This page considers the most common conversion errors, when doing conversions between the 3 measurement systems (Metric, Imperial, and USA). For more general information on the metric, USA and Imperial systems of measurement, click on Metric System Conversion.
The most common errors made when converting to or from the metric system include:
- Confusion between a fluid ounce and an ounce of weight.
- Confusion between Imperial measures and USA measures.
- Confusion between different units with the same name.
Converting Between imperial and USA Measures
There are a number of differences between the Imperial and USA systems of measurement. Some of these differences have been eliminated by evolving standards. In particular, in 1959 the Imperial and USA measurements of the inch (and all its multiples: foot, yard, mile, square foot, square yard, etc.) and the pound were standardized to the same values. The most common differences that are still encountered are:
- Measurements of liquid volume. All the common measures of volume have the same name in both the USA and Imperial systems, but most of them have different sizes. See Metric & Imperial Units for details.
- Measurements of weight. The Imperial pound and the USA pound have now been standardized to the same weight. However, the Imperial system also uses the "stone" (which is not used in the USA system) and the definitions of hundredweight and ton are different in the two systems. See Metric & Imperial Units for details.
- Measuring spoons. There are various differences for measuring spoons (teaspoon, tablespoon, dessertspoon, etc.), see Cooking Converter for details.
Measures with Multiple Definitions
In both the Imperial and the USA systems, a number of measures have multiple definitions. Fortunately, most of these ambiguous measures are not in common usage. Following is a summary, although we make no claims to it being complete:
- Acre. A "US survey" acre (43, 560 square feet) is larger than a "commercial acre" (36,000 square feet).
- Barrel. There are seven different barrel sizes used in the USA, with the size being dependent on the contents. Their names and metric equivalents are as follows: US cranberry (95.5 liters), US dry (115.628 liters), US liquid (119.24 liters), US federal (117.348 liters), US federal proof spirits (151.416 liters), US drum (208.4 liters), US petroleum (135 kg.), US petroleum statistical (158.99 liters).
- Bushel. There are many definitions of the bushel. Examples of USA bushels and their approximate metric equivalents are: Oats bushel (14.5 kg), Barley bushel (21.8 kg), Shelled corn or rye bushel (25.4 kg), Wheat/soybeans/potatoes bushel (27.2 kg), US volume bushel (35.239 liters). The Imperial volume bushel is equal to 36.36872 litres.
- Clothing and sizes. Shoe sizes are measured different for "men", "women" and "children". Likewise, clothing sizes. This is further complicated by different measurements from one country to another and different interpretations from one manufacturer to another. See Clothing and shoe sizes.
- Degree. In addition to degree Fahrenheit (for temperature), there are at least 12 other definitions of "degree". Ten of these define density, one defines angle (degree arc) and one defines temperature (degree Rankine).
- Foot. The foot in common use has been standardised (in 1959) to 30.48cm. There is also the "US survey" foot which is slightly larger at 30.480061cm.
- Mile. In addition to the "international" mile which is common usage (defined as 1.609344 Km.), there are also the "nautical" mile (1.852 Km.) and the "US survey mile" (approximately 1.609347 Km.).
- Ounce. An ounce can either be a measure of volume (in which case it is referred to as a "fluid ounce") or a measure of weight (in which case it is referred to as a "ounce weight"). If the term "ounce" is used without specifying fluid or weight, it is generally assumed to be "ounce weight". The USA fluid ounce (equal to 29.573 529 562 5 milliliters) is slightly larger than the Imperial fluid ounce (equal to 28.413 062 5 milliliters). In addition, there are multiple types of "ounce (weight)". The ounce (weight) in common usage is the "ounce avoirdupois" (29.349523125 grams). The "troy ounce" (31.1034768 grams) is used in various specialist applications, in particular the weights of precious metals. Consequently, an ounce (weight) of gold is not the same weight as an ounce (weight) of steel.
- Pint. A pint can be either a "liquid pint" or a "dry pint", although in common usage one assumes the liquid pint. The USA liquid pint (approximately 473.2 milliliters) is smaller than the UK liquid pint (568.261485 milliliters), and smaller than the USA dry pint (approximately 550.6 milliliters).
- Quart. A quart can be either a "liquid quart" or a "dry quart". The USA liquid quart is smaller than the UK quart. See above discussion of Pint.
- Year. There are six different definitions of the "year", although the differences are not large (except for the "leap year" and the "lunar year".
- Other. There are two versions of the BTU (British Thermal Unit) and the calorie; both of these are measures of energy. There are two versions of the Cable (a measure of length), footcandle, keg (US large beer and US small beer), minute (solar minute of 60 seconds, sidereal minute of 59.83617 seconds), pace (geometrical and military), pica. There are several definitions of "point". The term "pound" is normally used to describe weight, but is also used to describe the density of paper.