荷质比又称比荷、比电荷，是一个带电粒子所带电荷与其质量之比，其单位为C/kg。计算时，粒子无论带何种电荷，应一律代入正值计算。

电子电荷e和电子静止质量 m的比值e／m（电子比荷）为电子基本常量之一，可通过磁聚焦法、磁控管法、汤姆森法及双电容法等进行测定。现代精确测量电子比荷的值为1.75881962×10C/kg，质子比荷的值为9.578309×10C/kg，一般计算中取1×10C/kg.

1897年，约瑟夫·汤姆森透过测定阴极射线在磁场和电场的偏转，获得电子的电荷对质量的比值,且电子在带电粒子中具有最大质荷比。 1901年，沃尔特·考夫曼发现β射线（高速电子流）的荷质比随速度增大而减小，由于电子电荷守恒，因此实验表明电子的质量随速度的增加而增加，成为狭义相对论的实验基础之一。

The mass-to-charge ratio (m/Q) is a physical quantity that is most widely used in the electrodynamics of charged particles, e.g. in electron optics and ion optics. It appears in the scientific fields of electron microscopy, cathode ray tubes, accelerator physics, nuclear physics, Auger electron spectroscopy, cosmology and mass spectrometry. The importance of the mass-to-charge ratio, according to classical electrodynamics, is that two particles with the same mass-to-charge ratio move in the same path in a vacuum when subjected to the same electric and magnetic fields. Its SI units are kg/C. In rare occasions the thomson has been used as its unit in the field of mass spectrometry.