The following calculators should help you find answers to the most common things we need to work out.

Calculators And Conversions.

Conversion/Calculator Just click on a link.
Sand bed volume calculator Here
Volume Calculator (For Hex tanks)

In US gallons, so you will need to divide by 1.2009 to get UK gall

 Here
Volume calculator (for rectangular tanks) Here
Flow rate  conversions. Here
Head loss Calculator Here
Calcium to Alkalinity or Alkalinity  to Calcium balance Here
Magnesium Supplement calculator Here
Calcium supplement Calculator Here
Temperature Here
Nitrogen Conversions Here
CO2 concentration Here
O2 concentration Here
Lunar Phase Here
Volume converter. Here
Water change Calculator Here
More to follow soon More to follow soon

Many thanks to Andy Hipkiss for the loan of his calculators.