Metric Thread Chart For Metric Bolt Sizes

A metric thread designation such as M8x1.25 encodes two key pieces of information: the letter M followed by a number indicates the nominal major diameter in millimeters (M8 = 8mm major diameter), and the number after the x indicates the thread pitch in millimeters (1.25 = 1.25mm distance between thread crests). For standard coarse threads, the pitch is often omitted since it is implied by the diameter (e.g., M8 alone implies M8x1.25).

Metric coarse threads (standard pitch) have fewer threads per unit length and are the default choice for general-purpose fastening. They offer better resistance to stripping and faster assembly. Metric fine threads have a smaller pitch (more threads per unit length), providing higher tensile strength, finer adjustment capability, and better vibration resistance. Fine threads are commonly used in automotive, aerospace, and precision instrumentation applications.

The tap drill size is approximately equal to the major diameter minus the pitch. For example, for an M8x1.25 thread, the tap drill size is 8 - 1.25 = 6.75mm, rounded to the nearest standard drill size of 6.80mm. The chart provides pre-calculated tap drill sizes for all standard metric threads. For critical applications, always refer to the tap manufacturer's recommendations as the exact size may vary based on material and thread class.

The pitch diameter (also called the effective diameter) is the diameter of an imaginary cylinder that passes through the thread profile where the widths of the thread ridge and the groove are equal. It is the most critical dimension for determining whether a bolt and nut will mate correctly. If the pitch diameters do not match within tolerance, the assembly will be too loose or too tight. The pitch diameter is always between the major and minor diameters.

A tapping drill creates the pilot hole for internal threads (nut or tapped hole). Its diameter is close to the minor diameter, leaving enough material for the tap to cut the threads. A clearance hole is drilled larger than the bolt's major diameter so the bolt shank passes through freely without engaging threads. In a typical bolted joint, one piece has a clearance hole and the mating piece has a tapped hole.

Metric bolt threads are governed by ISO standards, primarily ISO 261 (general plan for ISO metric screw threads), ISO 262 (selection of sizes for screws, bolts and nuts), and ISO 724 (basic dimensions). The ISO metric thread profile has a 60-degree included angle, similar to the imperial Unified Thread Standard, but uses millimeter-based dimensions. Thread tolerance classes include 6g/6H (general purpose), 4g6g/4H6H (precision), and 8g/8H (loose fit).

No, metric and imperial (UNC/UNF) threads are not compatible and should never be mixed. Although both use a 60-degree thread angle, the pitch conventions differ (mm per thread vs. threads per inch), the diameter measurements use different units, and the thread profile proportions are slightly different. Attempting to mate a metric nut with an imperial bolt will result in cross-threading, incomplete engagement, or joint failure.

Thread pitch can be measured using a thread pitch gauge (a set of metal blades with known pitch values), a vernier caliper, or a ruler. With a caliper, measure the distance across 10 thread crests and divide by 10 to get the average pitch. With a pitch gauge, find the blade that fits perfectly into the thread grooves. The pitch is the axial distance from one thread crest to the next, measured in millimeters.