Tablet hardness unit

The units of hardness for tablets follows standards for materials testing - the International System of Units (SI – from Le Système International d'Unités)(1-4).

• Kilogram (kg) – The kilogram is recognized by the SI system as the primary unit of mass.
• Newton (N) – The Newton is the SI unit of force; the standard for tablet hardness testing. 9.807 Newtons = 1 kilogram.
• Pound (lb) – Technically a unit of mass but can also be used for force and should be written as pound force or lbf in this case. Sometimes used for tablet strength testing in North America, but it is not an SI unit. 1 kilogram = 2.204 pounds.

Historically, tablet hardness testing evolved using units derived from specific instrument readings.

• Kilopond (kp) – Not to be confused with a pound. A unit of force also called a kilogram of force. Still used today in some applications, but not recognized by the SI system. 1 kilopond = 1 kgf.
• Strong-Cobb (SC) – The Strong-Cobb unit is a legacy of one the first tablet hardness testing machines. Although the SC is arbitrary, it was recognized as the international standard from the 1950s to the 1980s. 1.4 Strong-Cobbs are approximately 1 kg (5).

Additional references

• Russ Rowlett, How Many? A dictionary of units and measurements
• The International System of Units (SI), Bureau International des Poids et Mesures
• J.E. Rees & P. J. Rue, 'Work Required to Cause Failure of Tablets in Diametrical Compression', Aston University, Birmingham, UK; Drug Development & Industrial Pharmacy. 4(2) 131-156 (1978)
• American Society for the Testing of Materials (ASTM), Designation: E4–07, 'Standard Practices for Force Verification of Testing Machines'. www.astm.org
• McCallum, A., Buchter, J. and Albrecht, R. (1955), Comparison and correlation of the strong cobb and the monsanto tablet hardness testers. Journal of the American Pharmaceutical Association, 44: 83–85. doi: 10.1002/jps.3030440208