Understanding OIML Guidelines: Accuracy Classes

Jun 02, 2025 by Sarah Kraatz Blog, Technical Resources Tags:

OIML, short for “Organisation Internationale de Métrologie Légale” (International Organization of Legal Metrology), was established on October 12, 1955, in Paris. This organization is dedicated to harmonizing and standardizing measurement techniques at an international level. Its main task is to develop recommendations and guidelines that serve as a benchmark for the design of national and international measurement standards. These efforts support legal metrology and contribute to ensuring that measurement procedures and instruments are uniform and reliable worldwide. This plays a significant role in global trade fairness and in building trust in measurement results across borders.

While the EU Directive 2014/31 provides a regulatory framework for accuracy classes within the European Union, the International Organization of Legal Metrology (OIML) plays a crucial role in shaping global standards for measurement accuracy. The OIML develops international recommendations and guidelines that serve as a reference for national and regional regulations, including those in the EU.

OIML Recommendations and their accuracy classes

The OIML has published several recommendations that align with the concept of accuracy classes of the EU Directive 2014/31. For example:

OIML R76: This recommendation covers non-automatic weighing instruments and defines accuracy classes (I, II, III, IIII) similar to those in the EU Directive.
OIML R111: This recommendation focuses on calibration weights and defines weight classes (E1, E2, F1, F2, M1, M2, M3) that correspond to the accuracy requirements of weighing instruments.

These OIML recommendations provide a global benchmark for accuracy classes, ensuring consistency in measurement practices across different countries and regions.

The Connection between accuracy classes and OIML weight classes

Accuracy classes for weighing instruments are closely linked to the OIML weight classes used for calibration. Calibration weights are categorized into classes (E1, E2, F1, F2, M1, M2, M3) based on their precision, and these classes correspond to the accuracy requirements of weighing instruments. For example:

Class E1: This class has the highest accuracy and is primarily used in highly precise laboratory environments.
Class E2: These weights are also used in precision applications, especially for calibrating measuring devices and verifying weights of class F1.
Class F1: Weights of this class are used for calibrating high-quality precision scales and for verifying weights of class F2.
Class F2: These weights are used in general industrial and commercial metrology. They are used for calibrating scales and for verifying weights of class M1.
Class M1: This weight class is typically used in the industry for calibrating commercial scales.
Class M2: These weights are intended for less accurate measurements and are often used in commerce or in other applications with lower accuracy requirements.
Class M3: This class has the least accuracy and is used for simple commercial and industrial applications where precision requirements are minimal.

The choice of the appropriate weight class depends on the required accuracy of the measurement and the type of scale or measuring device to be calibrated. Using OIML-certified weights ensures the accuracy and reliability of measurements in various application areas.

