There is an urgent need for a cost-effective, precise, and portable device for rapid and in situ measurement of the critical properties of an emulsion. Here, we report the development of such an optofluidic device for the measurement of mean droplet size (dMean) and droplet size distribution (DSD) of a water-in-oil emulsion. We formulated and detected water-in-oil droplets of much smaller dimensions (15μm) compared to the detection of larger droplets or plugs ( 100 μm to 300μm) reported in the literature, employing a cost effective and portable in-house built optical detection system. Use of the device for the measurement of the frequency of droplets from an on-chip droplet generator is demonstrated and validated using microscopy with excellent accuracy (2%). In addition, we provide some insight into the relatively high uncertainty in the collected signal in case of smaller droplets. The droplet size dD is characterized in terms of forward scatter signal and residence time T. We further argue that normalized residence time T of droplets in the detection zone which correlates linearly with droplet size dD is a better parameter to measure droplet size , compared to the forward scatter signal which vFSCM correlates nonlinearly with dD. Finally, the device is used to count the number of droplets of different size to predict dMean and DSD of emulsions. The results were compared with that obtained from traditional microscopy and a very good match (10-13%) was found, in contrast to previously reported non-portable off-chip methods that are 20-44% accurate. Thus, the reported device possesses high potential for accurate measurement of and DSD of emulsions in practical applications. © 2017 IOP Publishing Ltd.