Minecraft wiki entry for TNT, associated text: Once activated, TNT turns into an entity, which includes being affected by gravity. The new primed TNT is spawned at the center (+0.5,+0.5,+0.5) of where the TNT block was, like a cube with an edge length of 0.98. Its fuse lasts 40 redstone ticks (4 seconds/80 game ticks) if activated by redstone or fire, or a random number between 10 and 30 game ticks (0.5 to 1.5 seconds) if it is destroyed by another explosion.