Source: Omni Calculator
The site is free to use, and several of the calculators explain exactly how the calculation is done. This site lends itself to science and math classes to help students check their work, but don’t let the word calculator peg the site to particular classes. I could see English classes use the site to verify calculations in writing, or Art classes calculating the amount of materials needed to a project.
Omni Calculator is really impressive, check it out!