A real manufactured surface is not perfectly smooth. Its measured profile contains geometric variation at several length scales.

Surface texture is commonly separated into:

• roughness: relatively short-wavelength irregularities associated with the process, such as tool marks;
• waviness: longer-wavelength variation caused by effects such as vibration, deflection or thermal distortion;
• form error: still larger-scale departure from the intended geometry.

A common roughness measure is the arithmetic mean roughness $R_a$. If $z(x)$ is the profile height relative to the mean line over evaluation length $L$,

$$R_a=\frac{1}{L}\int_0^L |z(x)|,dx.$$

For discrete sampled heights $z_i$,

$$R_a\approx \frac{1}{n}\sum_{i=1}^n |z_i|.$$

If five equally spaced deviations are $-2,-1,0,1,2\ \mu\mathrm m$, then

$$R_a=\frac{2+1+0+1+2}{5}=1.2\ \mu\mathrm m.$$

Two surfaces with the same $R_a$ need not have the same shape, so one scalar cannot describe every functional aspect of texture. Sealing, friction, fatigue and appearance can depend on peaks, valleys, directionality and spacing as well as average amplitude.

Surface-texture requirements should therefore be specified because function needs them, not simply because a smoother surface seems preferable. Finer finishes usually require additional process capability, time or finishing operations.