The amount of air leakage through window gaps in buildings depends on the width and length of the gaps, the pressures across the buildings and the resistances in the air circuit. Reviews the ranges of these factors for Britain and gives results of measurements within these ranges for standard metal and wooden windows and for weather-stripped windows. Gives two methods of estimating the air flow through gaps in complete windows. The first method, based on measurements of flow throughshort uniform lengths is accurate, but tedious, and the second method based on the best fitting equation, reproduced as a nomogram provides a quick but less accurate estimate. Shows how air flow through windows in buildings may be estimated by the use of the nomogram.