Metric and Inch Fastener Conversion Guide
This guide provides reference dimensional conversions between metric and inch fastener units. All conversions are for reference only. A dimensional conversion does not mean that metric and inch fasteners are interchangeable. Thread system, pitch or TPI, standard, grade, head dimensions, matching components, and application requirements must be confirmed separately.
Direct Answer for Buyers
When buyers send RFQs with inch dimensions, we can review the requirements and provide reference metric conversions for communication. However, a dimensional conversion is a reference only. It does not mean that metric and inch fasteners are interchangeable.
For example:
- 1/2 inch equals 12.7 mm, but 1/2-13 UNC is not the same product as M12 × 1.75.
- 3/8 inch equals 9.525 mm, but a 3/8-inch fastener must not automatically be replaced by an M10 fastener.
Buyers should always provide the original inch specifications, applicable standard, thread series, TPI, grade, and material rather than submitting only an approximate converted metric dimension.
Differences that prevent automatic substitution include:
- Thread system (metric vs. inch)
- Pitch or TPI
- Thread profile (60-degree included angle but different root and crest geometry)
- Thread tolerance class
- Head dimensions and drive type
- Material grade or property class
- Matching nuts and washers
- Coating allowance affecting thread fit
- Applicable DIN, ISO, ASTM, ASME or buyer standard
- Application and assembly requirements
Inch to Millimeter Conversion
The exact relationship is:
millimeters = inches × 25.4For example:
- 1/2 inch = 0.5 × 25.4 = 12.7 mm
- 3/8 inch = 0.375 × 25.4 = 9.525 mm
- 1 inch = 1.0 × 25.4 = 25.4 mm
Inch ↔ Millimeter Converter
Reference conversion only. Decimal inches use the format 0.5 for 1/2 inch. This tool does not provide interchangeable product recommendations.
Millimeter to Inch Conversion
The exact relationship is:
inches = millimeters ÷ 25.4For example:
- 12 mm ÷ 25.4 = 0.4724 in
- 10 mm ÷ 25.4 = 0.3937 in
- 6 mm ÷ 25.4 = 0.2362 in
Millimeter ↔ Inch Converter
Reference conversion only. This tool does not provide interchangeable product recommendations.
Thread Pitch and TPI Conversion
Thread pitch is the distance between adjacent thread crests. For metric fasteners, pitch is measured in millimeters. For inch fasteners, threads per inch (TPI) is used.
The conversion formula:
metric pitch (mm) = 25.4 ÷ TPITPI = 25.4 ÷ metric pitch (mm)Examples:
- 13 TPI = 25.4 ÷ 13 = 1.954 mm pitch
- 20 TPI = 25.4 ÷ 20 = 1.27 mm pitch
- 1.75 mm pitch = 25.4 ÷ 1.75 = 14.5 TPI (reference only)
ISO metric threads and Unified inch threads both use a nominal 60-degree basic thread angle, but they are not interchangeable because diameter, pitch or TPI, tolerances, thread details and matching components differ.
TPI ↔ Metric Pitch Converter
Reference conversion only. Equivalent pitch does not mean interchangeable threads.
Metric, UNC and UNF Thread Differences
| Characteristic | Metric Coarse | Metric Fine | UNC (Unified Coarse) | UNF (Unified Fine) |
|---|---|---|---|---|
| Thread system | Metric (ISO 68, DIN 13) | Metric (ISO 68, DIN 13) | Inch (ASME B1.1) | Inch (ASME B1.1) |
| Diameter unit | Millimeters | Millimeters | Inches (fractional) | Inches (fractional) |
| Pitch designation | mm per thread | mm per thread | Threads per inch (TPI) | Threads per inch (TPI) |
| Thread profile angle | 60° | 60° | 60° | 60° |
| Common standards | DIN 13, ISO 261 | DIN 13, ISO 261 | ASME B1.1, ASTM | ASME B1.1, ASTM |
| Typical grade references | Class 4.8, 5.8, 8.8, 10.9, 12.9 | Class 4.8, 5.8, 8.8, 10.9, 12.9 | Grade 2, Grade 5, Grade 8 | Grade 2, Grade 5, Grade 8 |
| Thread fit tolerance | 6g/6H common | 6g/6H common | Class 2A/2B common | Class 2A/2B common |
This comparison is for reference only. Metric and inch product requirements should be confirmed based on the buyer's drawing, standard, BOM, or RFQ requirements.
