# How small the earth should be for it to become a black hole

How small would the earth have to be squashed so that it would become a black hole?

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You can define a Schwarzschild Radius based solely on total mass, i.e. $R_s = \frac{2GM}{c^2}$. If you plug in the mass of the earth, the radius is about 9 mm --- which is how small you would have to compress it to make a black-hole.