How much liquid needs to be added to make the total volume up to 1 liter?

To determine how much liquid needs to be added to reach a total volume of 1 liter, subtract the current volume from 1 liter. For example, if you have 300 milliliters, you would need to add 700 milliliters to reach 1 liter. Simply adjust the amount based on the existing volume to find the correct volume to add.