Irma set multiple records for intensity, especially at easterly longitudes, time spent at such an intensity, and its intensity at landfall. When Irma reached Category 5 intensity with winds of 175 mph (280 km/h) at 11:45 UTC on September 5 at 57.7°W, it became the easternmost Atlantic hurricane of this strength on record, surpassing Hurricane David of 1979. By 00:15 UTC on September 6, Irma reached peak intensity with () winds and a minimum pressure of . This ties it with Hurricane Mitch of 1998 and Hurricane Rita of 2005 as the sixth-strongest Atlantic hurricane by wind speed. Only five other Atlantic hurricanes have been recorded with wind speeds higher than Irma: Hurricane Allen of 1980, which had maximum sustained winds of 190 mph (305 km/h), and the 1935 Labor Day hurricane, Hurricane Gilbert of 1988, Hurricane Wilma of 2005, and Hurricane Dorian of 2019, all of which had peak winds of 185 mph (295 km/h). At the time, Irma was also the strongest hurricane ever recorded in the Atlantic Ocean outside the Caribbean Sea and Gulf of Mexico; later surpassed by Hurricane Dorian, and was the strongest Atlantic hurricane since Wilma in terms of maximum sustained winds, and the most intense in terms of pressure since Dean in 2007. In addition, Irma achieved one of the longest durations of Category 5 strength winds, and the third-highest accumulated cyclone energy (ACE) index for a tropical cyclone in the Atlantic basin, with a value of 64.9 units. Only the 1899 San Ciriaco hurricane and Hurricane Ivan in 2004 achieved higher values.