Common Fractional Inch Reference Table
| Fractional Inch | Decimal Inch | Millimeters |
|---|---|---|
| 1/8 in | 0.125 | 3.175 mm |
| 3/16 in | 0.1875 | 4.7625 mm |
| 1/4 in | 0.25 | 6.35 mm |
| 5/16 in | 0.3125 | 7.9375 mm |
| 3/8 in | 0.375 | 9.525 mm |
| 7/16 in | 0.4375 | 11.1125 mm |
| 1/2 in | 0.5 | 12.7 mm |
| 5/8 in | 0.625 | 15.875 mm |
| 3/4 in | 0.75 | 19.05 mm |
| 7/8 in | 0.875 | 22.225 mm |
| 1 in | 1.0 | 25.4 mm |
Reference conversion only. Fractional inch dimensions are converted using the relationship 1 inch = 25.4 mm. These conversions are for dimensional reference and do not indicate product interchangeability.
Why Similar Dimensions Are Not Interchangeable
A common misconception is that a metric fastener and an inch fastener with similar converted dimensions can be used interchangeably. This is not correct. The following factors make each product specific to its thread system:
- Thread pitch difference: M12 × 1.75 has 1.75 mm pitch. 1/2-13 UNC has approximately 1.954 mm pitch. The 0.2 mm difference changes thread engagement and load distribution.
- Thread profile: Metric and Unified threads share a 60° angle but differ in root radius, crest flat, and allowance for coating.
- Tolerance system: Metric threads follow ISO tolerance classes (e.g. 6g, 6H). Inch threads follow ASME B1.1 classes (e.g. 2A, 2B).
- Head dimensions: A metric hex bolt head and an inch hex cap screw head have different across-flat and height dimensions.
- Matching components: Metric nuts, washers, and tapped holes are sized for metric threads. Inch components are sized for inch threads.
- Standard requirements: A fastener made to DIN, ISO, ASTM or ASME must meet the dimensional and mechanical requirements of that specific standard. Conversion does not change the standard requirement.
- Grade and material: Metric property classes (8.8, 10.9) are not equivalent to inch grades (Grade 5, Grade 8). Mechanical properties differ.
- Coating allowance: Plating thickness is calculated on the original thread dimensions. Coating a converted dimension fastener may cause thread interference.
Final specifications should be confirmed according to the buyer's original drawing, BOM, applicable standard, unit system, sample, or RFQ requirements.
Product-Specific Conversion Considerations
Bolts
When converting bolt dimensions, confirm each of the following separately:
- Nominal diameter
- Thread pitch or TPI
- Overall length and thread length
- Head width across flats and height
- Grade or property class
- Applicable DIN, ISO, ASME, ASTM or buyer standard
- Matching nut and washer requirements
Nuts
- Thread system (metric or inch)
- Pitch or TPI matching the mating bolt or stud
- Nut class matching the bolt grade
- Nut height, across-flat dimensions
- Applicable standard (DIN 934, ISO 4032, ASTM A563, etc.)
- Finish and coating compatibility with mating parts
Machine Screws
- Head type (pan, flat, round, truss, etc.)
- Drive type (Phillips, slotted, hex, Torx, etc.)
- Thread type (metric, UNC, UNF)
- Thread pitch or TPI
- Length
- Diameter tolerance
- Material and finish
Self-Tapping Screws
- Thread form (thread cutting, thread forming, etc.)
- Point type (sharp point, blunt point, drill point)
- Pre-drilled hole size requirement
- Base material type and thickness
- Drive and head type
- Material, hardness, and coating
Stud Bolts / Double-End Studs
- Overall length
- Thread length at each end
- Thread pitch or TPI
- Grade or property class
- Matching nut requirements at each end
- Applicable standard (DIN 975, DIN 976, ASTM A193, etc.)
Threaded Rods
- Thread system must be stated: Metric, UNC, UNF, 8UN, etc.
- Diameter
- Pitch or TPI
- Length
- Grade or material
- Finish
- Applicable standard
Washers
- Nominal bolt size does not equal actual inside diameter
- Inside diameter is larger than the bolt diameter for clearance
- Outside diameter and thickness vary by series (small, normal, large)
- Hardness requirement (hardened or non-hardened)
- Applicable standard (DIN 125, DIN 9021, ISO 7089, ASME B18.22.1, etc.)
RFQ Checklist
Use this checklist when preparing an RFQ that involves both metric and inch dimension references.
- Metric or inch unit system
- Nominal diameter
- Thread pitch or threads per inch
- Thread series (metric coarse, metric fine, UNC, UNF, 8UN, etc.)
