Einstein's gravitational constant, `kappa` , is the coupling constant from Einstein's field equation which can be written as follows:
`G^(alpha * lambda) = kappa * T^(alpha * lambda)`
In this equation, `G^(alpha * lambda)` is the Einstein tensor and `T^(alpha * lambda)` is the stress-energy tensor.
This expression depicts the curvature of space and time. This stress-energy tensor, `T^(alpha * lambda)`, defines the disturbance of spacetime and this, in turn defines gravity.
Note that the constant `kappa` is directly proportional to Newton's gravitational constant G.
We can calculate the value of `kappa` a couple of ways:
`kappa = (8 * pi * G) / c^2 approx 1.866 * 10^-26 m/kg`, which expresses the stress-energy tensor in units of mass density.
Alternatively, T can be expressed as energy density and this means:
`kappa = (8 * pi * G) / c^4 approx 2.076579 * 10^-43 s^2/(m *kg)`