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Weights and Measures

By

The Manual's Editorial Staff

Last full review/revision Mar 2010| Content last modified Sep 2013
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In medicine, precise measurements are necessary—for example, when various substances are measured in laboratory tests to evaluate health or make a diagnosis. Different units of measure may be used depending on the substance. Usually, the metric system, based on multiples of 10, is used to measure the following:

  • Mass: Grams measure mass, the amount of matter in an object. Mass is similar to weight, but weight is affected by gravity.

  • Volume: Liters measure volume, the amount of space an object occupies.

  • Length: Meters measure length.

Prefixes, indicating which multiple of 10 is meant, can be attached to the basic unit, such as meter (m), liter (L), or gram (g). Using prefixes helps make a number more readable. Commonly used prefixes include kilo (k), deci (d), centi (c), milli (m), and micro (μ).

Other units measure different properties of a substance. For example, a mole (mol) is the amount of a substance that contains the same number of particles (molecules or ions) that is in 12 grams of carbon. Thus, regardless of the substance, 1 mole always contains the same number of particles. However, the number of grams in 1 mole varies greatly from substance to substance. One mole equals the molecular (atomic) weight of a substance in grams. For example, the molecular weight of sodium is 23, so 1 mole of sodium equals 23 grams. A molecule of table salt (sodium chloride) consists of one atom of sodium and one atom of chlorine (which has a molecular weight of 35 grams). Thus, one mole of sodium chloride weighs 23 grams + 35 grams = 58 grams.

Osmolarity is a measure of the number of particles in a liter of liquid, and osmolality is a measure of the number of particles in a kilogram (kg) of liquid. Because 1 liter of water weighs 1 kg, osmolarity and osmolality are the same for substances dissolved in water. An osmole is the amount of a substance that dissolves in liquid to form 1 mole. For example, because table salt dissolves into sodium and chloride in water, one mole of table salt dissolved in 1 liter of water results in 1 mole of sodium and 1 mole of chloride. Thus, its osmolarity is 2 osmoles per liter, and its osmolality is 2 osmoles per kg.

Equivalents (Eq) and milliequivalents (mEq) measure a substance’s ability to combine with another substance. A milliequivalent is roughly equivalent to a milliosmole.

Formulas are used to convert a measurement from one unit to another. The same amount can be expressed in terms of different units. For example, the concentration of calcium in the blood is normally about 10 milligrams in a deciliter (mg/dL), 2.5 millimoles in a liter (mmol/L), or 5 milliequivalents in a liter (mEq/L).

The units used for medical tests vary depending on the substance being measured. The units that are traditionally used in the United States are called conventional units. Conventional units usually express concentration as weight per volume, and the volume can vary. The International System of Units (SI units) always expresses concentration as moles per liter.

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