How many kilometers is the universe across?

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.

So, the estimate is within

If by universe you mean everything that surrounds us, including space outside of our galaxy, then the answer is unknown. Since the universe is constantly growing and expanding, we will never be able to measure it precisely. At the moment, our "observable universe" has a diameter of 28 billion light-years, which is a very large number when converted to kilometers.

A single Astronomical Unit, or AU, is equivalent to 150 million kilometers. There are 63241.1 AU in a single light year, and the visible universe has a diameter of 28 billion light-years.

Here's the math, then!

1,770,750,800,000,000 AU is equal to 63241.1AU x 28,000,000,000ly.

150,000,000km x 1,770,750,800,000,000AU = 2.6561262e+23

Therefore, 2.6561262e+23km is your answer.

The observable universe is estimated to be about 93 billion light-years (about 8.8 × 10^26 meters) in diameter. This corresponds to roughly 880 billion trillion kilometers.