Metric Screw Size Chart: Thread Pitch, Tap Drill & Clearance Holes
Quick answer: Metric screw sizes are written as diameter × thread pitch × length. For example, M6 × 1.0 × 20 describes a 6 mm nominal thread diameter, a 1.0 mm distance between thread peaks, and a 20 mm fastener length.
Use the chart below to compare common ISO metric thread sizes, standard coarse pitches, typical cutting-tap drill sizes, and clearance holes. Then follow the links to find the corresponding metric fasteners and tools stocked by Metric Screw & Tool Company.
Metric Screw Size Chart
The values below cover commonly used ISO metric coarse threads. Tap-drill recommendations can vary with the tap style, desired thread percentage, work material, and shop practice. Clearance-hole requirements also vary by fit and application. Confirm the governing drawing or specification before machining a critical component.
Metric size | Nominal diameter | Coarse pitch | Typical tap drill | Close clearance | Normal clearance |
M1.6 | 1.6 mm | 0.35 mm | 1.25 mm | 1.7 mm | 1.8 mm |
M2 | 2.0 mm | 0.40 mm | 1.60 mm | 2.2 mm | 2.4 mm |
M2.5 | 2.5 mm | 0.45 mm | 2.05 mm | 2.7 mm | 2.9 mm |
M3 | 3.0 mm | 0.50 mm | 2.50 mm | 3.2 mm | 3.4 mm |
M4 | 4.0 mm | 0.70 mm | 3.30 mm | 4.2 mm | 4.5 mm |
M5 | 5.0 mm | 0.80 mm | 4.20 mm | 5.2 mm | 5.5 mm |
M6 | 6.0 mm | 1.00 mm | 5.00 mm | 6.2 mm | 6.6 mm |
M8 | 8.0 mm | 1.25 mm | 6.80 mm | 8.4 mm | 9.0 mm |
M10 | 10.0 mm | 1.50 mm | 8.50 mm | 10.5 mm | 11.0 mm |
M12 | 12.0 mm | 1.75 mm | 10.20 mm | 13.0 mm | 13.5 mm |
M14 | 14.0 mm | 2.00 mm | 12.00 mm | 15.0 mm | 15.5 mm |
M16 | 16.0 mm | 2.00 mm | 14.00 mm | 17.0 mm | 17.5 mm |
M18 | 18.0 mm | 2.50 mm | 15.50 mm | 19.0 mm | 20.0 mm |
M20 | 20.0 mm | 2.50 mm | 17.50 mm | 21.0 mm | 22.0 mm |
M22 | 22.0 mm | 2.50 mm | 19.50 mm | 23.0 mm | 24.0 mm |
M24 | 24.0 mm | 3.00 mm | 21.00 mm | 25.0 mm | 26.0 mm |
How to Read a Metric Screw Size
A designation such as M6 × 1.0 × 20 contains three useful measurements:
M6: The thread has a nominal outside diameter of 6 mm.
1.0: The thread pitch is 1.0 mm—the distance from one thread peak to the next.
20: The nominal fastener length is 20 mm.
If the pitch is omitted, the fastener is often understood to use the standard coarse pitch for that diameter. Do not rely on that assumption when ordering a replacement or working from an incomplete drawing. Measure the pitch or verify the specification.
Metric Thread Pitch: Coarse vs. Fine
Metric threads are identified by pitch in millimeters rather than threads per inch. A smaller pitch number means the thread peaks are closer together.
Coarse threads are the common general-purpose choice. They are usually faster to assemble and more tolerant of minor damage and contamination.
Fine threads provide more threads over a given engagement length and allow smaller axial movement per revolution. They are used only when the design calls for them.
Examples of common coarse and fine combinations include M6 × 1.0 and M6 × 0.75, M8 × 1.25 and M8 × 1.0, M10 × 1.5 and M10 × 1.25, and M12 × 1.75 with 1.5 or 1.25 mm fine pitches.
A fine-thread nut will not properly mate with a coarse-thread screw of the same nominal diameter. Confirm both diameter and pitch before ordering.
How to Select a Metric Tap Drill
A tap drill creates the hole that will be threaded. For many common cut-tap applications, a useful starting approximation is:
Tap-drill diameter ≈ nominal thread diameter − thread pitch
For M6 × 1.0, that approximation gives 6.0 − 1.0 = 5.0 mm. For M10 × 1.5, it gives 8.5 mm.
This is a starting point, not a universal machining instruction. The correct hole can change with the desired percentage of thread, tap geometry, workpiece material, plating allowance, tolerance class, and whether a cutting or forming tap is used. Form taps generally require a different hole size than cutting taps. Follow the tap manufacturer’s recommendation for production work.
Close, Normal, and Loose Clearance Holes
A clearance hole allows the screw or bolt to pass through without cutting mating threads into that component.
Close fit: Provides limited clearance and more precise positioning, but requires accurate alignment.
Normal fit: The general-purpose choice for ordinary assembly.
Loose fit: Provides extra installation clearance for less precise alignment, coatings, or field assembly.
The clearance values in the chart are practical reference values. Always use the hole diameter required by the engineering drawing, applicable standard, or equipment manufacturer.
How Metric Screw Length Is Measured
Most screws and bolts are measured from the bearing surface beneath the head to the end of the fastener. Countersunk flat-head screws are normally measured overall because the head sits in the countersink. Specialty fasteners can follow different conventions, so verify the applicable product standard.
Common Metric Fastener Standards
The thread size alone does not define the complete fastener. Head geometry, drive, thread length, strength, material, and dimensional tolerances are controlled by the relevant product specification.
DIN 912 / ISO 4762: Hexagon socket head cap screws
DIN 7984: Low-head socket cap screws
DIN 933 / ISO 4017: Hexagon head screws with full thread
DIN 931 / ISO 4014: Hexagon head bolts with partial thread
DIN 7985 / ISO 7045: Cross-recessed pan-head screws
DIN 965: Cross-recessed countersunk flat-head screws
DIN 125 / ISO 7089 or ISO 7090: Normal-series plain washers, depending on form
DIN 433 / ISO 7092: Small-series plain washers
DIN 9021 / ISO 7093-1: Large-series plain washers
DIN and ISO documents that describe similar products should not automatically be treated as dimensionally interchangeable. Check the current standard, required dimensions, material, property class, finish, and governing drawing before substituting one designation for another.
How to Identify an Unknown Metric Screw
Measure the outside thread diameter with a caliper.
Use a metric thread-pitch gauge to identify the pitch.
Measure the length using the correct convention for the head style.
Identify the head style and drive type.
Read any strength, material, or manufacturer markings.
Compare the dimensions with the applicable DIN, ISO, or JIS standard.
Do not force a nearly matching nut onto a screw. Metric and inch threads—and coarse and fine metric threads—may begin to engage while still being incompatible.
Shop Metric Fasteners and Tools
Metric Screw & Tool Company stocks more than 42,000 metric hardware items for maintenance, repair, manufacturing, machinery, automotive, and industrial applications.
Need help identifying a metric fastener? Call 1-800-METRIC-1 or browse the Metric Screw & Tool product catalog.
Technical References
ISO 261:1998, ISO general purpose metric screw threads—General plan (confirmed current in 2024)
ISO 724, ISO general-purpose metric screw threads—Basic dimensions
ISO 273, Fasteners—Clearance holes for bolts and screws
University of Wisconsin Physics Instrument Shop, Common Metric Tap Drill Sizes
Technical note: This chart is a general reference, not a substitute for the applicable engineering drawing, current standard, tap manufacturer’s data, or qualified engineering review. Verify requirements before machining or using a fastener in a safety-critical joint.
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