粒径是指如岩石或其他固体单一个粒子的物理尺度。

颗粒物质的大小范围可由极小的胶体，经黏土、粉砂、砂、砾，至岩石。

不同尺度范围的名称以巫登–温特瓦分级(Udden_Wentworth scale)来定义。克伦宾 φ 尺度，巫登 - 温特瓦分级的变型，为以下式计算的对数尺度：

在其他的一些分类法内，「砾」是指一切大过砂(>2.0 mm)的物质，包含着上表中的「细砾」、「小卵石」、「大卵石」及「岩块」。在此一分类法中，「小卵石」的尺度范围在4至64公厘之间(−2 to −6 φ)。

Particle size, also called grain size, refers to the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which refers to the size of a single crystal inside a particle or grain. A single grain can be composed of several crystals. Granular material can range from very small colloidal particles, through clay, silt, sand, gravel, and cobbles, to boulders.