If you have responsibility for a collection of chemicals, or a complete chemical storeroom, you should be keeping an accurate inventory which should be updated annually. In doing your inventory you will want to estimate how much material you have left in stock so that you can make an informed decision about future purchases. I keep a collection of plastic chemical storage bottles on-hand for just this occasion. The collection includes the most popular sizes that my suppliers use to distribute their wares. When the contents of a bottle have been consumed I will clean it throughly, mark through the information on the label and save it in a box. When inventory time comes around I use the empty bottles to tare an electronic balance and then place a matching bottle from my storeroom shelves on the balance and voila, a quite accurate measure of how much material remains on my storeroom shelf. Of course this only works for solids; unless you want to do some calculations involving density. For liquids I calibrate a bottle by incrementally filling it with water and making marks on the exterior for each increment; 100 mL or 250 mL work nicely depending on the capacity of the bottle. I suggest you use bottles that have contained materials of low intrinsic hazard.

*or any other chemical