Filldraw square in tikz

I am trying to filldraw a square with sides 7sqrt(7). I am not getting the desired output.

begin{tikzpicture}
filldraw(0,0) rectangle (7*sqrt{7},7*sqrt{7}); 
end{tikzpicture}

documentclass[tikz, border=1cm]{standalone}
begin{document}
begin{tikzpicture}
  filldraw(0,0) rectangle ({7*sqrt(7)},{7*sqrt(7)});
end{tikzpicture}
end{document}