Travellers at Singapore Changi Airport who are in transit and who are not seeking entry into Singapore, do not need to apply for a visa if they remain in the transit area and do not cross immigration. Travellers are advised to inquire with the respective airlines in their itinerary if they are required to cross immigration in order to change terminals or to formally check-in again before continuing with the subsequent leg of their journey. This is particularly so for travel on Low Cost Carrier (LCC) or connecting to a LCC as they may need to seek immigration clearance and obtain their onward boarding passes at the respective Departure Check-in Hall. In such a situation, visitors will be required to seek immigration clearance and satisfy the prevailing entry requirements, which include possessing an entry visa, where applicable. Visitors are advised to find out the transfer details from their airline before embarking on their journeys. If the airline confirms that a traveler is able to obtain his / her onward boarding pass for connecting flight out of Changi Airport without going through immigration clearance, he / she may remain within the Changi Airport transit area until it is time for his /her next connecting flight.
Transit visa is not required if layover is not more than 12 hours
however if you want to step out of Immigration area or you want to explore singapore you will need Singapore visa.