Calibration weights are mainstays in most labs as they help ensure that lab balances are giving you accurate readings. However, they are not all made the same. Calibration weights are assigned a class which is based on the accuracy or tolerance of the weight. There are different sets of classes of weights, including ASTM, OIML, and NIST. In addition, there are certifications granted to different weights.
In this guide, we explain the different classes of calibration weights and the certificates that can be purchased alongside them. We also look at other factors to consider when purchasing weights such as style, material, and construction.
Classes of Weights
There are three main calibration weight class systems that you’ll come across:
NIST: National Institute of Standards and Technology (NIST) Class F weights are often used in industrial settings to verify Class III, Class IIII, and non-designated scales. They may also be used in warehouses and manufacturing settings to calibrate scales used for the shipping or production of large products. NIST Class F weights aren’t typically used in laboratories as they are not accurate enough to verify the scales used for most laboratory applications.
ASTM: ASTM International (formerly American Society for Testing and Materials) is a nonprofit non-governmental organization that develops voluntary consensus standards. It has developed 10 calibration weight classes as guided by document ASTM E 617: ASTM Class 000 thru ASTM Class 7. The higher the class number, the higher the level of tolerance (and less accurate) the weight will be. Most laboratory applications require ASTM weights of Class 4 or below. ASTM class weights are the most common type of calibration weights used in US laboratories.
OIML: The Organisation Internationale de Métrologie Légale or International Organization of Legal Metrology (OIML) is an intergovernmental organization that provides standards and systems with the goal of harmonizing legal metrology procedures. From lowest (most accurate) to highest tolerance, the OIML classes are E1, E2, F1, F2, M1, M2, and M3. Most laboratory applications require OIML weights of Class F2 or below. OIML class weights are more commonly used outside the US.
Some manufacturers of calibration weights create their own class systems. For example, Troemner’s UltraClass weights are classed as UltraClass, UltraClass Gold, and UltraClass Platinum. However, Troemner provides a reference to explain what these classes equate to in terms of ASTM and OIML.
Tolerance Levels
The following tables shows the tolerance levels for the different classes of weights, starting with OIML.
OIML:
| E1 | E2 | F1 | F2 | M1 | M2 | M3 | |
| mg | mg | mg | mg | mg | mg | mg | |
| 5000 kg | 25000 | 80000 | 250000 | 800000 | 2500000 | ||
| 2000 kg | 10000 | 30000 | 100000 | 300000 | 1000000 | ||
| 1000 kg | 1600 | 5000 | 16000 | 50000 | 160000 | 500000 | |
| 500 kg | 800 | 2500 | 8000 | 25000 | 80000 | 250000 | |
| 200 kg | 300 | 1000 | 3000 | 10000 | 30000 | 100000 | |
| 100 kg | 160 | 500 | 1600 | 5000 | 16000 | 50000 | |
