Standard Metric Bolt Diameter Chart

ISO 262:2023 covers selected commercial metric thread sizes over the 1 to 100 mm diameter range. Head/wrench dimensions are covered separately later on this page, tied to specific product standards.

M6 means D = 6 mm. M12 means D = 12 mm. M20 means D = 20 mm. The actual manufactured external thread can measure slightly below its basic nominal diameter because thread tolerances apply.

M8 x 1.25 means nominal diameter = 8 mm, pitch = 1.25 mm. M10 x 1.5 means nominal diameter = 10 mm, pitch = 1.5 mm. M12 x 1.75 x 60 is a full designation including length: M12 = diameter, 1.75 = pitch, 60 mm = bolt length.

ISO 965-1:2026 provides the current tolerance system for ISO general-purpose metric screw threads. Do not reject an M12 bolt merely because a caliper reads slightly under 12.00 mm.

ISO 262 selects preferred diameter and pitch combinations from the broader ISO metric thread plan; not every millimeter increment is a standard commercial size.

Nearest diameter does not mean interchangeable fastener; thread systems and pitch are different.

M12 = 12 mm, while 1/2 in. = 12.7 mm, a difference of 0.7 mm. More importantly, metric and Unified inch fasteners use different thread pitch, thread profile designation, and nut compatibility. See the US Bolt Sizes Chart for the complete inch-series reference.

An M12 bolt may exist with a coarse pitch or one or more fine pitches, depending on the standardized series. M12 alone identifies the nominal diameter, but not always the complete thread.

For full thread geometry, see the Metric Thread Chart.

Do not assume a single universal fine pitch for every diameter; fine-pitch availability varies and must be explicitly specified in the designation.

If only M12 is written in many contexts, the standard coarse pitch is generally implied.

ISO 262:2023 covers selected diameter and pitch combinations for bolts, screws, studs, and nuts from 1 mm through 100 mm, selecting these combinations from ISO 261. ISO 262 determines preferred commercial thread size combinations, not bolt-head geometry.

ISO 261:1998 remains current after confirmation in 2024, and it defines the broader general plan for ISO metric threads, from which ISO 262 selects commercial fastener sizes. Keep the distinction clear: ISO 261 is the broad metric thread plan, while ISO 262 is the selected commercial bolt, screw, stud, and nut sizes drawn from it.

ISO 724:2023 defines the basic dimensions of ISO metric screw threads in accordance with ISO 261, including major diameter, pitch diameter, minor diameter, and basic profile dimensions. This is the numerical reference behind the thread-diameter data used throughout this page.

ISO 68-1:2023 defines the basic and design profile for general-purpose metric threads. Metric thread form is standardized, and nominal diameter alone does not describe full thread geometry.

ISO 965-1:2026 specifies the tolerance system for ISO general-purpose metric screw threads, superseding the prior 2013 edition. This explains why actual measured major diameter does not always equal exactly the nominal diameter. Common external and internal tolerance-class concepts exist within this framework, though full thread-fit calculations are beyond the scope of this identification-focused chart.

Partially threaded metric bolts can have an unthreaded shank. Body diameter is controlled by the applicable product standard and tolerances. For ordinary ISO hex bolts, use ISO 4014 data where the product falls within scope; ISO 4014:2022 covers coarse-pitch hexagon-head bolts from M1.6 through M64, product grades A and B.

ISO 4017:2022 covers coarse-pitch hexagon-head screws from M1.6 through M64, the fully threaded counterpart to ISO 4014’s partially threaded bolts.

An M12 bolt means a 12-mm nominal thread diameter, not a 12-mm wrench or a 12-mm head. Actual head dimensions depend on the product standard, the generation of ISO standard, and head type. This page does not create a universal M-size-to-head table without tying it to a specific fastener standard.

Some older DIN-based products and current ISO products can differ in hex-head dimensions for certain sizes, so identify the governing standard. See the Socket Size Chart for the complete reference.

ISO 4014:2022 covers steel and stainless hexagon-head bolts with coarse metric threads from M1.6 through M64, product grades A and B. ISO 262’s metric thread range extends beyond ISO 4014’s particular hex-bolt product scope, all the way to M100, so these two standards should not be confused with one another.

ISO 4017:2022 similarly covers coarse-pitch hexagon-head screws from M1.6 through M64, product grades A and B. Thread diameter standard does not equal fastener product standard; each product family has its own defined scope.

Metric property classes such as 4.6, 8.8, 10.9, and 12.9 describe mechanical properties, not diameter. M12 8.8 means M12 = diameter and 8.8 = property class. Do not confuse M8 with 8.8; these numbers answer completely different questions. For grade and marking data, see the Bolt Grade Chart.

In a notation such as M12 x 1.75 x 80, M12 = diameter, 1.75 = pitch, and 80 mm = length. The length figure is always separate from the diameter designation.

Exact hole sizing depends on close, normal, or large clearance class, the design standard, and assembly requirements. This page does not publish one universal “hole equals M size plus 1 mm” rule, since that would be inaccurate across sizes and applications.

For an M12 bolt, compatible nominal hardware is typically an M12 nut with matching pitch and an M12 washer series, but nut across flats is larger than 12 mm and washer ID is larger than 12 mm. See the Nut Size Chart, Washer Size Chart, and Metric Thread Chart for complete mating dimensions.

Zinc plating, galvanizing, rust, flattened thread crests, and damage can all affect a caliper reading. Measure multiple crest locations, avoid damaged sections, use pitch as a second identifier, and do not rely on one caliper reading alone.