Symbol kDa. A non-SI unit of mass used to express molecular mass, especially for large molecules, such as proteins and polysaccharides. It is equal to 1000 daltons. A dalton is equal to one twelfth of the mass of an atom of carbon-12, or 1.660 33 × 10−27 kg. It is named after British chemist and physicist John Dalton (1766–1844).