Detroit is about 4.4 times smaller than London.

London (including the 32 boroughs of greater London) is approximately 1,572 sq km, while Detroit (map shows its 92 named neighbourhoods; the area covers the whole city) is approximately 359 sq km, making Detroit 22.86% the size of London.
Tip: This to-scale comparison of London vs. Detroit uses the Mercator projection, which distorts the size of regions near the poles.
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