Stupid maths shortcut #1 - Volume of a sphere

I'm afraid I'm not very good at maths. Never have been. Back when other kids were learning their multiplication tables I was daydreaming about dinosaurs or being an astronaut, or a dinosaur astronaut. However, my friends all think I am good at maths simply because I can do fairly difficult arithmetic quickly in my head. But the only reason I ever learnt to do this is because I am bad at maths. Because I never paid attention in class I had to develop all kinds of mental tricks to cover up the fact that I didn't know what I was doing.

Most of my shortcuts probably won't make much sense to anyone but me, such as turning numbers into dates and subtracting the Wall Street Crash from the Boxer Rebellion, but I guess there are a few that might help other people.

One lazy shortcut I had was for finding the volume of a sphere. Rather than remembering pi and the correct formula, I would just find the volume of a square of the same diameter as the sphere, which is easy (the diameter cubed, or simply diameter by diameter by diameter), and cut the result in half and round up to get a quick approximate answer. To be more precise, a sphere will have 52% of the volume of a cube of the same diameter. (Or to be really precise, 52.35988%, but this is supposed to be a short cut.)

Perhaps the more mathematically gifted are laughing at me right now, but I hope this is of help to someone, particularly students.