Lahan is a municipality situated in the Siraha district of the Madhesh region in Nepal. Located at an elevation of 111 meters above sea level, it sits at the geographical coordinates of 26.717 latitude and 86.483 longitude. The city follows the Nepali time zone with a UTC offset of +05:45 hours.
The municipality is characterized by its high population density, ranking as the 36th densest in Nepal. It has a population of 102,031 residents distributed across 24 wards, which is the highest number of wards for any municipality in the country. Lahan is accessible via the east-west highway and is located approximately 261 km east of Kathmandu via the B.P. Highway and E-W Highway, or 424 km east via the Narayanghat-Mugling Highway.