- Overall length
- Thread length
- Applicable standard (DIN, ISO, ASTM, ASME, etc.)
- Original buyer specification before conversion
- Material grade or property class
- Surface finish and coating requirement
- Matching nuts, washers, or other components
- Quantity by SKU
- Packing and labeling requirements
- Inspection document requirements
Prices, MOQ, packing, documentation, and lead time are confirmed after manual RFQ review.
Common Conversion Mistakes
- Assuming M12 is equivalent to 1/2 inch because the diameters are close
- Replacing a 3/8-inch fastener with an M10 fastener
- Converting only the diameter while ignoring thread pitch or TPI differences
- Using metric coarse pitch to substitute for UNC, or metric fine pitch for UNF
- Submitting only a converted metric dimension without the original inch specification
- Assuming washer nominal size equals actual inside diameter
- Treating ISO and ASME thread dimensions as interchangeable without standard confirmation
- Ignoring coating build-up when converting thread dimensions
- Using a dimensional conversion to select nuts without verifying thread system and class
- Accepting a supplier's converted specification without reviewing the original standard or drawing
Metric and Inch Fastener Conversion FAQ
Practical answers for buyers preparing RFQs with mixed unit references.
Can you convert inch fastener sizes to metric sizes?
Yes. We can review RFQs in either metric or inch units and provide reference dimensional conversions. However, a converted diameter does not mean the products are interchangeable. Thread series, pitch or TPI, tolerances, head dimensions, material grade, matching components and the applicable standard must be confirmed before quotation and production. For example, 1/2-13 UNC is not the same product as M12 × 1.75.
What information should buyers provide when the RFQ uses inch dimensions?
Please provide the nominal diameter, thread series such as UNC or UNF, threads per inch, overall length, applicable standard, material, grade, finish and drawing where available. Do not provide only an approximate metric conversion.
What is the difference between pitch and TPI?
Pitch is the distance between adjacent threads measured in millimeters, used for metric fasteners. TPI stands for threads per inch, used for inch fasteners. The conversion is: metric pitch in mm = 25.4 / TPI. For example, 13 TPI equals approximately 1.954 mm pitch.
Can UNC and metric coarse threads be substituted?
ISO metric threads and Unified inch threads both use a nominal 60-degree basic thread angle, but they are not interchangeable because diameter, pitch or TPI, tolerances, thread details and matching components differ.
Can UNF and metric fine threads be substituted?
ISO metric threads and Unified inch threads both use a nominal 60-degree basic thread angle, but they are not interchangeable because diameter, pitch or TPI, tolerances, thread details and matching components differ.
Does nominal bolt size equal washer inside diameter?
No. Washer inside diameter is typically larger than the nominal bolt size to allow assembly clearance. For example, an M12 washer has a larger inside diameter than 12 mm. Washer dimensions should be confirmed by the applicable washer standard, not by converting the bolt diameter.
How should inch fastener length be stated?
Inch fastener length should be stated in inches with decimal or fractional precision as required by the applicable standard. Do not provide only a metric approximation. Include nominal diameter, thread series, TPI, grade, finish, and the applicable standard.
Can metric and inch products be included in one mixed order?
Yes. Metric and inch products can be included in the same RFQ or mixed order when each SKU is clearly defined with its own standard, unit system, thread details, grade, finish, quantity, and packing requirements.
Is M12 equivalent to 1/2 inch?
No. M12 has a nominal diameter of 12 mm. 1/2 inch equals 12.7 mm. Although the diameters are close, M12 and 1/2-13 UNC are different products with different thread systems, pitch, thread profile, tolerances, head dimensions, and applicable standards. A dimensional conversion is a reference only.
Can a 3/8-inch fastener be replaced by an M10 fastener?
No. 3/8 inch equals 9.525 mm, while M10 has a nominal diameter of 10 mm. The 0.475 mm difference is significant for thread fit and assembly. Additionally, the thread series, pitch, profile, tolerance, head dimensions, and applicable standard are all different. Fasteners must not be replaced based on approximate dimensional conversion.
Related Sourcing Links
Use these pages with the conversion guide when preparing a buyer RFQ, reviewing product categories, or verifying specifications.
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Send your product list, drawings, metric or inch specifications, standards, quantities, finish requirements, and packing expectations to xyan060816@gmail.com.
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Final specifications should be confirmed according to the buyer's original drawing, BOM, applicable standard, unit system, sample, or RFQ requirements.
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