##hypotenuse = l * sqrt2##

An isosceles right triangle has a ##90^@## angle between its two equal sides let’s say ##l##.

If the hypotenuse is ##hyp## you can use Pythagoras’ Theorem to write

##hyp^2 = l^2 + l^2##

##hyp = sqrt(l^2 + l^2)##

##hyp = sqrt(2l^2) = l * sqrt(2)##

So in order to find the hypotenuse of an isosceles right triangle you need to multiply the length of one of two equal sides by ##sqrt(2)##.