The known Universe, that is.

According to this little gem: http://scaleofuniverse.com/

1) The diameter of the known Universe is roughly 9.3*10²⁶ meters.

2) And, the diameter of a grain of sand is roughly 5*10^-4 meters or 0.5 millimeters

3) The volume of a sphere is 4/3*ΠR³

4) The volume of the known Universe is 4/3*3.1416*(4.65*10²⁶)³ cubic meters, or 4.211613*10⁸⁰ cubic meters

5) The volume of a grain of sand is 4/3*3.1416*(2.5*10^-4)³ cubic meters, or 6.545*10^-13 cubic meters

6) Thus, the number of grains of sand that would fit in the known Universe, assuming they all packed together leaving no space in between, would be:

4.211613*10⁸⁰ / 6.545*10^-13 , or 6.4349*10⁹²