Great!

But what about volume, in both SI and US. And weight! Have a bag of M&M's and give it to the person who can best estimate the volume in 60 seconds.

And flow rate, in GPM, LPM and pounds per hour.

And, from the weight of sand in your filter, the average grain size (0.50mm), and the mass density of quartz, and an assumption of a grain shaped like an equilateral 4-sided pyramid (doubled) . . . calculate the number of sand particles in your filter!