Nominal value	OIML R111-2004 Error limits = permissible tolerances “Tol ± mg”
	E1	E2	F1	F2	M1	M1-2	M2	M2-3	M3
			cater to OIML accuracy class I instruments		suitable for OIML accuracy class II weighing instruments
5 000kg	–	–	± 25 g	± 80 g	± 250 g	± 500 g	± 800 g	± 1600 g	± 2 500 g
2 000 kg	–	–	± 10 g	± 30 g	± 100 g	± 200 g	± 300 g	± 600 g	± 1 000 g
1 000 kg	–	± 1600 mg	± 5 000 mg	± 16 g	± 50 g	± 100 g	± 160 g	± 300 g	± 500 g
500 kg	–	± 800 mg	± 2 500 mg	± 8 000 mg	± 25 g	± 50 g	± 80 g	± 160 g	± 250 g
200 kg	–	± 300 mg	± 1 000 mg	± 3 000 mg	± 10 g	± 20 g	± 30 g	± 60 g	± 100 g
100 kg	–	± 160 mg	± 500 mg	± 1 600 mg	± 5 000 mg	± 10 g	± 16 g	± 30 g	± 50 g
50 kg	± 25 mg	± 80 mg	± 250 mg	± 800 mg	± 2 500 mg	± 5 g	± 8 000 mg	± 16 g	± 25 000 mg
20 kg	± 10 mg	± 30 mg	± 100 mg	± 300 mg	± 1 000 mg	–	± 3 000 mg	–	± 10 000 mg
10 kg	± 5,0 mg	± 16 mg	± 50 mg	± 160 mg	± 500 mg	–	± 1 600 mg	–	± 5 000 mg
5 kg	± 2,5 mg	± 8,0 mg	± 25 mg	± 80 mg	± 250 mg	–	± 800 mg	–	± 2 500 mg
2 kg	± 1,0 mg	± 3,0 mg	± 10 mg	± 30 mg	± 100 mg	–	± 300 mg	–	± 1 000 mg
1 kg	± 0,5 mg	± 1,6 mg	± 5,0 mg	± 16 mg	± 50 mg	–	± 160 mg	–	± 500 mg
500 g	± 0,25 mg	± 0,8 mg	± 2,5 mg	± 8,0 mg	± 25 mg	–	± 80 mg	–	± 250 mg
200 g	± 0,10 mg	± 0,3 mg	± 1,0 mg	± 3,0 mg	± 10 mg	–	± 30 mg	–	± 100 mg
100 g	± 0,05 mg	± 0,16 mg	± 0,5 mg	± 1,6 mg	± 5,0 mg	–	± 16 mg	–	± 50 mg
50 g	± 0,03 mg	± 0,10 mg	± 0,3 mg	± 1,0 mg	± 3,0 mg	–	± 10 mg	–	± 30 mg
20 g	± 0,025 mg	± 0,08 mg	± 0,25 mg	± 0,8 mg	± 2,5 mg	–	± 8,0 mg	–	± 25 mg
10 g	± 0,020 mg	± 0,06mg	± 0,20 mg	± 0,6 mg	± 2,0 mg	–	± 6,0 mg	–	± 20 mg
5 g	± 0,016 mg	± 0,05 mg	± 0,16 mg	± 0,5 mg	± 1,6 mg	–	± 5,0 mg	–	± 16 mg
2 g	± 0,012 mg	± 0,04 mg	± 0,12 mg	± 0,4 mg	± 1,2 mg	–	± 4,0 mg	–	± 12 mg
1 g	± 0,010 mg	± 0,03 mg	± 0,10 mg	± 0,3 mg	± 1,0 mg	–	± 3,0 mg	–	± 10 mg
500 mg	± 0,008 mg	± 0,025 mg	± 0,08 mg	± 0,25 mg	± 0,8 mg	–	± 2,5 mg	–	–
200 mg	± 0,006 mg	± 0,020 mg	± 0,06 mg	± 0,20 mg	± 0,6 mg	–	± 2,0 mg	–	–
100 mg	± 0,005 mg	± 0,016 mg	± 0,05 mg	± 0,16 mg	± 0,5 mg	–	± 1,6 mg	–	–
50 mg	± 0,004 mg	± 0,012 mg	± 0,04 mg	± 0,12 mg	± 0,4 mg	–	–	–	–
20 mg	± 0,003 mg	± 0,010 mg	± 0,03 mg	± 0,10 mg	± 0,3 mg	–	–	–	–
10 mg	± 0,003 mg	± 0,008 mg	± 0,025 mg	± 0,08 mg	± 0,25 mg	–	–	–	–
5 mg	± 0,003 mg	± 0,006 mg	± 0,020 mg	± 0,06 mg	± 0,20 mg	–	–	–	–
2 mg	± 0,003 mg	± 0,006 mg	± 0,020 mg	± 0,06 mg	± 0,20 mg	–	–	–	–
1 mg	± 0,003 mg	± 0,006 mg	± 0,020 mg	± 0,06 mg	± 0,20 mg	–	–	–	–

What are Permissible Error Limits?

The permissible error limit for test weights defines the maximum allowable deviation from the actual mass of a test weight used for calibrating scales. These limits are set in the OIML guideline R 111 for different accuracy classes. The tolerance is equated with the measurement uncertainty of the test weight. For example, a 1 g test weight can illustrate how manufacturing tolerances differ from class E1 to M3. For a class E1 test weight, the error limit is ± 0.010 mg, i.e., 0.00001 g. In class F1, the tolerance is ± 0.10 mg, while in class M3, it is ± 10 mg.

How OIML supports the EU Directive

The OIML and the EU collaborate closely to harmonize measurement standards. Here’s how OIML supports the EU Directive on accuracy classes:

Standardization: OIML recommendations serve as a basis for EU regulations, ensuring that accuracy classes are consistent with international standards.
Mutual Recognition: OIML certificates and test reports are recognized in the EU, facilitating the free movement of goods and reducing trade barriers.
Technical Expertise: OIML provides technical guidance and best practices that help EU member states implement and enforce accuracy class requirements.
Global Harmonization: By aligning EU standards with OIML recommendations, the EU ensures that its measurement practices are compatible with those of other countries, promoting international trade.

Both the EU Directive 2014/31/EU and OIML recommendations play vital roles in defining accuracy standards. While the EU directive focuses on regional regulations, OIML ensures global standardization, fostering trust and consistency in international trade.

For those looking to purchase compliant weighing instruments, selecting a scale based on the correct accuracy class is essential. Visit our analytical balances and precision balances to find the right device for your needs.