The intermix ratio is the ratio in which matter and antimatter are combined. A warp core is the most familiar device where the intermix ratio is essential.
A question in Wesley Crusher's Starfleet Academy entrance exam in 2364 was: "The matter and antimatter tanks on a Galaxy-class starship are nine-tenths depleted. Calculate the intermix ratio necessary to reach a star base 100 light years away at warp factor eight." This was a trick question as there is only one ratio with matter/antimatter: 1:1. (TNG: "Coming of Age")
The new warp core of the refitted USS Enterprise (NCC-1701) required new fuel equations to balance the intermix. There was an antimatter imbalance that the flow sensors of the warp core couldn't correct. (Star Trek: The Motion Picture)
Background
In Star Trek: The Next Generation Technical Manual the intermix ratio is explained further. According to the text, there is only one ratio with matter/antimatter: 1:1, when traveling at warp 8 or faster. However when traveling at lower speeds the intermix ratio is different. Intermix ratio of 25:1 is used when the core is online in an idle state generating energy only to the electro-plasma system of the ship. When the speed of warp 1 is established the ratio is changed to 10:1. As the speed of the ship is increased so is the ratio until it reaches 1:1 at warp 8.
- While the Star Trek: The Next Generation Technical Manual is not canon, it is a Memory Alpha permitted resource.