Urban slums and unplanned settlements in India depend on highly on-site sanitation systems (OSS). Fecal Sludge (FS) produced in OSS is one of the most dangerous pollutants that have the potential to spread many diseases. The Government of India is emphasizing proper collection, treatment, and disposal of FS by launching various programs and schemes (such as NUSP, AMRUT, smart city mission, UIDSSMT, etc.). The private sector plays a major role in the collection of FS, while very little if any, treatment exists for the FS collected. The design and sustainability of any FS treatment depend on the accurate knowledge of the quality and quantity of FS at the city/town level. This paper systematically reviews the existing methods of FS quantification and its drawbacks in Indian conditions. In the literature on FS, two theoretical approaches (i) FS production method (FSP), (ii) FS collection methods (FSC) are being used for quantification. Most of the Indian practices use the FSP method for FS quantification, which is similar to conventional wastewater quantification. However, in India, the data on the amount of excreta produced and sludge accumulation rate (SAR) in different OSS are seldomly found. This results in the usage of data available in developed countries leading to over/underestimation of FS on which treatment facilities are designed. This study concludes that the design of FSM for Indian conditions requires basic research on excreta produced and its degradation properties in different OSS. © 2021, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.