Minecraft wiki entry for Minecart_with_TNT, associated text: The explosion has a base power of 4, the same as regular TNT, but the game also adds a random bonus value up to 1.5 times velocity, but no higher than 7.5. This means that with a speed of 5 or higher the power will be a random value between 4 and 11.5. When triggered by an activator rail or by damage, the bonus value is calculated using the horizontal velocity of the minecart. When hit by a flaming arrow the velocity of the arrow is used instead. When triggered by fall damage, the fall distance divided by 10 is used.