The square inch is a unit of area and is an imperial unit with the symbol in².
One square inch is equal in area to a square with edges one inch in length.
The square inch is a common unit of measurement in the UK and US.
The barn is a unit of area and is used in particle physics with the symbol b.
It represents approximately the area of the cross-section of a uraneum nucleus, so is very small compared to most other recognised units of area.
It is also used to measure the interaction of a nucleus with an electric field gradient in nuclear quadrupole and magnetic resonance.