The approximate Schwarzschild radius of Earth is 9 millimeters.
The Schwarzschild radius is the radius an object of a given mass would have if it were compressed into a non-rotating black hole with no electric charge. It is calculated with the formula rₛ = 2GM/c², so the result depends only on mass, the gravitational constant, and the speed of light.
For Earth, whose mass is about 5.97 × 10²⁴ kilograms, the calculation gives approximately 8.87 millimeters, conventionally rounded to 9 millimeters. Earth is nowhere near being a black hole: its actual average radius is about 6,371 kilometers, vastly larger than its Schwarzschild radius.
A common mix-up is confusing the Schwarzschild radius with the diameter of the event horizon. For a simple Schwarzschild black hole, the event horizon lies at that radius, so its diameter would be roughly 18 millimeters. For comparison, the Sun's Schwarzschild radius is about 3 kilometers, while the Moon's is approximately 0.1 millimeter. The tiny value for Earth illustrates how little its mass is compressed under ordinary conditions.