Why Do Metric and Imperial Standards Exist? A Complete Guide to Differences, Applications, and How to Choose
- 翰君 陳
- Nov 19
- 4 min read
In hardware parts, machining, and equipment maintenance, it’s common to encounter a confusing situation: you only want to find a screw or a pin, yet the specifications appear in two completely different formats — one written as M4, M6, M8 (metric), and another as 1/4"-20 or #8-32 (imperial).
Many people wonder:“Why aren’t these specifications unified?”“Since they look similar, can they substitute for each other?”
The answer is: No.Metric and imperial systems differ in units, notation, and thread structure — they are two entirely different systems.
This article will clearly explain the differences and help you understand how to choose correctly.
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1. Why Do Metric and Imperial Systems Exist?
Metric System (Metric)
Originated in France in the 18th century
Based on meter (m) and kilogram (kg)
Most widely used globally
Uses decimal system and easy conversion
Imperial System (Imperial)
Developed during the British Industrial Revolution
Adopted widely in the United States
Uses inch, pound, etc.
Strongly embedded in American machines and industrial supply chains
Why do both systems still coexist today?
Because early industrialized countries set their own standards, and many machines are still in use decades later.Therefore, technicians frequently encounter both metric and imperial standards during maintenance or retrofitting.
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2. Metric vs Imperial: Key Differences
Although metric and imperial often look like different ways of writing sizes, they actually differ fundamentally:
1. Unit of measurement
Metric: millimeters (mm)
Imperial: inches (inch)
Example: 6 mm ≠ 1/4" (6.35 mm) → Close, but NOT interchangeable.
2. Thread notation
Metric (example)
M6 × 1.0
M8 × 1.25
Imperial (example)
1/4"-20
#8-32
Metric specifies pitch in millimeters.Imperial specifies TPI (threads per inch).
3. Thread pitch definition is completely different
Metric → Pitch = distance between threads (mm)
Imperial → TPI = number of threads per inch
These two concepts are incompatible.
4. Thread angles differ
Metric thread angle: 60°
Imperial (BSW): 55°
Different geometry = cannot engage properly.
5. Why you must not mix them
Even if dimensions look close, mixing leads to:
Mismatched pitch
Wrong thread angle
“Feels tight” but actually damages the female thread
Loosening or stripping over time
Loss of machine precision and potential equipment failure
Metric and imperial threads are 100% non-interchangeable.
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3. Global Usage and Industry Distribution
Metric (Metric System)
Category | Description | Reason |
Major countries | Taiwan, Japan, Korea, China, Germany, France, EU | Engineering & manufacturing entirely metric |
Machining & CNC | CNC, molds, precision parts | Tooling and gauges all metric |
Automation equipment | Taiwan / Japan / Germany | Structure and threads metric |
Electronics & consumer goods | PCB, 3C, home appliances | Global supply chain standard |
Semiconductor (mostly) | Japanese/European machines | Mainly metric with some US components |
Imperial (Imperial System)
Category | Description | Reason |
Major countries | USA / Canada | Entire industry built on imperial |
Aerospace / military | UNF / UN threads | Long-standing standards |
Hydraulics / piping | NPT, BSP, SAE | Pipe threads mostly imperial |
US-made automation modules | Fixtures, rails, actuators | US brands use imperial |
Old equipment | Machine tools, industrial machines | Legacy imperial standards |
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4. Common Real-World Scenarios
Scenario | Metric or Imperial? | Explanation |
Taiwan machining & self-made parts | Metric | Tooling and drawings are metric |
Repairing US equipment | Imperial | Typically uses UNC/UNF |
Mixed automation lines | Mainly metric + some imperial | American modules are imperial |
Hydraulics/piping | Imperial | NPT/BSP/SAE |
Semiconductor equipment | Mixed | Main body metric, some tooling imperial |
Japanese/European machinery | Metric | Rarely uses imperial |
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5. Why You Must Not Mix Metric and Imperial
Main reasons:
Different pitch system (pitch vs TPI)
Different angles
Damages threads
Causes loosening and failures
Leads to scrap and expensive repairs
Mixing metric and imperial is never recommended.
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6. How to Choose the Correct System
Step 1: Identify equipment origin
USA → Imperial (UNC/UNF)
Japan / Germany / Taiwan → Metric
Unknown → Go to Step 2
Step 2: Check markings
M6 / M8 / M10 = Metric
1/4"-20 / #8-32 = Imperial
No markings → Step 3
Step 3: Measure thread pitch
Pitch 1.0 / 1.25 = Metric
20 TPI / 32 TPI = Imperial
Cannot determine? → Ask supplier
Three Golden Rules
Rule 1：For repair → always use original standardUS machine = imperialOther machines = metric
Rule 2：Pitch vs TPI is the most accurate method
Rule 3：If unsure, don’t guess — ask a supplier
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7. Conversion Table (Common Values)
1 inch = 25.4 mm
1/4" = 6.35 mm
3/8" = 9.525 mm
1/2" = 12.7 mm
⚠️ Conversion ≠ compatibility
Converted values cannot be used as actual thread standards.
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8. Why Choose Sheng Fong Precision?
Sheng Fong Precision specializes in mixed metric & imperial supply chains, helping engineers quickly confirm the correct standard and avoid costly mistakes.
✔ Complete metric & imperial inventory
M series, UNC/UNF, BSW/BSF
✔ Specification identification service
We understand Japan/Germany/US equipment differences and match the correct thread system.
✔ Standardized product coding + clear illustrations
Avoids part confusion and incorrect installation.
✔ Local inventory + fast delivery
Supports urgent repair needs.
✔ Replacement solutions for foreign brands
McMaster-Carr / MISUMI / PEM compatible items available.
📩 Need help identifying metric or imperial threads?LINE（ID：@s9000） — We’ll help you confirm size, pitch, and compatible alternatives.
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