http://health.ratzrus.co.uk/cagecalc/
That calculator also takes into account the shelf space. It might give you a more realistic calculation.
According to the rattycorner calculator, my new Martins cage (24Wx14Dx60H) can hold 5 rats. According to the UK Fancy Rats calculator, it can hold 5.83 rats (so nearly 6). According to the ratzrus calculator, link above, it can hold 5 or 6 maximum.
But, according to the rattycorner calculator, my 34Wx22Dx19H cage can hold up to 4 rats. And on the UK Fancy Rats calculator, it says 4 rats as well. However, the same cage, according to the ratzrus calculator, can only hold 2 because it only has 1 small shelf and the bottom floor space.
So it's height isn't really much of a factor. You can have a seriously tall cage and it could say you can keep 10 rats in it, but if you have only 1 shelf and the floor space, that isn't a realistic number. It's all about the wise use of space and floor area.