Hm, that is, "hundred meters", belongs to the hectare unit, but it can be regarded as "100m unit" in the popular sense, 1hm= 100 meter.

So the square of hm is "100 square meters".

1 km2 =100 km2.

Extended data:

The unit of length refers to the basic unit for measuring spatial distance, which is formulated by human beings to standardize the length.

In the international system of units, the standard unit of length is "meter", which is represented by the symbol "m". Generally speaking, "meter" is generally defined as the length of the distance traveled by light in vacuum within the time interval of 1/299792458 s. The unit of length adopted in China is consistent with the international system of units, that is, "meter" is the legal unit of length measurement in China.

Commonly used length units are: kilometer {kilometer} (km), decimeter (dm), centimeter (cm), millimeter (mm), micron (μm), nanometer (nm), picometer (pm) and femtometer (fm).

Source: Baidu Encyclopedia-Length Unit