Given the plethora of units in the German-speaking part of Europe in Kant's day, it is difficult to ascertain exactly which reference standard he was using on any given occasion. We may assume, however, that for the most part he used Prussian measures.

As Kant explains in § 8 of the Physical Geography, one ‘geographical mile’ was taken as 1/15 of a degree, measured along the Earth's circumference. Therefore there are 15 × 360 = 5,400 ‘geographical miles’ in the circumference of the Earth. Taking the modern value of 7,927 English miles for the equatorial diameter of the Earth, a geographical mile is thus 4.612 English miles. This figure may be used for the purposes of comparing Kant's astronomical data with modern measurements, as is done in Appendix ii. (It will be noted that good agreement is found for the diameter of the earth, which shows that the conversion factor of 4.612 is appropriate.) Also, since 7,927 is a known modern value, we may use it to gauge the value of the eighteenth-century units of length employed by Kant, even though we may not know exactly which standard he was referring to at any particular point in his text. It may be noted, however, that according to Ronald Zupko one Rhineland foot (as used in Prussia) was equal to 12.36 English inches (or 1.030 English feet). We have, therefore, a small discrepancy, but the information is adequate to gain a reasonably accurate estimate of which standard of length Kant was using.