A portable computer comprising a planar base member having a circular, logarithmically spaced scale thereon indicative of a first value and having an opening disposed therethrough, a circular disc rotatably mounted in superimposed relation on the base member and having a logarithmically spaced scale disposed on a front face thereof which is indicative of a second value. The circular disc also has a logarithmically spaced scale on the back face thereof in alignment with the opening through the base member and being indicative of a third value which is the quotient of the first and second values so that when the circular disc is rotated relative to the base member with a predetermined first value aligned with a predetermined second value, then the resulting third value will be aligned in the opening in the base member. Preferably, the computer also includes an additional logarithmically spaced scale and an equally spaced scale on the circular disc member and also includes a second circular disc member having a fixed indicator means and an equally spaced scale thereon for cooperating with the first disc member to compute additional values. The portable computer may be specifically constructed for use by a motorist and the above scales include units indicating various motorist''s values.