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The Scoville scale measures the concentration of capsaicin in peppers. An extract of the vegetable is diluted with sugar water and tasters rate the spiciness. The concentration is reduced until the majority of respondents no longer feel the burning sensation.
For example, 1,000 Scoville means that the pepper extract needs to be diluted 1,000 times, at which point its spiciness disappears. Bell peppers have 0 Scovilles, jalapeños have 3-4 thousand, and pepper spray for bear protection reaches 2.2 million Scovilles. Pepper X is theoretically hotter than even that defense tool.