How many miles up is the Earth's atmosphere?
Earth's atmosphere can be broken down into six layers-- the troposphere (lowest), stratosphere, mesosphere, thermosphere, ionosphere and finally the highest layer, the exosphere.
At the top of the exosphere, atmospheric particles merge into the solar wind. This occurs at 10,000 km.
Some consider the exosphere as not part of the atmosphere so the base of the exosphere would be the top. That is at 700 km.
From space the halo of the Earth is visible at about 400 km. That would be another possibility.
Below the Karman line lies 99.99997% of the atmosphere, and international it is observed as where outer space begins. This is at 100 km.
The base of the atmosphere is the surface of the Earth. To go with the majority (and international convention) and say that the Karman line is the top, we would then have an atmosphere of 100 km.