Hex Head Bolts Wrench Size To Use

Hello,
I bought some hex 3/8 bolts and nuts.
Yes, the O.D. of the bolt appears to be 3/8.
But a wrench that is marked 3/8 does not fit the head of the bolt.
Is there some "standard" where you can tell beforehand, for a given diameter (size) of bolt, what wrench to use for its head ?
Thanks, Bob
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On 05/11/2015 03:45 PM, Bob wrote:

Is there some web site where an inquisitive person can search for arcane stuff?
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SAE standards for bolt / wrench sizes are available on the net or in any mechanics text. 1.4"bolt 7/16 wrench 5/16" bolt 1/2" wrench 3/8" bolt 9/16" wrench etc
The old witworth /bsf system is the only system that marked the wrench with the bolt size - and 1 3/8" witworth was different than a 3/8" bsf if I remember correctly.
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On 5/11/15 3:45 PM, Bob wrote:

You double the numerator and add the denominator. Let me walk you through this one to show how easy it is. 3*2+8mm.
Before I figured that out, I always used slipjoint pliers. I still use them when I don't have my calculator with me.
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J Burns presented the following explanation :

You would be banned from most professioal workshops. B-)
A much better way if you don't know is to just choose the one that fits nice and snug. B-)
--
John G Sydney.

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