Units of measurement

Many different units of measurement are used throughout Equus.

Equestria
Equestria has two main units of length: the royal hoof and the standard hoof.

Royal hoof
The royal hoof (rH) was introduced as a way of standardizing measurements by using the exact length of Princess Celestia's hoof from back to front as a unit, although later on it was more precisely defined based on planetary measurements which are less prone to change.

A royal hoof is defined by the Equestrian Institute of Measurement to be precisely 1/150,000,000 of the circumference of Equus.

The average mare is 4.5 royal hooves tall and the average stallion is 5 royal hooves tall.

Standard hoof
The standard hoof (sH) was introduced as the hooves of an average Pony are significantly smaller than those of Celestia.

It approximates the length of an average Pony hoof as 3/4ths of a royal hoof, making it 1/200,000,000 of the circumference of Equus by definition.

The average mare is 6 standard hooves tall and the average stallion is 6.7 standard hooves tall.

Multiples
Legs are equivalent to 3 hooves. The standard leg (sL) equals 3 standard hooves and the royal leg (rL) equals 3 royal hooves.

Legs are also called paces as the length of a Pony's leg is similar to the distance covered by taking two steps.

Steps (sS and rS) or half-paces are equivalent to 1.5 hooves.

The length of 60 standard paces is known as a toss (T).

The length of 60 tosses is known as a walk (W).

The length of 60 walks is known as a journey (J).

Weight
The bit (b) is the standard unit of weight.

It is defined as the weight of a gold bit made of pure gold, which is 1/4th of a royal hoof in diameter and 1/16th of a royal hoof in width.

Volume
The basic unit of volume in Equestria is the Equestrian gallon or cubic standard hoof.

It has a subdivision, the Equestrian cup (1/60 of an Equestrian gallon) and a multiple, the Equestrian barrel (60 Equestrian gallons).

Time
The basic unit of time is the second. A second is 1/60th of a minute, a minute is 1/60th of an hour and an hour is 1/24th of a day.

A moon is equivalent to 29.5 days and a sun is equivalent to 365.25 days.

Equestria uses the Selenial Calendar, which pegs the latest Canterlot eclipse as day 1 of the year 1001. Previously, the Helial Calendar was used, which pegged the day 1 of the year 1 at the coronation of Princess Celestia.

Qilinia
Qilinia uses the imperial hoof (iH), whose length is pegged to the royal hoof.

Griffonstone
Griffons use claws (c), which are equivalent to the length of a fully extended Griffon claw between the furthest tips of the talons. A claw is approximately 7/3rds of a standard hoof or 7/4ths of a royal hoof.

Yakyakistan
Yakyakistan uses hooves. The yak hoof (yH) isn't standardized, but Yak hooves are similar in length to a Pony hoof so a measurement in yak hooves usually doesn't need conversion to standard hooves.

Changeling Lands
Changeling hives traditionally used the hoof of their queen as a measurement unit. These units became standardized among hives as time passed and remained in use even as some hives lost their queens.

A changeling royal hoof (cH) is roughly 11/8ths of a pony standard hoof.