Minecraft wiki entry for TNT, associated text: Once spawned, primed TNT is given a vertical velocity of 0.2 blocks per tick, and a horizontal velocity of 0.02 blocks per tick in a random direction. Given these velocities, the TNT travels 0.166 blocks (or 6 block pixels) horizontally before it stops, given there is no block in the way. When the countdown timer expires, the TNT explodes. If in the air, TNT falls roughly 77 blocks before exploding once it is ignited. The explosion has an explosive force of 4.