[stlnative]

It is 18” (probably 9” each way)

On a clear day you can see for thirty miles. Don’t be surprised if you feel the arch swaying, it is designed to move up to 18” and to survive an earthquake.

http://www.apj.co.uk/destinations/gateway_arch.asp

[LucyJo]

I’ll just admire it from the ground, thank you!