Instead of working with the Hubble parameter, a common practice is to introduce the dimensionless Hubble parameter, usually noted h, and to write the Hubble's parameter H₀ as being

H₀ = 100 h km·s

Second

The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

⁻¹·Mpc⁻¹, all the uncertainty relative of the value of H₀ being then relegated on h.

Note that other conventions may be found in the literature.