What metric to US customary unit conversions should I know for traveling to the US?

In addition to the other answers, soft drinks are sold by the US fluid ounce, commonly called ounces (oz). 1 litre is 35.2 USfloz, 330ml (UK soft drink can) is 11.6 USfloz. Cans seem to be sold by 12floz.

One USfloz is 28.4ml

• If traveling in the mountains, it’s good to know that maps and signs show elevation in feet. 1000 feet is about 300 meters. So if you plan to climb a mountain that’s 9000 feet above sea level, that will be about 2700 meters.

• It’s also nice to remember that highway speed limits are usually around 60 miles per hour, and there are 60 minutes in an hour; i.e. one mile per minute. So if you’re going to drive somewhere that’s, say, 90 miles away, you can immediately estimate that it will take you about an hour and a half. This is of course subject to traffic, road conditions, etc, but it gives you an order of magnitude.

• Slightly off the subject, but remember that the continental US has four time zones: Pacific / Mountain / Central / Eastern Standard Time are UTC-8/7/6/5, plus one hour during Daylight Saving Time (roughly mid-March to early November). (Most areas observe Daylight Saving Time but a few do not, most notably the state of Arizona.) So that’s a three-hour distance from one end of the country to the other. On a long journey, check for time zone boundaries you may cross; they don’t necessarily coincide with state lines. Times are usually quoted in the local time zone, but for something that applies to the whole country (e.g. live television broadcasts), it’s typical to use Eastern Time, standard or daylight as appropriate, since that’s the most populous time zone.

I tend to just use rough estimations when I travel to the US. On a holiday, you rarely need the exact equivalent:

1. 1l = 0.25 gallons
2. 1m = 1 yard
3. 100ml = 4 fluid ounces (same goes for grams to ounces)
4. 1.5km = 1 mile
5. 1kg = 2 pounds

For temperature, subtract 30 from the Fahrenheit and divide by 2 – not 100% accurate, but it’ll certainly help you dress accordingly! (90F = ~30C, 70F = ~20C)

Just remember, if the imperial system gets too much, Canada is rarely more than a couple of hours away by plane!

Something I haven’t seen in the other answers:

There’s a conversion for prices you need to think about. Each state (and city) sets its own sales tax. The prices shown in a store do not include this tax. Tax rates can be as low as 0 or as high as 10%.

gasoline prices shown do include the tax.

The only conversion that seems to me that you should really know is the currency conversion.

Other than that: I am German and I lived a year in the USA and I went to High School there and if you’d asked me about how big a person who is 6 foot tall is in meters I’d now by instinct somehow know that this is rather big for a person and since big persons are somewhere in the broad 2 meters area I’d go for a guess there and if you give me 20 seconds I’ll calculate it for you. Fine. But other than that I found that the unit conversions are rarely necessary. If you drive in your car and there is a speed limit then your speed indicator must not show more than the number on the speed limit sign. Who cares what unit it is? For judging your speed from a safety perspective I guess you will not look up the angle and radius of a turn up ahead and then calculate the safe turning speed but rather judge your speed by looking outside and using your experience.

If you want to compare the prices at the gas stations: They are all in dollar per gallons. Since going to the UK for buying gas will not be an option there is little use in converting gallons to liters except for academic purposes of knowing that gas in the USA is really cheap. Also the fuel consumption of your car will be indicated in miles per gallons (which is somewhat strange if you are used to liters/100km because it is not only other units but it is the inverse fraction) so if you need to know how far your car will travel after buying 5 gallons of gas there is no need for kilometres either.

Also you will not need to know how many miles a kilometer for time estimations because all distances are given in miles and your speeds are given in miles per hour (except South Dakota (and other states?) where all (many?) road signs for distances actually list both miles and km). What is actually useful to know is the general speed limit which varies from state to state. For freeways that varies between 55mph to 75mph with 65 mph being the most common if I remember correctly. This allows you to estimate the time you need for long distance travel.

In the supermarket you will find that next to all imperial units metric units are also present. Oftentimes a much smaller font but they are there. So if you buy a gallon of milk it says “1 gallon (4.78 liters)” on the package.

One time when I was in the US I thought that now it would be good if I know the imperial system better than I do and that was when I was renting skis and the salesman wanted to know my weight and height. After thinking a moment I apologized and said I needed a few seconds to convert units and then he told me that the table he uses for determining the correct settings features both metric and imperial units so I could go ahead with metric units.

There may be a use for unit conversions if you ask people for distances as a pedastrian so you know whether your destination is in a suitable walking distance but then the estimations given by other people are not more accurate in the US than other countries, so knowing that “a mile is considarably more than a km” and “a foot is considerably less than a meter” and “a yard matches about a meter” will be perfectly fine.

If you like most people carry a smart phone with you, then just download and save the Imperial units page from Wikipedia (or a conversion app). If you meet the need to convert units, then you can still have a look.

So to answer your question which conversions you should know:

• A mile is more than a km
• A yard is about a meter
• Three foot are a yard.

