How many kilometers is the universe across?
Truth is we don't really know.
If you mean the universe as in everything around us, including outside of our galaxy, then its unknown. The universe is ever-expanding and growing, so we will never have an exact measurement. Currently our 'observable universe' is 28 billion light-years in diameter. When you calculate that to kilometers, well you get a really really big number.
One Astronomical Unit, or AU for short, is 150 million km. And in a single light year there is 63241.1 AU. And we have 28 billion light-years in diameter that we can see.
So! Here is the math!
63241.1AU x 28,000,000,000ly = 1,770,750,800,000,000AU
1,770,750,800,000,000AU x 150,000,000km = 2.6561262e+23
So your answer is 2.6561262e+23km.
1 billion trillion
From the virtual center of our universe, the distance of the Earth is estimated to be 13.8 billion light years (bly).
Double this can be used as the least diameter of our universe. Recent research had taken it up to 93 bly. I consider this as upper bound for the diameter. So, the diameter is within (27.6, 93) bly.
#"1b" = 10^9 "ily" = "9.46 trillion km"#
So, the estimate is within
#9.46(27.6, 93) "billion trillion km" = (261, 880) "billion trillion km"#