Metric Thread Sizes (ISO 261)

Every standard metric thread size — coarse and fine pitch side by side — with pitch, pitch diameter, minor diameters, tap drill and tensile stress area.

Data verified 2026-09-29 · based on ISO 261:1998

Quick Answer

Metric Thread Sizes — Coarse and Fine (ISO 261)

All dimensions are basic — the theoretical ISO 68-1 profile with no tolerance-class allowance. Tap drill is sized for approximately 75% thread engagement and is the nearest standard metric drill at or below the theoretical pilot diameter. Tensile stress area follows ISO 898-1: As = π/4 × (d − 0.9382P)². Rows inside each series are ordered by nominal diameter; where one diameter appears twice within a series the coarser pitch is listed first.

Coarse Pitch vs Fine Pitch — Which to Specify

The coarse series is the default. For any nominal diameter ISO 261 defines exactly one coarse pitch, and it is the pitch you get when the designation names only the diameter. Coarse threads have a deeper thread form, tolerate plating and slightly damaged threads better, and assemble faster because fewer turns are needed. They are the right choice for almost all general fastening into steel and cast iron.

The fine series defines additional smaller pitches for the same diameters. A finer pitch gives a larger minor diameter — the hole through the middle of the bolt is bigger — so a fine-thread bolt is stronger in tension than the coarse bolt of the same nominal size. It also gives a shallower helix, so the fastener resists loosening under vibration better and allows finer adjustment. The costs are that fine threads cross-thread more easily, are more sensitive to plating thickness, and are harder to tap in deep holes.

Practically: use coarse for the general case, and reach for fine where the thread must resist vibration, where wall thickness is limited and you cannot afford the coarse thread's deeper cut, or where the joint needs precise adjustment. Compare the Tensile area column above — the difference between M10x1.5 (58.0 mm²) and M10x1.25 (61.2 mm²) is entirely the effect of the finer pitch.

How to Read This Table

Size is the full designation — nominal diameter × pitch. Pitch is the axial distance between thread crests in millimetres, not the number of threads per inch. To convert, threads per inch = 25.4 ÷ pitch, so a 1.5 mm pitch is 16.9 TPI.

Pitch dia, Ext. minor and Int. minor are basic diameters: the pitch diameter is where the thread flanks are theoretically equal in width, and the two minor diameters differ because the external thread has a rounded root and the internal thread a flat one. A part measured on the shop floor will differ from these by the tolerance-class allowance — 6g for external, 6H for internal in the default metric system.

Tap drill is the pilot hole for cutting the internal thread, chosen for roughly 75% engagement. Tensile area is the effective cross-section used for strength calculations and for tightening torque, and it is smaller than the nominal diameter would suggest because the thread reduces the section.

Frequently Asked Questions

What is the difference between M10 and M10x1.5?

None — they are the same thread. The ISO 261 coarse series defines one pitch per diameter, and for a 10 mm nominal diameter that pitch is 1.5 mm. M10 implies the coarse pitch; M10x1.5 states it explicitly. The explicit form is preferred on drawings because it cannot be misread.

What is the metric equivalent of a 1/4-20 UNC thread?

There is no exact equivalent. A 1/4-20 UNC thread has a 6.35 mm major diameter and a 1.27 mm pitch (20 TPI). The closest metric sizes are M6x1 (6 mm diameter, 1 mm pitch) and M8x1.25 (8 mm diameter, 1.25 mm pitch) — neither matches on both diameter and pitch. Metric and inch threads also differ in thread angle and flank form, so a substitute will not have the same strength or fit.

What tap drill do I need for a metric thread?

The pilot diameter is the nominal diameter minus the pitch — for M10x1.5 that is 8.5 mm, for M10x1.25 it is 8.8 mm. Then round down to the nearest standard metric drill. The Tap drill column above gives the resulting size for every thread in the table, sized for approximately 75% thread engagement.

Why is the tensile stress area smaller than the nominal diameter?

Because the thread removes material from the bolt's cross-section. ISO 898-1 defines the stress area as As = π/4 × (d − 0.9382P)², which averages the minor and pitch diameters rather than using the major diameter. For M10x1.5 that gives 58.0 mm², where a plain 10 mm bar would have 78.5 mm² — a reduction of about 26%. Using the nominal diameter for strength calculations will overestimate capacity.

Is a fine thread stronger than a coarse thread?

The bolt is, the assembly is not necessarily. A finer pitch leaves a larger minor diameter, so the bolt's tensile stress area is larger and it carries more load before breaking. But the thread is shallower, so there is less material engaged in the nut and stripping the internal thread becomes the likely failure mode instead. Fine threads also cross-thread more easily and are more sensitive to plating.

Related

• Metric Coarse Thread Chart
• Metric Fine Thread Chart
• Tap Drill Size Chart
• UNC Thread Sizes
• NPT Thread Sizes
• Thread Dimension Calculator
• All thread data sheets

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

Data Sources

Cross-checked against:

• Every row is identical to the corresponding row on the coarse or fine pitch page — this table is the union of those two published datasets, not an independent extraction
• The coarse and fine rows are validated by the same build-time checks as their source pages
• Tensile stress areas cross-checked against the ISO 898-1 worked values

Derived values — the following values on this page are calculated, not taken directly from the standard:

Dimensions are basic sizes. Add the tolerance-class allowance — 6g external, 6H internal by default — before inspecting a part to these numbers. Tap drill sizes are sizing guidance, not a specification: engagement percentage is a design decision that depends on the material being tapped.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.