Almost all countries in the world use the metric system, officially known as the International System of Units (SI). Before the introduction of the metric system, there was no worldwide standard in measurements, and every geographic region or country used their own system. Some of these historic measurement systems are still in use, for example variations of the old English system (or its descendants U.S. customary and the imperial system) of weights and measures survive in various forms in the United States, United Kingdom, Ireland, Canada and Liberia, while Myanmar still plows its own furrow. In all these countries except for the U.S., the metric system is widely understood. In the U.S., you’ll find the metric system only used in scientific, military, and most medical contexts, while in the UK and Canada, usage is more mixed. In scientific usage, the metric system is used exclusively in all countries of the world.
Some countries that are officially metric use non-standard units in everyday speech. While most of them are “metricated” (e.g. a German Pfund (“pound”) being exactly 500 grams or a Dutch ons (“ounce”) being exactly 100 grams), some are not, and the vague definitions of what exactly is meant by a “pound” or a vara (“rod”, a Latin American unit of distance, somewhere between 0.8 and 1.1 meters) give you a sense of the confusion that led to the introduction of the metric system in the first place. Canada is also officially fully metricated, though imperial units continue to be widely used by older Anglophone Canadians in daily conversation. The rest of the Anglosphere (such as Australia, Ireland and New Zealand) switched to metric beginning in the 1970s and are now fully metric in daily use, though the imperial system still survives to varying extents in colloquial usage. Some special uses still apply non-metric units almost globally (such as inches for bicycles and television sets, feet in aviation, and knots and nautical miles in maritime contexts and aviation), although few are of interest to the average traveller.
Since use of the metric system is taught in schools in physics and chemistry classes, younger generations in most of the world exclusively know metric units, and historic units gradually die out. When traveling in The Netherlands for example, the elderly may still use ounce in spoken language, but teenagers won’t have any idea what you’re talking about. Exceptions exist in fields where particular units are well established. Jewelers typically measure the weight of diamonds in carat (which is 0.2 g), so entering a shop and asking for a diamond ring of 2.4 g will definitely raise some eyebrows. Another example is the energy contained in food, which is traditionally measured in calories. Food items will often list the value in kcal (kilocalories) and metric unit kJ (kilojoules) side by side.
There are so many historic units that listing them all on this page would be nearly impossible, and of limited use to the traveler. The goal here is to list the most commonly used historic units and their conversions to metric equivalents to give the traveler a rough idea of quantity. Note that not all conversions are exact, some are approximations even though the “=” is used below.
For everyday applications (e.g. weather), you can approximate with very simple math. To convert Celsius to Fahrenheit, double the number and add 30. To convert Fahrenheit to Celsius, subtract 30 and divide in half.
The exact formulas are similar, but are harder to calculate in your head:
And to go the other direction: