Metric nuts and bolts are the current standard across the globe. In the UK, they have replaced older standards using imperial units, such as the British Standard Whitworth (BSW) standard. Metric bolt sizes are specified in millimetres and include the diameter, pitch, and length of a bolt.
Diameter
The first number in a metric bolt specification refers to the bolt's nominal outer diameter. For example, a bolt specified as "M6-1.0 x 30" fits a 6.0-mm diameter hole. The "M" indicates that it is a metric size. Keep in mind that metric bolts are usually slightly smaller than their specified diameter. For example, a M16 bolt is often 15.97 mm in diameter. This creates a small "clearance" that allows some flexibility for misaligned parts. In general, larger bolts are also stronger, though the bolt's material also plays an important role in its strength. Stainless steel bolts are weaker than hardened steel bolts of the same diameter.
Pitch
In the metric specification, the pitch refers to the distance between two adjacent threads in millimetres. The M6-1.0 bolt has a pitch of 1.0 mm. To convert pitch to "Threads per Inch" (TPI), multiply by 0.03937 to change the metric pitch to inches and then divide 1 by the resulting number. For example, a bolt's 1.0 mm pitch equals 0.0394 inches and results in a 25.40 TPI. The pitch also indicates whether it is a "fine" or "coarse" bolt. Metric fine bolts have a pitch of 1.5 mm or smaller. Bolts with a diameter of 24 mm and larger are exceptions, as a 2.0 mm pitch indicates a fine thread. The most common metric threads are coarse. If the bolt size does not specify pitch, assume it is coarse.
Length
The length of a bolt is the last number in its metric size specification. Using the same example, the M6-1.0 x 30 bolt has a length of 30 mm. Most bolts, including cheese, hex, pan, socket, button, and low socket head types, are measured from under the head to the tip of the bolt. However, the length of oval and flat head bolts includes their head height.
Other Metric Markings
Metric bolts are made to DIN or ISO standards, and numbers indicate different fasteners. For example, the DIN 6921 is a hex flange head fastener. Besides the lowest grades, most metric bolts and nuts have markings that indicate their strength. Two numbers separated by a decimal point are usually marked on the bolt head but may also be on the side. For example, a bolt marked "9.8" can withstand a load of 90 kg per square millimetre. However, it starts to stretch at 80 per cent of that load.