Remember back in the 1970s when we were all going to go on the metric system? What ever happened to that? Oh, yes: Reagan.
Whether you need to figure out how far 150km is in miles (93), how many centimeters in a footlong hot dog (30cm), how to order a half a pound of cheese for your picnic (225 grams), or just how ridiculously hot 40° Celsius is so you can complain in your postcards home (105° F), here are the answers.
If you're wondering what your size 12 American body is to that little black dress in a Paris boutique (now don't faint, but here you're a size 42), or whether your size 10 feet need a 35 or a 55 in a handcrafted Italian leather shoe (neither; you’re a 44), that's all on a different page of clothing sizes.
At least these are easy enough to do in your head. A kilometer (km) is equal to 0.62 miles, and a mile is 1.61 km.

OK so it's not THAT easy. Still, doing a rough calc in your head isn't that hard. For kilometers to miles, just break it into two bits: 50% plus 10% (which totals 60%, or 0.60), then round up a smidgen for that extra 2% (the 0.02).
That sounds more complicated than it is: The road signs says Rouen is 80 km away. OK, so to get 50% you just cut that in half, which is 40. Tuck that away in a crevice of your cerebellum for a moment.
Now take 10% of the original 80—which is 8—then add that to the 40 and you get 48. Round up "a smidgen" to cover that extra 2% and you get 50. That means 80 km is roughly 50 miles (actually, it's 49.6, so close enough). Here, I'll make it easy for you with the table up to the right.
Frankly, you'll rarely ever need to convert the other way, unless you want to say to someone in Europe something along the lines of "I live 20 miles outside of Boston," only put it in kilometers terms so he understands where your suburb is.
No biggie. Ten miles is about 16 kilometers, so tell him you live 32 km from Boston. (This is assuming, of course, he has more than a vague sense of where Boston is in the first place.) That cheat sheet is also above to the right.
Thank heavens for the yardstick. You can point with it, you can rap people in the knuckles with it, you can even measure stuff with it, so long as the stuff in question is less than three feet long.
And it's also about one meter long, so just picture a yardstick and you know what a meter looks like. Well, roughly.
Actually, 1 meter is 3.3 feet—a bit longer than a yard. So 10 meters is equal to 33 feet, or 11 yards, which is enough for a first down (only they play soccer in Europe, so that little joke wouldn't work). 100 meters is 330 feet (or 110 yards), 1,000 meters is 3,300 feet (or 1,100 yards). One thousand meters is also 1 kilometer, which is 0.62 miles; see above.

I can't think of a time when I've had to use inches or centimeters in any discussion in Paris, much less needed to convert between the two, but just to be thorough:
One inch is equal to 2.54 centimeters (make it two and a half).
One cm is 0.4 inches.
That means 12 inches is 30.48 cm (just pretend it's 30). We call that "a foot," but Europeans don’t have a separate word for something of that length; they'd just say "30 centimeters" or maybe "a third of a meter."
Then again, since they don’t eat hot dogs (just würstel), I guess they don't need a term for footlong.

Unlike the U.S., where only cocaine ever comes in kilos, in France you'll be ordering lots of (perfectly legal) things by the "key."
Actually, when it comes to food you'll more frequently order by the gram, because who needs 2.2 pounds (1 kilo) of anything?
Conveniently enough, 100 grams is just about the perfect amount, per person, of cheese, salamis and other cured meats, fruit, or whatever else you desire in putting together a picnic.
No, you don't order a "112.5grammer" at McDonald's in Europe; they know what a "Quarter Pounder" is. (Well, a Royale with Cheese"...but I digress.) However, if I catch any of you giving into temptation and ducking into that McDonald's while you're in Paris, so help me I'll verbally thrash you to within an inch...er, 2.54 centimeters...of your greasy friesaddicted life.
Actually, this point was brought up by a greengrocer arrested in Britain a few years ago when jolly old England finally decided to crack down and force feed the metric system to its people. The bloke was still selling his bananas by the pound, not the kilo, at a local market so he was hauled off and fined.
The jovial fellow became something of a minor cause cèlèbre, the "Metric Martyr," and was fond of pointing out the hypocrisy that they would persecute a small fry like him when McDonald's was left to flagrantly flout the new laws by refusing to rename the Quarter Pounder.

Actually, it kinda is—sold by the quart, that is.
One liter equals roughly a quarter of a gallon (i.e. a quart). Not that you'll catch any Frenchman using such a barbarous term. No, they drink wine by the liter over there, that you very much—which may explain why they're so happy much of the time.
OK, so bottles of wine are actually usually 0.75 liters, not full liters, but let's not quibble.
If that's too much vino for your blood, you can always order table wine by the halfliter (about four glassesworth, called un demi in French) or quarterliter (two glasses; un quart in French).
° F  ° C 
32  0 
40  5 
50  10 
60  15 
65  18 
70  20 
75  25 
80  27 
85  30 
90  32 
95  35 
100  38 
Tell a doctor in Paris that you're running a fever of 102°, and he won't believe you—because 102° Celsius is equivalent to 216° Fahrenheit, which means your brain would be fried faster than that egg in the old "...this is your brain on drugs" commercial.
And if the local news reports that tomorrow's temperature will be around 32°, that's means it'll shorts and Tshirt weather, not down parka time—32° degrees Celsius is 90° Fahrenheit.
The bad news: there is no simple formula to convert Celsius to Fahrenheit and back. There is a formula, of course, but it ain't the sort of thing you'd want to do in your head. Take the Celsius amount, multiply it by 1.8, then add 32. See, wasn't that easy?
If math is one of the things you're taking a vacation from, the chart on the right can help you ballpark things.