It has never been clearly stated what city Calvin lives in, or even whether the city is an original creation of Bill Watterson's.
Among the few certainties, we know that:
- Calvin lives in the United States.
- The town is vaguely similar in some regards to Watterson's home town of Chagrin Falls, Ohio.
- It is explicitly stated that Calvin lives neither in Florida, California, North Dakota, Arizona, Alaska, Hawaii or The Yukon.
- He lives in a state where it snows in the winter.
- Arizona is generally to the west.
- Buckeyes can be found in his neighborhood, which are native to the Midwest.
In one comic Calvin becomes a giant and walks downtown (he also went there on his Flying Carpet). There, we see many skyscrapers and the area appears to be up against the ocean or some other large body of water. The city was probably once a harbor town and could likely be Cleveland, as there is also a Natural History museum like the one seen in the strip. Also, when Calvin floated away on his balloon, he mentions a river and a town triangle, and on the back of The Essential Calvin and Hobbes a giant Calvin is seen rampaging through the town triangle in Chagrin Falls, Ohio all the while holding Chagrin Falls Popcorn Shop.
In another comic, Calvin's father said that it took an hour and a half of driving to get to the beach. Because Calvin is close to downtown (where his dad works) and lives in a rural area, he seems to live in the suburbs of a big city. That city is likely Cleveland and the beach is probably on Lake Erie.
Also, during the trip back from Mars, Hobbes states that they live near the letter "E" in "[United] States", which on most maps is in Illinois, indicating they likely live in the Midwestern United States.
In one strip, Calvin had a map of Montana that he carried as part of a "Be Prepared Kit". Hobbes mentioned how "close" his state was to Yukon on a globe, however, his conception of distance is probably flawed.
It is possible that Calvin lives in Montana, the midwest, Washington state, or Oregon. But all the facts appear to line up with northeastern Ohio.