Hubble’s Law states that the recessional velocity of a galaxy (V) is equal to the product of the Hubble’s Constant (H) and the distance to that galaxy (D), or V = HD

Hubble’s law is usually expressed as a relationship between redshift and distance.

The exact value of the Hubble constant, H, is very important in cosmology, as it leads to an estimate of the age of the universe, helps test theories of Dark Matter and Dark Energy, etc. Thus a great deal of effort has gone into working it out.

On 21 March 2013, an European-led research team estimated it to be 67.80 ± 0.77 kilometers per second per megaparsec; this is about 21 km/sec per million light years. What this means is that an object a million light years away would be receding from us at 21 km/sec; an object 10 million light years away at 210 km/sec, etc.