I want to know how many grains of sand are in a cubic mm, because I want to work out how many grains of sand fit in my bedroom (using maths rather then filling it with sand and then counting for the rest of my life.)

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How can we do that? Well, we can just gather up a bit of sand and count the grains. But instead of actually gathering a cubic centimeter of sand and counting each individual grain (which, while accurate, would also be tedious and slow), let’s make a quick and dirty estimate by lining up a bunch of grains in a row and measuring the length of the line in centimeters. The number you get will depend upon the type of sand, where it’s from, and a bunch of stuff like that—but if you do this you should get somewhere between 15 and 25 grains per centimeter (please email me and let me know what you get!). So if we assume that an average of 20 grains will fit along each side of a 1 cubic centimeter box of sand, we find that there are about 20 x 20 x 20 = 8,000 grains per cubic centimeter. Which, after a bit of fun converting units, we find is equivalent to 8,000,000,000 grains of sand per cubic meter.

- See more at: http://www.quickanddirtytips.com/education/math/how-many-grains-sand-are-earth%E2%80%99s-beaches?page=all#sthash.lUCmLW0t.dpuf

I assume you understood, but didn't read it all.

I didn't do this the information came from

http://www.quickanddirtytips.com/education/math/how-many-grains-sand-are-earth%E2%80%99s-beaches?page=all#sthash.lUCmLW0t.dpuf

ANY calculation is going to be an estimation unless your willing to fill the room and count it all. - even then if you did it twice you will get a different reading.

You HAVE to be realistic.

- read the article.

Second, the volume of any furniture or other stuff will have to be accounted for. The volume of those would have to be removed from the total.

Finally, your putting a square peg in a round hole. There will be voids where the cubes don't fill completely because of the odd shapes. Those will add up and be yet another error factor.

SINCE you ARE being picky, its usually considered at around 6000 stars in both hemispheres, for someone with good vision in a dark location.

"good vision", "dark location", blah, blah, blah. I'm a city boy, Steve -- I'm lucky if I can see Venus, let alone anything else :-)