But it’s not really important. There are more important things. E.g. traffic rules. E.g. in most states you are allowed to make a turn against a red traffic light after stopping and yielding to pedestrians if you do not need to cross traffic (this is if you turn right or if you turn left from a one-way street into another one-way street (ok, not that important either because if you just sit there and wait for a green light nobody is going to behead you either, but just as example). At least in oregon also if you are turning left from a two-way street into a one-way street). See e.g. the Oregon Drivers manual for things to know.

As a counterpoint to the other (useful) answers, I would like to invite you not to worry too much.

The most useful units to master are probably miles (to plan your trip) and miles-per-hour (for speed limits). But you already know miles and road signs and your car’s speedometer will both use mph anyway. So you can always relate the two without fully grasping the unit and knowing how mph relate to kph is not extraordinarily important.

As mentioned in Thorsten’s answer, the 12-hour clock can also take a little bit of effort getting used to for people coming from central Europe but here again, coming from the UK, it would not be new for you.

Many other things can be a bit disconcerting but spending only three weeks in the country as a tourist, you should be able to cope easily without knowing anything. Buying and cooking can be done visually and you can always look outside to choose your clothes. A few (small, in this case) surprises have to be expected when discovering a foreign country anyway.

The only situation in which I find myself actually converting units is when I am using cookbooks from different countries (mostly after coming back, as I often buy a few as keepsake…)

Important for car (if you rent one, it will automatically have mph, but it is still useful how fast you are going for e.g. braking distance):

All values are rounded for convenience; the most frequent values are

Speed

30 mph = 50 km/h
55 mph = 90 km/h
65 mph = 105 km/h

Every car has cruise control. If you want to explore the USA by car, I recommend strongly to use it because the long distances together with the consistent speed limit and no action (In Europe you are overtaking, adjusting the speed because the speed limit changes, changing the lane and leave the highway) causes you to violate the speed limit easily and the punishment is prohibitive for European standards.

Temperature

Fahrenheit in Celsius: Subtract 30 and halve the value

30 – 40 °F = 0 – 5°C cold
40 – 50 °F = 5 – 10°C cool
50 – 60 °F = 10 – 15°C fresh
60 – 70 °F = 15 – 20°C coolish
70 – 80 °F = 20 – 25°C normal
80 – 90 °F = 25 – 30°C warm

Distance

1 inch = 2,5 cm (width of a thumb)
1 foot = 12 inches = 30 cm (a big foot, multiply with 3/10)
1 yard = 3 foot = 90 cm (a bit shorter than a m)
1 mile = 1760 yards = 1,6 km ( multiply with 5/8)

Time

The 24h system is not used widely in the US, they are (except the
military) still using the a.m. (before midday) and p.m (after midday) system. Everything which is not 12:XX is easy (1:29 p.m. = 13:29, 5:30 a.m. = 5:30), but the 12:XX are tricky because they are changing the a.m./p.m. during midday and midnight. Example: From one minute to midday it is 11:59 a.m., 12:00 p.m., 12:01 p.m……1:00 p.m. and at midnight the change to 12:00 a.m. occurs.

It is good to know that the calendar begins with Sunday (in contrast to Monday in Europe) and dates are expressed in the month/day/year format.

Drinking and car filling:

The US and UK pint and quarts are different, I forgot that

1 fluid ounce = 30 ml (US) used for small portions
1 pint ~ 470 ml (US) / 550 ml (UK) (half a liter)
1 quart ~ 900 ml (US) / 1,1 l (UK) (nearly a liter)
1 gallon = 3,8 l (important for filling the car and yes, petrol/gas is very cheap).

Tire pressure

1 psi (pounds/inch²) = 1/15 bar = 7 kPa (1 bar = 100 kPa ~ atmospheric pressure)

15 psi = 1 bar = 100 kPa
30 psi = 2 bar = 200 kPa
etc.

Weights:

1 pound = 450 g (1 kg = 2.2 lb)
1 ton = 1 ton

Miscellaneous

The floors are numerated with 1/G, 2, 3, 4…instead of the European G, 1, 2, 3…

US pints and gallons are about 80% the size of UK pints and gallons due to the US pint being 16 US fluid ounces and the UK pint being 20 UK fl. oz. See
http://en.wikipedia.org/wiki/Pint
The about is because the US fl. oz. is about 4% larger than the UK fl.oz.

As a tourist in the USA, using hotels and restaurants, I needed inches, miles, temperatures in Fahrenheit and the odd liquid measurements like gallons and quarts.

When doing supermarket shopping you can chose your containers and go be guessing. But if you want to be sure how much you buy, you should know all regular container sizes.

If you are helped in a shop you can ask for how much you need in ounces and/or pounds, but you can also use the tourist way and ask for a bit of whatever and indicate with your fingers or hands how big, or make the ‘add on’ sign for a bit more. Worked for me in countries where I did not speak the language, should work as well in the USA (as long as it is not a supermarket.)

But I have met people traveling the USA who had not bothered learning any of the measurements and did survive the experience.