How to measure
I would recommend measuring everything in cm. This way you get results in ml (cm3/CC). However, if you want results in litres (dm3), measure in dm (as in 10 cm).
The calculations are independent of metric vs imperial, so if you have to use inches or feet, please do so. The volume should then be in inch3 or feet3 or whatever measurement you enter.
The formulas do not do fractions, so you have to calculate them into a decimal value.
Note Use the same system for all measurements in the formula.
Diameter vs radius
All the math containing a circle is based on the idea of r, or radius. This is the distance from the centre of the circle to the edge.
This might be difficult to measure, so I recommend measuring the Diameter. This is from edge to edge crossing the centre point of the circle.
All the input fields when dealing with circles are based on the input of the Diameter value.
All the heights are measured perpendicular to the top and base, never at an angle.
Think of it as if you are on top of the item you want to measure. Drill a hole at your feet and drop the end of the measuring tape into this hole. Let it drop until it hits the bottom. This is your height.