What are Calibration Weight Classes?

What are Calibration Weight Classes?

We are asked many questions at Scales Outlet, and one of the most common questions we get is "what calibration weight do I need?" There is much to unpack with this question and varies greatly depending on the user's application. Most people know how much weight they need, but many have no idea what class of weight they need or that classes even exist. We've put together this blog to help point those searching for the perfect weight in the right direction.

Calibration weights have a class assigned based on their accuracy or tolerance. There are governing bodies that determine classes including ASTM, NIST, and OIML and each has different requirements. We'll explore ASTM and NIST classes which are the most commonly used class systems in the United States.


The NIST, or National Institute of Standards and Technology, is an organization under the United States Department of Commerce. The NIST promotes " U.S. innovation and industrial competitiveness by advancing measurement science, standards, and technology in ways that enhance economic security and improve our quality of life." Their vision includes creating and maintaining measurement solutions and standards in the US.

The most common weight class for industrial use is NIST Class F. Many local weights and measures agencies, scale service firms, and private companies in the US use Class F weights as a means of verifying Class III, Class IIII, and non-designated scales and devices for accuracy. Warehouses and manufacturers typically use Class F weights for calibrating scales used in the production or shipping of larger products or for internal quality control.


ASTM International (formerly American Society for Testing and Materials) is a non-profit organization "that provides a forum for the development and publication of international voluntary consensus standards for materials, products, systems, and services." This non-governmental agency has set forth 10 classes for calibration weights. As the number of the class increases, the tolerance allowed also increases.

ASTM Class 7

Primarily used in industrial applications that require rough measurements, ASTM Class 7 has the highest tolerance allowable in the ASTM classification system. Calibration weights with an ASTM Class 7 are suitable for Class III, Class IIII, and non-designated scales like floor scales, shipping scales, and other industrial devices.

ASTM Classes 4 - 6

ASTM Class 4, ASTM Class 5, and ASTM Class 6 calibration weights are primarily used to check and calibrate Class I and Class II balances. Precision balances and those scales found in student laboratories typically require calibration weights within these classes. These scales typically have readability, or divisions, lower than 0.1 grams (i.e., 0.5g, 1g, etc.).

ASTM Class 3

Moderate precision balances, or those with readability from 0.1 to 0.01 grams, generally require an ASTM Class 3 calibration weight. These types of scales, like the Ohaus Adventurer AX2202N/E, are commonly found in commercial laboratory environments.

ASTM Class 2

Higher precision balances, or those with readability between 0.01 and 0.001 grams (1 milligram), require an ASTM Class 2 weight for calibration purposes. High precision balances, like the Ohaus Explorer EX423N, are used in the pharmaceutical and chemical compounding industries.

ASTM Class 1

Analytical and semi-micro balances have readabilities of 0.1 mg and 0.01 mg respectively. It is recommended to use an ASTM Class 1 weight when calibrating devices that can weigh out to a fraction of a gram. ASTM Class 1 weights may also be used as references when calibrating other weights with higher tolerances. Users should wear gloves or tweezers when handling these weights as any oils or dirt from their hands can alter the weights' tolerances.

ASTM Class 0 - 000

Almost exclusively, metrology laboratories use ASTM Class 0, ASTM Class 00, and ASTM Class 000 as reference standards for calibrating other weights and are often expensive. Controlled environments are required when using these classes of calibration weights because of their tight tolerances. These classes of weights should be handled delicately with tweezers as they are often minute in size.

Obsolete Classes

Below is a useful conversion chart to equate obsolete classes with current standards:

Obsolete Class Current Class
Class M ASTM Class 0, OIML E2
Class S ASTM Class 1, 2
Class S1 ASTM Class 3, OIML F1
Class P ASTM Class 4, OIML F2
Class Q ASTM 5
Class C ASTM Class 6, 7, NIST Class F
Class T ASTM Class 7


Whether you're in manufacturing or biotechnology, almost every industry use scales at one point or more in their operations. With that comes the need for calibrating and maintaining those scales for accuracy for which a calibration weight is needed. If you made it this far in the blog, more than likely you were trying to find the right calibration weight for your scale. Hopefully, we provided enough insight into the world of calibration weight classes that you were able to find the correct weight for your application in our store. If not, we're here to help. Message us via chat, give us a call, or send an email and our scale technicians can help guide you to the right weight. 

Jan 24th 2019 Josh M

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