One of our clients had an invoice rejected by a buyer because it was formatted to print in A4 (210mm by 297mm) rather than US letter size (215.9mm by 279.4mm) and would not fit their US filing system.

A good way to have Americans not keep your business card is to have it printed in UK size (85mm by 55mm) and not US (89mm by 51mm) so it will not fit a US wallet or file.

Of course, you can almost always forget the metric system in the US. Do not specify ten centimeters (US spelling), but instead use 3.93701 inches.

Ten kilograms in the UK is 22.0462 pounds in the US.

A 50-hectare (or 123.553 acres) field in the UK would be 1.93051 square miles (or 123.553 acres) in the US.

You might put 56.7812 liters (US spelling) of petrol in your UK car, but an American would put in 15 gallons of gas (or gasoline). (A US gallon is smaller than an Imperial gallon.)