400 = 24 x 52.
400 = 202, a perfect square.
400 = 70 + 71 + 72 + 73. Hence, 400 is 1111 in base 7.
400 has a representation as a sum of two squares: 400 = 122 + 162.
A full circle is 360 degrees or 400 grads.
Years divisible by 400 are leap years, following adoption of the current Gregorian calendar.
Source: Numeropedia