| 50 kg | 25 | 80 | 250 | 800 | 2500 | 8000 | 25000 |
| 20 kg | 10 | 30 | 100 | 300 | 1000 | 3000 | 10000 |
| 10 kg | 5 | 16 | 50 | 160 | 500 | 1600 | 5000 |
| 5 kg | 2.5 | 8.0 | 25 | 80 | 250 | 800 | 2500 |
| 2 kg | 1 | 3 | 10 | 30 | 100 | 300 | 1000 |
| 1 kg | 0.5 | 1.6 | 5 | 16 | 50 | 160 | 500 |
| 500 g | 0.25 | 0.8 | 2.5 | 8.0 | 25 | 80 | 250 |
| 200 g | 0.1 | 0.3 | 1.0 | 3 | 10 | 30 | 100 |
| 100 g | 0.05 | 0.16 | 0.5 | 1.6 | 5.0 | 16 | 50 |
| 50 g | 0.03 | 0.10 | 0.3 | 1.0 | 3.0 | 10 | 30 |
| 20 g | 0.025 | 0.08 | 0.25 | 0.8 | 2.5 | 8.0 | 25 |
| 10 g | 0.020 | 0.06 | 0.20 | 0.6 | 2.0 | 6.0 | 20 |
| 5 g | 0.016 | 0.05 | 0.16 | 0.5 | 1.6 | 5.0 | 16 |
| 2 g | 0.012 | 0.04 | 0.12 | 0.4 | 1.2 | 4.0 | 12 |
| 1 g | 0.010 | 0.03 | 0.10 | 0.3 | 1.0 | 3.0 | 10 |
| 500 mg | 0.008 | 0.025 | 0.08 | 0.25 | 0.8 | 2.5 | |
| 200 mg | 0.006 | 0.020 | 0.06 | 0.20 | 0.6 | 2.0 | |
| 100 mg | 0.005 | 0.016 | 0.05 | 0.16 | 0.5 | 1.6 | |
| 50 mg | 0.004 | 0.012 | 0.04 | 0.12 | 0.4 | ||
| 20 mg | 0.003 | 0.010 | 0.030 | 0.10 | 0.30 | ||
| 10 mg | 0.003 | 0.008 | 0.025 | 0.08 | 0.25 | ||
| 5 mg | 0.003 | 0.006 | 0.020 | 0.06 | 0.20 | ||
| 2 mg | 0.003 | 0.006 | 0.020 | 0.06 | 0.20 | ||
| 1 mg | 0.003 | 0.006 | 0.020 | 0.06 | 0.20 |
ASTM:
|
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 | |
| mg | mg | mg | mg | g/mg | g/mg | g/mg | g/mg | |
| 5000 kg | 100 g | 250 g | 500 g | 750 g | ||||
| 3000 kg | 60 g | 150 g | 300 g | 450 g | ||||
| 2000 kg | 40 g | 100 g | 200 g | 300 g | ||||
| 1000 kg | 20 g | 50 g | 100 g | 150 g | ||||
| 500 kg | 10 g | 25 g | 50 g | 75 g | ||||
| 300 kg | 6 g | 15 g | 30 g | 45 g | ||||
| 200 kg | 4 g | 10 g | 20 g | 30 g | ||||
| 100 kg | 2 g | 5 g | 10 g | 15 g | ||||
| 50 kg | 63 | 125 | 250 | 500 | 1 g | 2.5 g | 5 g | 7.5 g |
| 30 kg | 38 | 75 | 150 | 300 | 600 mg | 1.5 g | 3 g | 4.5 g |
| 25 kg | 31 | 62 | 125 | 250 | 500 | 1.2 g | 2.5 g | 4.5 g |
| 20 kg | 25 | 50 | 100 | 200 | 400 | 1.0 g | 2 g | 3.8 g |
| 10 kg | 13 | 25 | 50 | 100 | 200 | 500 mg | 1 g | 2.2 g |
| 5 kg | 6 | 12 | 25 | 50 | 100 | 250 | 500 mg | 1.4 g |
| 3 kg | 3.8 | 7.5 | 15 | 30 | 60 | 150 | 300 | 1.0 g |
| 2 kg | 2.5 | 5 | 10 | 20 | 40 | 100 | 200 | 750 mg |
| 1 kg | 1.3 | 2.5 | 5 | 10 | 20 | 50 | 100 | 470 |
| 500 g | 0.6 | 1.2 | 2.5 | 5 | 10 | 30 | 50 | 300 |
| 300 g | 0.38 | 0.75 | 1.5 | 3 | 6 | 20 | 30 | 210 |
| 200 g | 0.25 | 0.5 | 1 | 2 | 4 | 15 | 20 | 160 |
| 100 g | 0.13 | 0.25 | 0.5 | 1 | 2 | 9 | 10 | 100 |
| 50 g | 0.06 | 0.12 | 0.25 | 0.6 | 1.2 | 5.6 | 7 | ... |
| 30 g | 0.037 | 0.074 | 0.15 | 0.45 | 0.9 | 4 | 5 | 44 |
| 20 g | 0.037 | 0.074 | 0.1 | 0.35 | 0.7 | 3 | 3 | 33 |
| 10 g | 0.025 | 0.05 | 0.074 | 0.25 | 0.5 | 2 | 2 | 21 |
| 5 g | 0.017 | 0.034 | 0.054 | 0.18 | 0.36 | 1.3 | 2 | 13 |
| 3 g | 0.017 | 0.034 | 0.054 | 0.15 | 0.3 | 0.95 | 2.0 | 9.4 |
| 2 g | 0.017 | 0.034 | 0.054 | 0.13 | 0.26 | 0.75 | 2.0 | 7 |
| 1 g | 0.017 | 0.034 | 0.054 | 0.1 | 0.2 | 0.5 | 2.0 | 4.5 |
| 500 mg | 0.005 | 0.01 | 0.025 | 0.08 | 0.16 | 0.38 | 1 | 3 |
| 300 mg | 0.005 | 0.01 | 0.025 | 0.07 | 0.14 | 0.3 | 1 | 2.2 |
| 200 mg |