Definition : The kilogram is the unit of mass; it is equal to the mass of the International Prototype of the Kilogram.
The International Prototype of the Kilogram (IPK) is kept at International Bureau of Weights and Measures (BIPM) and it is considered as exactly “1 kg” immediately after cleaning and washing by the method used at the BIPM.
The National Physical Laboratory, India (NPLI) is the custodian of National Prototype of the Kilogram, copy No. 57 (NPK-57), which was provided by the BIPM in 1958 after its first calibration in 1955. The NPK-57 has been recalibrated in 1985, 1992, 2002 & 2012 so far at the BIPM. It is served as primary standard in India.
The mass value of NPK-57 is disseminated to NPL mass standards from 1 mg to 2,000 kg, solid density standards and other derived parameters. Our Mass Standards has been successfully gone through the Peer Review in 2004 & 2008 and our current CMCs on mass, volume, density and viscosity have been approved by the BIPM in 2011 and available in Appendix C in BIPM website.
The Mass Standards have participated in number of key comparisons and bi-lateral comparison organized by the CCM and APMP for the reaffirmation of the CMCs.