Can't find the right socket size for this bolt?

I have a piece of patio furniture that has a bolt loose. I pulled out my standard toolkit which includes a standard 1/4 inch square drive socket set (6 point). I tried the biggest one that it had included which was 3/8 and it wasn't big enough...so I went out to Lowes and bought the next size up socket which... show more I have a piece of patio furniture that has a bolt loose. I pulled out my standard toolkit which includes a standard 1/4 inch square drive socket set (6 point). I tried the biggest one that it had included which was 3/8 and it wasn't big enough...so I went out to Lowes and bought the next size up socket which would be 7/16 and now that's too big. But from my understanding, there is nothing between 3/8 and 7/16 right? Please help!
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