And in the last two places are the financial calculations that determine how much you can afford to borrow and how large a repayment you can expect on a mortgage or loan. I would have thought that these would be useful, particularly in the current economic climate. Mind you, maybe everyone already knows that they can't afford their mortgage, and nobody is interested in getting a new one. However, my guess is that there are already a lot of financial wizardry sites out there with calculators of their own, and money to burn on SEO. To put it another way, people want these calculations, but they don't end up finding them on CalcTool.
The real surprise is the dire performance of astronomy calcs, in particular those determining parameters of orbits. Circular orbits, satellites of the Earth, and planetary orbits all have calcs... that reside firmly in the bottom ten. This lack of interest I can't explain, and I'd be interested to hear any ideas on the subject.
Many of the rest are less surprising, being rather simple, boring or obscure. Let's face it, who goes to the web to calculate a logit function? Gravity, parallel plate capacitors and drug dosage all get a place here, as does the calc which finds how many tiles you need to cover your floor or wall. So if you're caught short on that front, don't come running to me for sympathy.