Let's assume there are 10 blades of grass per square centimeter. This means there are 100.000 blades of grass per square meter and 100.000.000.000 blades of grass per square kilometer.
The surface area of earth is 5.1 * 108 km2
The surface of earth is roughly 70.8% water (Ibid), which means that 29.2% is land. Of these 29.2% approximately 20% is grass
This means the surface area of all grass areas combined is:
Area_of_grass = 5.1 * 108 km2 x 29.2 % x 20 % = 29 million km2
Hence,
blades = 1011 blades of grass / km2 * 29 million km2 = 3 * 1018 blades of grass
This is approximately 418 million blades of grass per person alive.
