400

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