What makes a chili hot is the amount of capsaicin it contains, and the Scoville test was devised by one Wilbur Scoville, a pharmacist, in 1912, to measure just that. The Scoville Scale has remained the generally accepted rating of how "hot" hot peppers really are.

The number of Scoville heat units was originally determined by how many parts of sugar water it takes to dilute an extracted sample of a given chile so that its heat can no longer be detected. The tests were performed by a panel of human testers who would taste each sample in increments, so they were rather subjective. Today, it's all done by machines using a process invented by James Woodbury, and involves dissolving dried chiles in ethanol saturated with sodium acetate; the resulting liquid is measured for heat in labs using HPLC (high performance liquid chromatography) testing. Even these tests are not definitive, however, for the heat level of any chile will vary depending on climate and growing conditions.

The generally accepted ratings are:

According to the Guinness Book of World Records, the red savina habanero is the hottest pepper in the world, with a rating of 570,000 Scoville heat units. Other average pepper ratings:

Find out more at Chile Pepper Hell:

avalyn says "I'm not sure if Guinness still recognizes the world's hottest pepper, but if they do and it's still the red savina, it's wrong. The hottest is actually the Indian bhut joloka (aka the Dorset naga), at a million SHU. Search for "bhut joloka" on youtube, there's quite a few amusing videos of people consuming them and trying not to die."