Not quite a mental calculation, but still in the spirit: One rough estimation I often find myself using is that a US dollar bill is six inches long. Using eithe

Not quite a mental calculation, but still in the spirit: One rough estimation I often find myself using is that a US dollar bill is six inches long. Using either folding or flipping, you can approximate most distances in the wave-your-arms-about scale. Be it thinking about a new physical device or ball-parking blinds at Home Depot, I use this all the time.