Birmingham is about 4.3 times smaller than Houston.

Houston is approximately 1,659 sq km, while Birmingham (map shows its 87 named neighbourhoods; the area covers the whole city) is approximately 381 sq km, making Birmingham 22.96% the size of Houston.
Tip: This to-scale comparison of Houston vs. Birmingham uses the Mercator projection, which distorts the size of regions near the poles.
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