The objective of Agar.io is to grow a cell by swallowing both randomly generated pellets, which slightly increase a cell's mass, and smaller cells without being swallowed by larger cells.^[4] The browser version currently holds four game modes: FFA (Free-for All), Teams, Experimental, and Party. The mobile version of the game includes: FFA (Free-for All) and Rush Mode. The goal of the game is to obtain the largest cell; players restart when all of their cells are swallowed by another player. Players can change their cell's appearance with predefined words, phrases, symbols or skins.^[5] The more mass a cell has, the slower it will move.^[6] Cells gradually lose mass over time.^[7]

Viruses split cells larger than them into many pieces (16 or less, depending on the mass) and smaller cells can hide underneath a virus for protection against larger cells. Cells in 16 pieces can eat viruses without splitting, though it's usually dangerous running around in sixteen pieces. Viruses are normally randomly generated, but players can make new viruses by feeding a virus, i.e. ejecting a small fraction of a player's cell's mass into the virus a few times, causing the virus to split up and hence create another virus.