Formula depended from table with two diffrent units

Hi, i don't have alot of experience in excel but im trying to learn on my own using youtube and google.

Right now im trying to create a easy way to calculate the energy amount from the diffrent fuels. I have made a table with 40 diffrent fuels and the energy amount from all these. The thing thats makeing my calculation complicated is that not all fuels have the same unit. Some use volume (m3) and other weight (kg)(number 2).

Right now right now I have managed create a scrollable numbered list and connected the numbers to a formula box i.e. when you find the right fuel you wright in the number in the formula box and the value and name of the fuel follow (number 1). The problem now is how do I connect the amount to the forumla box?

In the formula box I two diffrent cells for the amount, one in m3 and the other in kg (number 2). How can i make a formula in (number 3) the formula box so excel understand what cell to multiply? How I didn't make it to hard to understand, follow the picture with the numbers. :)