can have numerous signs and all of them that have nonmetric distance/speed measurement would have to be changed. It has really never been about American reluctance to switch to metric. It is the money.

Does this help you at all with conversions?

Yes

No

A little


Does any of this surprise you?

Yes

No

What are the units of measurement in the metric system?
Millimeters, centimeters, meters, kilometers, grams, kilograms, and liters are the most used ones. There are more, but the biggest point is that the metric system is a base10 system. Within the metric system, to convert from, say, millimeters to centimeters to meters, all you need to do is move the decimal point.
That is different than the US standard system, where 12 inches equals a foot, 3 feet equals a yard, 5,280 feet equal a mile, and so forth. In the standard system, a person can’t just move the decimal to convert from one to the other.
Thank you for the metric lesson.
It is really a fairly simple way to convert from one system to the other. I use the conversions a lot more often than people might think.
In the UK we switched to metric in many respects a long time ago, but retained road distances in miles, including speed limits. The US always had “metric” currency, with 100 cents to a dollar, but the UK did not before we switched to a decimal system in 1971.
I have to confess that I usually think in feet and inches rather than metres, although anyone educated in recent years wonders what on earth I am talking about!
That is a great point and it is a reason that the reason for not switching to metric in distances isn’t because of public opinion. Americans would adapt to the metric system easily enough, especially considering that the conversion isn’t at all difficult. Science has been metric for some time, even in the US.
Although there was a time – still may be for all I know – when timber for building and DIY in the UK was measured in metric for length but imperial for crosssection. You would therefore ask for “two metres of three by two”!
On this side of the pond, lumber is normally sold by US standard. If I go to the hardware store and tell them that I want a two by four by eight, I’ll get a board that is 2 inches by 4 inches by eight feet long.
Actually, it isn’t really 2 inches by 4 inches. It is more like 1 7/8 inches by 3 7/8 inches. The idea was that an eighth of an inch is lost in milling, but that isn’t actually true. In fact, some lumber mills use lasers to cut the wood. There is next to no wood loss and they still cut it an eighth of an inch less.