USA industry is now using metric fasteners extensively.  The remainder of the world uses ISO metric fasteners almost exclusively, due to their superiority in proportions, fatigue strength, pitch, size and specification designations, and international availability.

The Thomas Register lists hundreds of firms under Metric Fasteners, Metric Screws, and Metric Bolts.  The Industrial Fasteners Institute (IFI) has guides for fastener types and producers.

Metric fasteners for all aerospace applications are made readily and easily available at "rock" bottom price to all NASA and contractor employees via NASA GSFC Fastener Inventory.

Metric fastener size designation.

ISO metric fastener material strength property classes (grades).  As given in ISO 898-1, ISO metric fastener material property classes (grades) should be used.  For example, fastener material ISO property class 5.8 means nominal (minimum) tensile ultimate strength 500 MPa and nominal (minimum) tensile yield strength 0.8 times tensile ultimate strength or 0.8(500) = 400 MPa.  (In a few cases, the actual tensile ultimate strength may be approximately 20 MPa higher than nominal tensile ultimate strength indicated via the nominal property class code.  Consult Table 10, below, for exact values.)  Many anchor bolts (L, J, and U bolts, and threaded rod) are made from low carbon steel grades, such as ISO classes 4.6, 4.8, and 5.8.

Preferred diameters.  Preferred nominal diameters for bolts and threaded rod are as listed below.  The fourth series listed below should be limited to unusual requirements when none of the preceding series can be used.  Reference individual standards prior to specification.  Sizes M5 to M45 are commonly used in construction.

Bolt versus screw definition.  The correct definition of bolt and screw is as follows.  Bolts are headed fasteners having external threads that meet an exacting, uniform bolt thread specification (such as M, MJ, UN, UNR, and UNJ) such that they can accept a nontapered nut.  Screws are headed, externally-threaded fasteners that do not meet the above definition of bolts.  For full discussion of misdefinitions and corresponding confusion regarding these two words, see details.

Handy conversion factors.  Imperial conversion factors, verified accurate to the decimal places shown via multiple, independent, credible sources, are 25.4 mm/inch (exact), 4.4482216152605 N/lbf, 6.89475729318 MPa/ksi, 47.880259 Pa/psf, 112.98483 (N mm)/(in lbf), 157.08746 (N/m^3)/pcf, 16.0184634 (kg/m^3)/(lbm/ft^3), 27679.9047 (kg/m^3)/(lbm/in^3), 9.80665 (m/s^2)/gravity (exact).  Rounding these conversion factors to a few less decimal places, we have 4.448222 N/lbf, 6.89476 MPa/ksi, 47.8803 Pa/psf, 113.0 (N mm)/(in lbf), 157.087 (N/m^3)/pcf, 16.01846 (kg/m^3)/(lbm/ft^3), 27679.9 (kg/m^3)/(lbm/in^3).

Metric system (SI).  The abbreviation for the metric system is SI, the International System of Units (from the French, Systeme International d'Unites).  It evolved from the original French metric system and is currently being used virtually worldwide.  Long the language universally used in science and among technically adept individuals, SI has also become the dominant language of international commerce and trade.  All new USA standards (ASTM, ANSI, SAE, IEEE, ASME, etc.) are now written in metric, as the lead engineers in these organizations recognize the importance of trying to get the USA on track with technically advanced countries, in an effort to regain lost USA competitiveness in a global economy, as there is essentially no global market for the archaic, oddball, incompatible product dimensions USA arbitrarily comes up with, while they forfeit industries and jobs to third-world countries who have no problem understanding something so simple and fulfilling the need efficiently.  IEEE was intelligent enough to recognize this decades ago.  Japan also was intelligent enough to recognize simple matters such as this long ago.  This small country, defeated in WWII only 60 years ago, has since captured a large portion of the global economy due to their intelligent progress, and consequently has become a major global financier, while USA has become a world-class debtor to the tune of trillions due to inefficient business practices, low educational level, slackerism, and inability to solve or understand even simple problems such as metric conversion.