Scoville scale is used to measure the hotness or spiciness of a chilli pepper. The earlier method to calculate it was as follows: Start diluting the chilli extract with water. Let a panel of five judges taste it at regular intervals. The Scoville index is achieved when none of the judges can sense the hotness of the chilli anymore.
Naga Jolokia was tested hottest in the world at 1,040,000 SHU! Here is a comparative list of Scoville ratings to get some perspective on it.