Several studies have shown the efficacy of cataract surgery in lowering intraocular pressure (IOP), especially in angle closure glaucoma compared to open angle. A total of 156 eyes of 109 patients (without glaucoma or with medically controlled glaucoma) were included. Mean preoperative IOP was 15.6mmHg with 0.7 (range 0-4) glaucoma medications per eye. Eyes with peripheral anterior synechiae, secondary open angle glaucoma and presence of other conditions that could affect IOP were excluded. When adjusting for preoperative IOP, age, gender and ethnicity, the preoperative parameters that were statistically significant for predicting amount of IOP reduction after phacoemulsification at six months were anterior chamber depth (ACD), gonioscopy score, presence of angle closure and angle open distance (AOD). Eyes with angle closure have a higher IOP reduction and these eyes tend to have a shallower anterior chamber (reduced ACD). Furthermore, in eyes with open angle glaucoma, the narrower the angle (lower AOD or lower gonioscopy score), or the more anteriorly positioned the lens, the greater the IOP reduction after cataract surgery at six months. In the immediate postoperative period, increased IOP is expected in eyes with glaucoma due to the inflammation leading to worsening in the drainage function at the trabecular meshwork. The authors herein propose a number of formulae that could be applied to clinical practice to quantify the anticipated reduction in IOP after cataract surgery.