Jet grouting is widely used in geotechnical engineering for a variety of applications and is a well-proven technique. As with many techniques developed from a practical perspective, there is still scope for improvements both in construction practice and design. This paper focuses on one of the most crucial elements of quality control required with jet grouting operations, the diameter of the constructed column. First, the jet grouting method is explained and the main issues of the concept highlighted. A description follows of the techniques available for estimating column diameter, discussing their application and evaluation on site. There is a particular emphasis on two methods: inclined core drilling and a newly-developed thermic approach (Meinhard et al., 2010) both of which were implemented on two construction projects. Data from these case studies are reported and analysed extensively, in conjunction with influencing factors such as the ground conditions, to assess their effects on the achieved diameter. An empirical approach for evaluating the diameter of jet-grouted columns is developed based on various factors influencing their size such as monitor lifting speed and soil strength.