Nonwoven geotextiles have been used as filters for over four decades in geotechnical and geoenvironmental works. These products have a complex fibre matrix and their behaviour as filters depends on how this matrix interacts with fluids and soil particles. Therefore, uncertainties still exist regarding prediction of geotextile filter performance under severe and critical situations. This paper investigates how confinement and partial clogging influence the dimensions of openings in nonwoven geotextiles. Bubble point (BBP) tests were carried out on six nonwoven geotextiles under unconfined and confined (equivalent vertical stresses of up to 1000 kPa) conditions with and without partial clogging of the geotextile. The results obtained show significant influences of confinement and partial clogging on geotextile opening dimensions and retention capacity. The results of O-95 from bubble point tests on unconfined and virgin specimens compared well with results from hydrodynamic tests. Comparisons between BBP results and data from filtration tests under confinement are also presented and discussed. The repercussions of reductions in opening size caused by confinement and partial clogging on geotextile clogging potential and filter criteria are discussed.