The square root of 7 is the positive real number that, when multiplied by itself, gives the prime number 7.

It is an irrational algebraic number. The first sixty significant digits of its decimal expansion are:

which can be rounded up to 2.646 to within about 99.99% accuracy (about 1 part in 10000).

More than a million decimal digits of the square root of seven have been published.

The extraction of decimal-fraction approximations to square roots by various methods has used the square root of 7 as an example or exercise in textbooks, for hundreds of years. Different numbers of digits after the decimal point are shown: 5 in 1773 and 1852, 3 in 1835, 6 in 1808, and 7 in 1797. An extraction by Newton's method (approximately) was illustrated in 1922, concluding that it is 2.646 "to the nearest thousandth".

In plane geometry, the square root of 7 can be constructed via a sequence of dynamic rectangles, that is, as the largest diagonal of those rectangles illustrated here.

The minimal enclosing rectangle of an equilateral triangle of edge length 2 has a diagonal of the square root of 7.

Due to the Pythagorean theorem and Legendre's three-square theorem, ${\displaystyle {\sqrt {7}}}$ is the smallest square root of a natural number that cannot be the distance between any two points of a cubic integer lattice (or equivalently, the length of the space diagonal of a rectangular cuboid with integer side lengths). ${\displaystyle {\sqrt {15}}}$ is the next smallest such number.