The maximum sulfur content in ultra-low sulfur diesel (ULSD) is 15 ppm. In the U.S. standard, that means no more than 15 parts of sulfur by mass per million parts of diesel.
ULSD became important because sulfur poisons or reduces the effectiveness of modern exhaust-treatment equipment. Removing most sulfur made technologies such as diesel particulate filters and nitrogen-oxide control systems practical, helping sharply reduce emissions from diesel vehicles and engines.
The 15-ppm limit is often confused with the older U.S. low-sulfur diesel limit of 500 ppm. Another source of confusion is that some regions use a 10-ppm sulfur specification for road diesel, but the familiar North American ULSD benchmark is 15 ppm. Sulfur content is separate from cetane rating, lubricity, energy content, and the fuel’s renewable or petroleum origin.