Source: The Martian (2015) starring Matt Damon
Example:
I have doubled my battery life by scavenging Rover 1. But if I use the heater, I will burn through half my battery every day. If I do not use my heater, I will be slowly killed by the laws of thermodynamics. I would love to solve this problem right now, but unfortunately my balls are frozen. I can't. I'm calling it. I'm calling it.
What exactly do you think that means? Is this a variation on the expression call it a day?
The exact meaning is slightly ambiguous. When I hear "I'm calling it", I don't personally think of "I'm calling it a day". The character is not exactly saying, "Well, I'm stopping work for the day"; the overall meaning in this case seems to be something like "I am declaring my decision."
One meaning of to call something is to announce, proclaim, or declare it, and this is used in a number of phrases like
The last one is actually what I am most reminded of. When a patient dies despite attempts to save their life, it is standard (at least on TV and in movies!) for the doctor to say, "I'm calling it." (where it is a code in the medical sense) meaning, roughly, "I am declaring that nothing more can be done for this person. Nurse, please note the time of death so we can put it on the death certificate."
So when the character says "I'm calling it", what I interpret it to mean is "I am announcing my final decision on this matter."
In short, yes.
To understand this, you must consider the context. In this scene, if I remember correctly from the movie and book, Mark Watney (the main character of the book) is giving his rover a test-drive after making some modifications to his rover. He is trying to see how long the battery can last (after rewiring the rover to not use any power for heating, as that was wasting a lot of the battery). He is absolutely freezing and can't carry on any longer.
After he says "I'm calling it. I'm calling it," he turns on the vehicle and drives home.
Now in this context, though there are many possible meanings for "I'm calling it," he does mean, "I'm calling the experiment," or that he's done.
We use "call" quite heavily in English as you can see below, I think in this case the correct idiom would be "I'm calling it off" rather than "I'm calling it a day". To "call it a day" implies that the job is complete, "to call it off" suggests that it's a failure and needs to be aborted or cancelled. You could also just "call an end to it" or "call time on it" which are fairly neutral, or "call it quits" which basically implies giving up. In any of these cases he's saying in a short phrase that the decision to stop is his to take and that he's taking it.
http://dictionary.cambridge.org/dictionary/english/call
call noun (DECISION)
informal a decision: It was a tough call, but eventually I decided to give up my job. More investment? That's got to be your call - you're the one that's paying!
Both of the above answers are right, in a way. The beginning of the definition chain seems to be when you flip a coin to make a decision, one person has to call out their prediction, and if their prediction is right, the matter is decided in their favor. A call is therefore a prediction.
If you can game the prediction in some way, and know that one call is more likely than another, then there can be better and worse calls to make. This is why when, in a game of strategy, a player makes a move expecting the other player to make a certain move, but the other player makes a different move, it can be said to be a bad call. He predicted wrongly, and then made an incorrect move based on that prediction. In cases like this, making a call becomes making a decision, and that became one of the definitions of "calling it".
This seems to be the making a decision definition, maybe influenced by "cry mercy" because he can't bear the situation anymore.
It basically means you're ending the debate and coming to a decision on something, and moving forward with that decision & whatever subsequent actions (or repercussions) that follow from it. It's something that generally comes up when someone is hemming & hawing or weighing a decision, with a 3rd party person saying "we're out of time, let's just call it" - because a decision has to be made one way or the other, and often other people then need to move forward with the specifics of whatever the decision entails. In this specific instance, he's weighing the pro's & con's of using the heater. I haven't seen the movie but based on his comment about "my balls are frozen", I would assume that his "call" or decision is to use the heater, even though he will "burn through half his battery every day". He's decided to "call it" and use the heater, and then live with the outcome (potential problems) of whatever may happen as a result of it.
There is another usage of the term, when someone says "I'm calling it right now" or "you watch, I'm calling it" - this is making a prediction about something and you're going on record announcing it. This is not the usage Matt Damon was using in the excerpted quote.
Also: "Call it quits" - to end an activity
