i need to find the amount of gallons of different water heights on a Horizontal tank with Elliptical Cross-Sections

minor axis height is 4
minor axis length is 6
tank length is 16

What are the units of your length dimensions? Feet? meters? You need to tell us.

The area of the elliptical base is pi*a*b, where a and b are the major and minor axes. Multiply that by the height for the volume in cubic "whatevers."

Then convert that to gallons. There are 7.48 gallons in a cubic foot.