A **gram** is a unit of mass in the metric system defined as one thousandth (1 x 10^{-3}) of a kilogram. Originally, the gram was defined as a unit equal to the mass of one cubic centimeter of pure water at 4°C (the temperature at which water has maximum density). The definition was changed when the base units for the International System of Units (SI) were redefined by the 26th General Conference of Weights and Measures. The change went into effect May 20, 2019.

The symbol for the gram is the lowercase letter "g." Incorrect symbols include "gr" (the symbol for grains), "Gm" (the symbol for the gigameter), and "gm" (easily confused with the symbol for the gram-meter, g⋅m).

Gram may also be spelled gramme.

### Key Takeaways: Gram Definition

- The gram is a unit of mass.
- One gram is one thousandth the mass of one kilogram. The previous definition of the gram was the absolute weight of a 1-centimeter cube of pure water at 4 °C.
- The symbol for the gram is g.
- The gram is a small unit of mass. It is approximately the mass of one small paper clip.

## Examples of Gram Weight

Because a gram is a small unit of weight, its size may be difficult for many people to visualize. Here are common examples of objects that have about one gram of mass:

- A small paperclip
- A thumbtack
- A piece of chewing gum
- One US bill
- A pen cap
- One cubic centimeter (milliliter) of water
- A quarter teaspoon of sugar

## Useful Gram Conversion Factors

Grams may be converted into several other units of measurement. Some common conversion factors include:

- 1 gram (1 g) = 5 carats (5 ct)
- 1 gram (1 g) = 10
^{-3}kilograms (10^{-3}kg) - 1 gram (1 g) = 15.43236 grains (gr)
- 1 troy ounce (ozt) = 31.1035 g
- 1 gram = 8.98755179×10
^{13}joules (J) - 500 grams = 1 Jin (Chinese unit of measurement)
- 1 avoirdupois ounce (oz) = 28.3495 grams (g)

## Uses of the Gram

The gram is widely used in science, particular chemistry and physics. Outside of the United States, the gram is used to measure non-liquid cooking ingredients and produce (e.g., flour, sugar, bananas). Relative composition for food nutrition labels is stated per 100 grams of product, even within the United States.

## History of the Gram

In 1795, the French National Convention replaced the *gravet* with the *gramme* in the metric system. While the term changed, the definition remained that of the weight of one cubic centimeter of water. The word *gramme* came from the Latin word *gramma* which in turn derived from the Greek word *grámma*. The *grámma* was a unit used in Late Antiquity (around the 4th century AD) equal to two oboli (Greek coins) or one twenty-fourth part of an ounce.

The gram was a fundamental unit of mass in the centimeter-gram-second (CGS) system in the 19th century. The meter-kilogram-second (MKS) system of units was proposed in 1901, but the CGS and MKS systems co-exists throughout the early to mid 20th century. The MKS system became the system of base units in 1960. However, the gram was still defined based on the mass of water. In 2019, the gram was defined based on the kilogram. The kilogram has a mass almost exactly equal to that of one liter of water, but its definition has been refined, too. In 2018, Planck's constant was defined. This allowed definition of the kilogram in terms of the second and the meter. Planck's constant *h* is defined to be 6.62607015×10^{−34} and equal to one kilogram meter squared per second (kg⋅m^{2}⋅s^{−1}). Even so, standard masses for the kilogram still exist and are used as secondary standards for kilogram and gram weights. For all practical purposes, a liter of pure water has a mass of one kilogram and a milliliter of pure water has the mass of one gram.

## Sources

- Materese, Robin (November 16, 2018). "Historic Vote Ties Kilogram and Other Units to Natural Constants". NIST.
- National Institute of Standards and Technology (October 2011). Butcher, Tina; Cook, Steve; Crown, Linda et al. eds. "Appendix C – General Tables of Units of Measurement"
*Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices*. NIST Handbook. 44 (2012 ed.). Washington, D.C.: U.S. Department of Commerce, Technology Administration, National Institute of Standards and Technology. ISSN 0271-4027.