Merck Manual

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# Unit of Measure Conversions

By

Manuals Staff

Reviewed/Revised May 2023
Topic Resources

In the US, most laboratory test results are reported in what are termed conventional units; the rest of the world reports results in Système International d’Unités (SI) or international units (IU). The unit basis for SI is updated periodically by a panel.

Many SI units are the same as units used in the US system; however, SI units for concentrations are not. SI concentrations are reported as moles (mol) or decimal fractions of a mole (eg, millimole, micromole) per unit volume in liters (L). Conventional units are reported as mass (eg, grams, milligrams) or chemical equivalency (eg, milliequivalents) per unit volume, which may be in liters or decimal fractions of liters (eg, deciliters, milliliters). Results reported in amount per 100 mL (1 dL) are sometimes expressed as percent (eg, 10 mg/dL may be written as 10 mg%).

Moles, milligrams, and milliequivalents: A mole is an Avogadro’s number (6.023 × 1023) of elementary entities (eg, atoms, ions, molecules); the mass of 1 mole of a substance is its atomic weight in grams (eg, 1 mole of sodium = 23 g, 1 mole of calcium = 40 g). Similarly, the mass of a given quantity of substance divided by its atomic weight gives the number of moles (eg, 20 g sodium = 20/23, or 0.87, mol).

An equivalent is a unit that integrates charge and moles; 1 equivalent represents one mole of charges and is calculated by multiplying the number of moles of charged particles in a substance times the valence of that substance. Thus, for ions with a +1 or 1 charge (eg, Na+, K+, Cl), 1 mole is 1 equivalent (1 × 1 = 1); for ions with a +2 or 2 charge (eg, Ca2+), ½ mole is 1 equivalent (½ × 2 = 1), and so forth for other valence values. A milliequivalent (mEq) is 1/1000 of an equivalent.

The following can be used to convert between mEq, mg, and mmol:

mEq = mg/formula wt × valence = mmol × valence

mg = mEq × formula wt /valence = mmol × formula wt

mmol = mg/formula wt = mEq/valence

(Note: Formula wt = atomic or molecular wt.)

Alternatively, conversion tables are available in print and on the Internet.

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