Even given oxygen to breath, the human organism is quite insistent on a whole host of demands: food, water, comfortable temperature, gravity, absence of deadly radiation; the list goes on and on. Not only do robots not have any of those requirements, but there also is no problem with sending the probes on suicide one-way missions.Human astronauts would tend to complain about that.It can vastly cut down on the payload mass "wasted" on food (given the CELSS penalty mass).
Each of those three can be controlled by either an "open" system or a "closed" system.
Generally this takes the form of some sort of plants, who use sunlight to turn astronaut sewage and exhaled carbon dioxide into food plants and oxygen.
If you want more data on life support than you know what to do with, try reading this NASA document. For some great notes on spacecraft life support, read Rick Robinson's Rocketpunk Manifesto essay.
Naturally the kind way to do the math is when initially planning the mission.
Multiply the number of crew members by the duration of the mission in days to get the required number of person-days of consumables (and if you are wise you'll add an additional safety margin).