Ho ≈ 73.8 km/sec/Mpc
In 1929 Edwin Hubble estimated the value of the expansion factor of the Universe, now called Hubble's constant, to be about 500 km/sec/Mpc. His method is considered correct but his data was largely flawed.
Today the precise value remains uncertain but it is estimated to be in the range of 70 km/sec/Mpc. Recent measurements place the calcuated value at 73.8 km/sec/Mpc (+/- 2.4 km/sec/Mpc).
The value is important to cosmologists because it can be used to estimate the amount of time since the expansion began. Current estimates for the age of the universe since the initial moments of the big bang are about 13.8 billion years.