密勒指数是一种用来确定晶体方向的指数。要想计算出一个晶面的密勒指数（hkl)需要：

以一个正方体的顶面为例，它在坐标轴上的截距为「+∞(h)，+∞(k)，1(l)」，这样它们的倒数之比就为0:0:1,换句话说，用密勒指数表示就是(001)。密勒指数之所以使用倒数之比就是为了防止正无穷的出现。

对于完全一样的晶面则可以用带花括号的密勒指数来表示，比如在立方晶系中{100}就表示所有的晶面。

Miller indices form a notation system in crystallography for planes in crystal (Bravais) lattices.

In particular, a family of lattice planes is determined by three integers h, k, and ℓ, the Miller indices. They are written (hkℓ), and denote the family of planes orthogonal to , where are the basis of the reciprocal lattice vectors. (Note that the plane is not always orthogonal to the linear combination of direct lattice vectors because the reciprocal lattice vectors need not be mutually orthogonal.) By convention, negative integers are written with a bar, as in 3 for −3. The integers are usually written in lowest terms, i.e. their greatest common divisor should be 1.