Flathead Lake is about 2.9 times bigger than District of Columbia.

District of Columbia is approximately 177 sq km, while Flathead Lake is approximately 510 km² (197 sq mi), making Flathead Lake 188% larger than District of Columbia.
Tip: This to-scale comparison of District of Columbia vs. Flathead Lake uses the Mercator projection, which distorts the size of regions near the poles.
Our free app, MapSize by MyLifeElsewhere — compare any two places, to scale on your mobile device.