How do you tell if a bolt or nut is standard or metric, and what size wrench or socket to use? The charts below show which are interchangeable between metric and standard, aka imperial or SAE (Society for Automotive Engineers), millimeters vs inches, and how they compare in thousandths of an inch.
When a metric and standard size are similar, it's best to use the smaller of the two sizes if it will fit your bolt. Less than .005" is not usually noticeable. Over .010" tighter may not fit.

View or download and print chart from our website (I keep one in my toolbox):
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Word doc highlighted w/yellow: [ Ссылка ]

Which vehicles are SAE, Metric, or Mixed?
US vehicles pre-1970: All SAE
US 1970 - 90's: Mixed SAE & Metric
US vehicles after 2000: All Metric
Foreign non-US vehicles: All Metric

How to read bolt heads to determine if a bolt is metric or SAE:
No Markings or mfg. initials: Uncertain, likely SAE grade 2
SAE grade 3 to 8: 1 to 6 Radial Lines; 307A, A325; Stainless: F593C or similar
Metric: A number; 2, 4, 6, 8.8, 10.9, 12.9; 8.8 = grade 5; Stainless: A2-70 or similar

Mechanic Math example: 5/8" SAE wrench: 5 ÷ 8 = .625 inches (decimal equivalent in inches)
16 mm Metric wrench: 1 mm = .0394 inches x 16 = .630 inches, so the 16 mm is 5 thousandths of an inch bigger than 5/16”, which is close enough. Since the 5/8" wrench or socket is a tiny bit smaller, it will fit tighter on the bolt or nut (whether it is metric or standard) and reduce the chance of stripping, so if it fits, I'd use the 5/8".

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