In modern mechanical manufacturing, general tolerances play a critical role in ensuring interchangeability, manufacturability, and cost control. When engineering drawings omit specific dimensional tolerances, general tolerance standards define acceptable deviation limits. Among the most widely referenced standards globally is ISO 2768, while in China, the equivalent national standard is GB/T 1804.

For companies engaged in international manufacturing, CNC machining, or cross-border sourcing, common technical questions arise:

Is GB/T 1804 identical to ISO 2768?
Can they be used interchangeably?
What are the differences in tolerance values?
How should they be specified on engineering drawings?
Which standard should be applied for CNC machining projects?

This guide provides a complete technical comparison between ISO 2768 and GB/T 1804, including tolerance tables, structural differences, equivalence analysis, practical application advice, and frequently asked questions.

1. What Is ISO 2768 General Tolerance Standard?

1.1 Purpose of ISO 2768

ISO 2768 is an international standard developed to simplify engineering drawings by defining general tolerances for linear and geometrical dimensions when no individual tolerance is specified.

Instead of specifying tolerances for every dimension on a drawing, engineers may add a note such as:

General tolerances: ISO 2768-m

This means unspecified dimensions must comply with the tolerance limits defined in ISO 2768 under class “m”.

Main objectives include:

Reduce drawing complexity
Ensure consistency in manufacturing
Improve international compatibility
Lower design and inspection costs

1.2 Structure of ISO 2768

Part	Title	Scope
ISO 2768-1	Tolerances for linear and angular dimensions	Size tolerances
ISO 2768-2	Geometrical tolerances	Form & position tolerances

ISO 2768-1 Covers:

Linear dimensions
Angular dimensions
External and internal features

ISO 2768-2 Covers:

Straightness
Flatness
Circularity
Parallelism
Perpendicularity
Symmetry

1.3 ISO 2768 Tolerance Classes Explained

Class	Description	Typical Use
f	Fine	Precision components
m	Medium	General mechanical parts
c	Coarse	Fabricated structures
v	Very Coarse	Large welded assemblies

2. What Is GB/T 1804 Standard in China?

2.1 Background of GB/T 1804

GB/T 1804 is China’s national standard specifying general tolerances for linear and angular dimensions when individual tolerances are not indicated. It aligns closely with ISO 2768 and was developed to harmonize Chinese manufacturing practices with international standards.

GB/T stands for:

GB = Guobiao (National Standard)
T = Recommended standard

2.2 Structure of GB/T 1804

GB/T 1804 mirrors ISO 2768-1 in structure and includes the same tolerance grades:

f (Fine)
m (Medium)
c (Coarse)
v (Very Coarse)

2.3 GB/T 1804 Tolerance Classes

Class	Application
f	Precision machining
m	Standard CNC machining
c	Welded or non-critical parts
v	Large structural parts

3. Is GB/T 1804 Equivalent to ISO 2768?

3.1 Are They Technically Identical?

GB/T 1804 was developed based on ISO 2768 principles. In practice, the tolerance classes, dimensional ranges, and numerical values are closely aligned. However, they remain separate standards under different governing authorities.

3.2 Key Similarities

Feature	ISO 2768	GB/T 1804
Tolerance classes	f, m, c, v	f, m, c, v
Linear tolerance structure	Yes	Yes
Angular tolerance	Yes	Yes
Used in machining	Yes	Yes
Recognition	Global	China-focused

3.3 Key Differences

Aspect	ISO 2768	GB/T 1804
Governing body	ISO	SAC (China)
Legal scope	International	National
Language	English	Chinese

4. ISO 2768 and GB/T 1804 Tolerance Comparison Table

4.1 Linear Dimension Tolerance Comparison (Class m)

Nominal Length (mm)	ISO 2768-m (± mm)	GB/T 1804-m (± mm)
0.5 – 3	0.1	0.1
>3 – 6	0.1	0.1
>6 – 30	0.2	0.2
>30 – 120	0.3	0.3
>120 – 400	0.5	0.5
>400 – 1000	0.8	0.8
>1000 – 2000	1.2	1.2

4.2 Fine Tolerance (Class f)

Nominal Length (mm)	ISO 2768-f (± mm)	GB/T 1804-f (± mm)
0.5 – 3	0.05	0.05
>3 – 6	0.05	0.05
>6 – 30	0.1	0.1
>30 – 120	0.15	0.15

4.3 Coarse Tolerance (Class c)

Nominal Length (mm)	ISO 2768-c (± mm)	GB/T 1804-c (± mm)
3 – 6	0.3	0.3
6 – 30	0.5	0.5
30 – 120	0.8	0.8
120 – 400	1.2	1.2

4.4 Angular Tolerance Comparison (Class m)

Nominal Length (mm)	ISO 2768-m	GB/T 1804-m
≤ 10	±1°	±1°
>10 – 50	±0°30′	±0°30′
>50 – 120	±0°20′	±0°20′

5. ISO 2768-1 vs ISO 2768-2: What’s the Difference?

5.1 ISO 2768-1

Covers linear and angular dimensions and is used in nearly all general mechanical drawings.

5.2 ISO 2768-2

Covers geometrical tolerances such as flatness, straightness, parallelism, and perpendicularity.

Feature	Example Tolerance
Straightness	0.1 mm
Flatness	0.2 mm
Parallelism	0.2 mm

6. How to Specify ISO 2768 or GB/T 1804 on Engineering Drawings

6.1 Example Drawing Notes

General tolerances: ISO 2768-m

General tolerances: GB/T 1804-m

6.2 When No Tolerance Is Indicated

Always specify the applicable standard to avoid ambiguity in manufacturing and inspection.

6.3 Common Mistakes

Assuming standards are identical without checking revision
Using fine tolerance unnecessarily
Failing to specify geometrical tolerances

7. Application in CNC Machining

7.1 Choosing Tolerance for CNC Parts

Most CNC machining projects use ISO 2768-m or GB/T 1804-m for general parts.

7.2 Cost Impact of Tighter Tolerances

Tolerance Class	Cost Impact
v	Low
c	Low–Moderate
m	Standard
f	High

7.3 International Manufacturing Scenario

When exporting from China, foreign clients often specify ISO 2768, while Chinese factories default to GB/T 1804. Clarification ensures compliance.

8. Frequently Asked Questions (FAQ)

1. Is GB/T 1804 identical to ISO 2768?

They are highly similar and largely equivalent in tolerance values, but remain separate national standards.

2. What does ISO 2768-m mean?

It specifies medium class general tolerance for unspecified dimensions.

3. Can GB/T 1804 replace ISO 2768?

In most machining applications, yes, if values align and contractual agreements allow it.

4. Does China use ISO 2768?

Chinese manufacturers primarily use GB/T 1804, which is harmonized with ISO 2768.

5. What is general tolerance in machining?

It defines permissible dimensional deviation when specific tolerances are not individually indicated.

6. When should fine tolerance (f) be used?

For precision assemblies or tight functional fit requirements.

7. Is ISO 2768 mandatory?

It is not mandatory unless specified in technical documentation or contract.

9. Conclusion

ISO 2768 and GB/T 1804 are foundational general tolerance standards in global and Chinese manufacturing. Structurally and numerically, they are closely aligned, particularly in commonly used tolerance classes such as “m”.

For CNC machining and international sourcing:

ISO 2768-m is generally equivalent to GB/T 1804-m.
Always specify the standard clearly on drawings.
Select tolerance class based on functional needs and cost considerations.
Avoid unnecessary tight tolerances that increase manufacturing cost.

Understanding these standards improves communication, reduces disputes, and ensures dimensional consistency across international manufacturing projects.

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