What was the approximate value of the Hubble constant originally calculated by Hubble?

The approximate value of the Hubble constant originally calculated by Hubble was 500 km/s per Mpc. This meant that, in Hubble's early interpretation, a galaxy one megaparsec farther away would appear to recede about 500 kilometers per second faster.

Hubble derived the estimate by comparing galaxy distances with their measured recession velocities, inferred mainly from spectral redshifts. His 1929 analysis used a relatively small sample of galaxies and distance calibrations that were not yet reliable. In particular, the relationship between Cepheid variable stars and true distance was misunderstood, causing galaxies to appear closer than they really were and pushing the calculated expansion rate far too high.

Later observations steadily revised the value downward. Modern measurements generally fall near roughly 67–74 km/s per Mpc, depending on the technique used. Measurements based on the cosmic microwave background and those based on the local distance ladder do not agree perfectly, a disagreement known as the Hubble tension.

The unit is not a speed alone. “Per Mpc” indicates how recession velocity changes with distance; multiplying the constant by a galaxy's distance gives an approximate expansion-related velocity.