square centimeter

The square centimetre is the bench scale of area. A postage stamp is about 5 cm², a fingernail about 1 cm², and laboratory work quotes filter membranes, electrode faces and solar-cell areas in cm² because the samples themselves fit in a hand: photovoltaic research compares cells by current per cm², and microbiology counts colonies per cm² of plate. On this site it feeds the geometry solvers and any pressure or stress calculation where a force is spread over a small face.

The trap is the old pressure gauge. A great deal of installed equipment worldwide reads kgf/cm², the metric cousin of psi, and 1 kgf/cm² is 98.0665 kPa: close to 1 bar at 100 kPa and close to one atmosphere at 101.325 kPa, but equal to neither, and the three get casually interchanged at the 2% level. Nuclear physics, meanwhile, never left this unit behind. Reaction cross-sections are quoted in barns, and the barn is defined as 10⁻²⁴ cm², not in square metres, so every neutron data library in use today still carries the CGS square centimetre inside it, and accelerator luminosity is likewise reported per cm² per second.