Nantucket is about 1.4 times smaller than District of Columbia.

District of Columbia is approximately 177 sq km, while Nantucket is approximately 124 km² (48 sq mi), making Nantucket 69.94% the size of District of Columbia.
Tip: This to-scale comparison of District of Columbia vs. Nantucket uses the Mercator projection, which distorts the size of regions near the poles.
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