The gravitational constant (also known as the universal gravitational constant, the Newtonian constant of gravitation, or the Cavendish gravitational constant), denoted by the capital letter , is an empirical physical constant involved in the calculation of gravitational effects in Sir Isaac Newton's law of universal gravitation and in Albert Einstein's theory of general relativity. In Newton's law, it is the proportionality constant connecting the gravitational force between two bodies with the product of their masses and the inverse square of their distance. In the Einstein field equations, it quantifies the relation between the geometry of spacetime and the energy–momentum tensor (also referred to as the stress–energy tensor). The measured value of the constant is known with some certainty to four significant digits. In SI units, its value is approximately The modern notation of Newton's law involving was introduced in the 1890s by C. V. Boys. The first implicit measurement with an accuracy within about 1% is attributed to Henry Cavendish in a 1798 experiment. Definition According to Newton's law of universal gravitation, the attractive force () between two point-like bodies is directly proportional to the product of their masses ( and ) and inversely proportional to the square of the distance, , between their centers of mass: The constant of proportionality, , is the gravitational constant. Colloquially, the gravitational constant is also called "Big G", distinct from "small g" (), which is the local gravitational field of Earth (equivalent to the free-fall acceleration). Where is the mass of the Earth and is the radius of the Earth, the two quantities are related by: The gravitational constant appears in the Einstein field equations of general relativity, where is the Einstein tensor, is the cosmological constant, is the metric tensor, is the stress–energy tensor, and is the Einstein gravitational constant, a constant originally introduced by Einstein that is directly related to the Newtonian constant of gravitation: Value and uncertainty The gravitational constant is a physical constant that is difficult to measure with high accuracy. This is because the gravitational force is an extremely weak force as compared to other fundamental forces at the laboratory scale. In SI units, the 2018 Committee on Data for Science and Technology (CODATA)-recommended value of the gravitational constant (with standard uncertainty in parentheses) is: This corresponds to a relative standard uncertainty of (22 ppm). Natural units The gravitational constant is a defining constant in some systems of