Infiltration is the ingress of outdoor air under normal operating conditions through adventitious openings located in the façade of a building. The importance of reducing infiltration to save energy is highlighted by standards and building codes in many countries. The mean heating season, or other typical, infiltration rate is often inferred from a measurement of an air leakage rate at a pressure differential of 50 Pa from a whole building pressurization test, often made using a blower door. A simple linear relationship between the air leakage rate and the infiltration rate (infiltration equals air leakage divided by 20) is sometimes used and is known by many terms, such as the rule-of-20, leakage-infiltration ratio, Sherman’s ratio, or the Kronvall-Persily rule. In practice, the value of 20 is not applicable to all buildings and climates, and should be adjusted according to a number of factors, such as building height, shielding, air leakage path size, and climate. The origins of this relationship and its proper application have previously been unclear, which is problematic if a ratio is to be used with any confidence.This paper investigates the origins of leakage-infiltration ratios (LIRs) and shows that they emerged in the early 1980s from various studies (some unpublished) that measured airtightness and air change rates in a range of single-family dwellings and established empirical relationships between them. It is shown that there is no physical basis for LIRs, yet they are applied by building codes around the world and used to make policy decisions. Their widespread use is likely a function of their simplicity, yet they have significant limitations. Accordingly, it is recommended that their use be limited to quick estimations at best and that more technically-sound models be used to obtain more rigorous predictions of infiltration rates.