155

155 is a natural number that comes after 154 and before 156. In Roman numerals, it is written CLV.

In mathematics, 155 is a semiprime: it is the product of two distinct primes, 5 and 31. It has exactly four positive divisors: 1, 5, 31, and 155. The number is odd and its digits sum to 11. In common positional numeral systems, 155 is represented as 10011011 in binary, 233 in octal, and 0x9B in hexadecimal.

In measurement and weapons, 155 mm is a standard artillery caliber used by many military forces. It has formed a central part of Western field artillery since the mid-20th century, with several artillery pieces designed to fire 155 mm shells, including modern self-propelled and towed systems.

Historically, the year 155 CE falls within the reign of Roman Emperor Antoninus Pius, a period of relative stability in the Roman Empire.