https://www.nist.gov/pml/weights-and-measures/si-units-mass SI Unuoj: Maso SI Units: Mass https://www.nist.gov/pml/weights-and-measures/si-units-mass National Institute of Standards and Testing: SI Units metric kilogram standard copies SI Unuoj: Maso https://www.nist.gov/pml/weights-and-measures/si-units-mass https://www.nist.gov/pml/weights-and-measures/si-units-mass https://www.nist.gov/pml/weights-and-measures/si-units-mass