A milligram (mg) is a unit of weight/mass in the International System of Units (SI), the modern form of the metric system of measurement. Milligrams are commonly used for measuring smaller masses, such as dosages for medications, gold (1 carat = 200 mg), the mass of a mosquito, etc.

1,000,000 milligrams = 1 kilogram

1,000 milligrams = 1 gram

Milligrams are used throughout most parts of the world, since SI is the most widely used system of measurement. Even in the US, which primarily uses the US customary system of measurement for everyday purposes, milligrams are commonly used in the scientific and medical fields, among others. Common measurements of mass in the US customary system include ounces, pounds, and tons; below are their relationships to milligrams.

1 milligram = 0.0000353 (3.53e-5) ounces

1 milligram = 0.0000022 (2.2e-6) pounds

1 milligram = 0.0000000011 (1.1e-9) tons

As can be seen, even compared to two of the smallest units of mass in the US customary system, the milligram is a small measurement, so its use in everyday applications is limited, and it is not as commonly used as units such as grams and kilograms.

SI prefixes

The International System of Units (SI) makes use of SI prefixes (milli-, centi-, kilo-, etc.) to denote multiples or submultiples of a base unit that are related by a power of 10. The base unit of mass in SI is the kilogram. The kilogram is unique as a base unit in that it is the only base unit that already has an SI prefix, kilo-, indicating that 1 kilogram is 103 grams. All other base units in SI such as meters, seconds, moles, etc., do not have prefixes.

Since SI does not allow for the use of multiple prefixes (e.g. "kilomilligram") to modify a base unit, prefixes are added to the gram, rather than kilogram, to denote different multiples or submultiples of mass.

1 milligram = 10-6 kilograms

1 milligram = 10-3 grams

1 milligram = 10-2 decigrams

1 milligram = 10-1 centigrams

Remembering SI prefixes and what power of 10 they represent is helpful because it allows us to choose the most appropriate unit for whatever is being measured. For example, a 200 mg tablet is a relatively common size for certain types of medicine. Measuring a tablet of this mass in kilograms, while possible, would not be the most appropriate choice of unit, since 0.0002 kg is more difficult to work with than 200 mg.

It is also important to be able to convert between various measurements of mass in different systems, particularly from US customary units to SI, since SI is the standard used throughout most of the world.


Convert the following measurements to milligrams and kilograms.

1. 8 pounds:

There are 0.00000220 pounds in 1 milligram, or 453,592.4 milligrams in 1 pound, so:

8 × 453,592.4 = 3,628,739.2 mg

3,628,739.2 × 10-6 = 3.629 kg

2. 0.25 ounces:

There are 0.0000353 ounces in 1 milligram, or 28,349.52 milligrams in 1 ounce, so:

0.25 × 28,349.52 = 7,087.38 mg

7,087.38 × 10-6 = 0.00709 kg