What does the phrase "that's a given" mean?

When something is characterized by "that's a given" or "it's a given" you'll know that whatever the something is, it's so obvious that no one really needs to point it out. It's already accepted as fact:

"Paying all my bills used up most of my paycheck"
"Why are you surprised? That's a given at my house."

Take along an extra raincoat - that's a given if you want to stay dry in Seattle.

I'm surprised more people didn't show up for the party. Usually when word gets around that there's free food, it's a given that the place will be packed.

"Shultz must ask abt 150 creative and original questions every week"!
"Yea, the creative part - that's a given. You really think 150? In just a week? Are you sure?"
"Just look below. He says so himself - 20 a day, and more on good days. Hey, who's the bimbo who misspelled his name? Was that you? How could you be so careless?"
"Oh please! You know I can't spell my way out of a paper sack. It's a given - I'm gonna get won one or too wirds rong evry thyme, but that's awl. Isle trie hardur."

Hope this helps!

I hate to get all geeky here but...no I don't. I love to get geeky.

A "given" is an element of an algebraic equation, a value that is given so that one may figure out the value of another variable.

For example:
Given x=3-4y, find y:
2y + 3x = 4
2y + 3(3-4y) = 4
2y + 9 - 12y = 4
9 - 10y = 4
5 = 10y
y = 1/2
Now solve for x:
Given y = 1/2, find x:
x = 3 - 4y
x = 3 - 4(1/2)
x = 3 - 2
x = 1

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A "given" is also an element of a logical argument, in that it is a supposition that is accepted as true for the sake of an argument.

Obviously in common conversation it means roughly the same thing:
If we assume that it is a good idea to save for retirement, then we call that a given without really questioning it too deeply.

we say 'thats a giben' in Scotland, dont really know why, but its a very common word here.

can it sometimes also mean "needless to say"?

In addition to Jodie44’s response it can also mean that after the "given" there can still be debate. For example one could say that they wish to debate religion and if loving God is a "given" they can proceed to examine doctrine. One could argue some point with a spouse or family member because it's a "given" that this family has love. When love is the "given" then the gift of it is crucial to what takes place in any situation including conflicts and war. In this case if love isn't the "given" nothing is resolved. World conflict is proof of that now. Even peace has become another kind of war where love is not the “given”.