A standard Charpy V-notch specimen has a width (W) of 10mm and a notch depth of 2mm. The specimen thickness, B, can be 10mm ('full-size'), 6.7mm ('two-thirds' size), 5mm ('half-size') or even 2.5mm ('quarter-size'). To convert J(energy) to J/cm2 (specific energy), divide the energy absorbed by the area (in cm2) of the un-notched ligament, i.e. by ((W-0.2)xB)cm2.
For example, if the energy absorbed during testing of a full-size specimen is 100J, then the specific energy is 125J/cm2. For a two-thirds-size specimen, an energy absorption of 100J would be equivalent to (100/(0.8x0.67))=187J/cm2