This week's #WorldbuildingWednesdays is going to be about metrology! Not meteorology, metrology. Admittedly a more niche thing that won't captivate everyone's interest, but still.
It bears to reason that ponies probably use a different system of measurement than humans; they didn't have a French revolution to bring about the metric system and they don't have units of measurement based on human body parts for obvious reasons.
Likely they'd start using hooves. Using already established approximations for the sizes of ponies (similar to real life ponies, admittedly), we can approximate that these hooves are about 20 centimeters long. But they'd probably also want to standardize this measurement as pony hoof sizes vary.
It makes sense to start with Princess Celestia's hoof as a standard, but it is larger than the average pony's by about a third. It makes sense to have a unit of measurement based on Celestia's hoof (let's call it royal hoof) as a basis. (1/5)