<P> Frequent - flyer programs from commercial airlines in the United States are commonly managed using miles . This is likely to continue; in fact, some international airlines that used kilometers are switching to miles or using both, to simplify the redemption of miles with United States airline partners (Aeromexico switched from kilometers to miles, and Varig from Brazil has used miles since the program was named "Smiles"). Aircraft speed is often measured in knots and heights in feet in many countries . </P> <P> For both domestic and international routes, U.S. airlines set luggage size and weight restrictions in terms of customary units . Conversely, international airlines that fly to the U.S. often restrict baggage to metric sizes . </P> <P> The temperatures in aviation - related weather reports are given in degrees Celsius in the United States . </P> <P> For carry on luggage, the Transportation Security Administration restricts liquids to 3.4 fluid ounces (100 mL). Originally, the agency planned to restrict to 3 ounces but switched it to 3.4 oz / 100 mL after pressure from the European Union . </P>

Where does the us use the metric system