A seal is fluid-tight if the leakage is not noticed or if the amount of noticed leakage is permissible. The maximum permissible leakage for the application is known as the leakage criterion.

The fluid tightness may be expressed either as the time taken for a given mass or volume of fluid to pass through the leakage capillaries or as the time taken for a given pressure change in the fluid system. Fluid tightness is usually expressed in terms of its reciprocal, that is, leakage rate or pressure change.

Four broad classes of fluid tightness for valves can be distinguished: nominal-leakage class, low-leakage class, steam class, and atom class.

The nominal- and low-leakage classes apply only to the seats of valves that are not required to shut off tightly, as commonly in the case for the control of flow rate. Steam-class fluid tightness is relevant to the seat, stem, and body-joint seals of valves that are used for steam and most other industrial applications. Atom-class fluid tightness applies to situations in which an extremely high degree of fluid tightness is required, as in spacecraft and atomic power plant installations.

Lok introduced the terms steam class and atom class for the fluid tightness of gasketed seals, and proposed the following leakage criteria.

Steam Class

Gas leakage rate 10 to 100 μg/s per meter seal length.

Liquid leakage rate 0.1 to 1.0 μg/s per meter seal length.

Atom Class

Gas leakage rate 10−3 to 10−5 μg/s per meter seal length.

In the United States, atom-class leakage is commonly referred to as zero leakage. A technical report of the Jet Propulsion Laboratory, California Institute of Technology, defines zero leakage for spacecraft requirements.According to the report, zero leakage exists if surface tension prevents the entry of liquid into leakage capillaries.