Unidades Relativistas
Relativistic units are used in Theory of Relativity, consist of taking the speed of light as the dimensionless constant 1, ie

c = 300.000 km/s ≈ 1

Now all the physical formulas can be passed to relativistic units.

Pass a non-relativistic formula units relativistic units is easy, simply replace all factors of c for 1, for example

E = mc2 is, in relativistc units

E = m (equivalence of mass and energy).

Converse is a bit trickier, but not too much, simply complete the units, always remembering that the units of c are [m].[s]-1, for example

E = m

E = m <=> [kg][m]2[s]-2 = m

Then need a [m]2[s]-2 to complete, so the formula becomes

E = mc2

Where these units come from? The answer come by consider Minkowski space, note that

v = (Δ x2 + Δ y2 + Δ z2) / Δt2

if we want v = c = 1, then

(Δ x2 + Δ y2 + Δ z2) / Δt2 = 1

- Δt2 + Δ x2 + Δ y2 + Δ z2 = 0

Taking infinitesimal increments we obtain The Minkowski metric .