Clear containers are great but not everything comes packaged that way.
Here is the side of a steel oil drum - how do I know how much is left?
I use very nearly the same amount of oil for each batch of product I make. On the side of the drum, I write down a tick mark for each batch made that day. This makes it easy to quickly tally up how much total oil I've used. I can make about 38 batches from the entire drum, so when I get close to 38, I need to get ready to melt and open a new drum of oil.
This tick mark process doesn't require software or computers and if I miss writing down the amount used on a certain date, I can always check my batch records to see what was made. This method is faster than removing my oil pump to dip a long stick in to see where the oil level is.