Types of Weighing Scales & Which Laboratory Calibration Weights Each Requires

Every weighing scale — from a sub-milligram analytical balance in a pharmaceutical laboratory to a multi-tonne platform scale on a factory floor — requires calibration to remain trustworthy. But calibration is only as valid as the reference weights used to perform it. Select the wrong weight class for your scale type, and you may be certifying an instrument against a standard that is itself less accurate than the scale's own readability. The result is a calibration that appears complete but provides no real assurance of accuracy. Understanding the relationship between scale type and calibration weight class is the foundation of any reliable weighing program.

Why Scale Type Determines Calibration Weight Class

A weighing scale measures mass by comparing an unknown quantity against a known reference. During calibration, that reference is a certified weight. The calibration is only meaningful if the reference weight is more accurate than the scale itself — specifically, the weight's maximum permissible error (MPE) should be no greater than one-third of the scale's readability (its smallest displayed increment). This is known in metrology as the one-third rule, and it is the governing principle behind all weight class selection decisions.

If a balance reads to 0.1 mg, the calibration weight's MPE must not exceed 0.033 mg — a requirement that points toward OIML Class E2 or ASTM Class 1 weights. If a trade scale reads to 1 g, an OIML Class M1 weight with an MPE of 5 mg at 100 g provides a comfortable margin without the expense of laboratory-grade hardware. The scale type, therefore, is not merely a label — it directly determines the accuracy class, material specification, and handling requirements of every calibration weight used with it. For a comprehensive technical overview of this relationship, see our detailed calibration weights and scale calibration guide.

The Main Types of Weighing Scales Used in Laboratories

Laboratory weighing instruments span a wide range of readability, capacity, and application. For a broader look at how scale types map to accuracy requirements across laboratory and industrial environments, see our overview of scales and weights types. The six principal categories most commonly encountered in laboratory and regulated industrial settings are as follows.

Analytical Balances are the most precise instruments in routine laboratory use, with readabilities of 0.1 mg (0.0001 g) and maximum capacities typically between 100 g and 320 g. They are enclosed in a draft shield to eliminate the effect of air currents on the weighing pan — at 0.1 mg sensitivity, a mild air current is sufficient to corrupt a reading. Analytical balances are standard in pharmaceutical compounding, chemical research, and quality control laboratories where minute differences in mass directly affect experimental outcomes or product formulations.

Semi-Micro and Micro Balances extend precision further, with readabilities of 0.01 mg (10 µg) for semi-micro instruments and 0.001 mg (1 µg) for true micro balances. These instruments require vibration-isolation tables, temperature-controlled environments, and the most precise calibration weights available. They are found primarily in national metrology laboratories, elite research institutions, and specialized pharmaceutical environments where reference standard verification is required.

Precision Top-Loading Balances offer readabilities ranging from 0.001 g to 0.01 g with capacities from 200 g to several kilograms. They do not require a draft shield in most conditions and are faster to operate than analytical balances. They are widely used in food production, general chemistry, educational laboratories, and any application where milligram-level precision is needed but sub-milligram resolution is not required.

Industrial Platform and Floor Scales handle loads from a few kilograms to several tonnes, with readabilities typically ranging from 1 g to 500 g depending on capacity. These instruments prioritize ruggedness, throughput, and legal-for-trade compliance over ultra-high precision. They are used in manufacturing, warehousing, freight, and food processing. Platform scales incorporate electronic load cells — sensors that convert mechanical force into an electrical signal proportional to applied weight.

Jewellery and Precious Metal Balances operate at readabilities of 0.001 g to 0.01 g and typically measure in grams, carats (1 carat = 0.2 g), and troy ounces (1 troy ounce = 31.1035 g). High-end trade-certified jewellery balances are calibrated to OIML Class E2 or F1 reference weights and must meet legal-for-trade standards in commercial contexts.

Moisture Analyzers and Specialized Balances include instruments such as moisture analyzers, density determination kits, and viscosity balances. These instruments are built on analytical or precision balance platforms and share the same calibration weight requirements as their base instrument type. Their readability ranges from 0.001 g to 0.1 g depending on the application.

OIML and ASTM Weight Classes Explained

Two classification systems govern the vast majority of calibration weights in use worldwide. Understanding both — and their relationship to each other — is essential for any procurement or quality management decision involving weighing equipment.

The OIML system (Organisation Internationale de Métrologie Légale, R 111 standard) is used internationally and classifies weights from highest to lowest accuracy as: E1, E2, F1, F2, M1, M2, and M3. E1 represents the most precise class, with tolerances measured in fractions of a milligram even for kilogram-range weights. M3 is the least precise, suitable for general commercial and industrial weighing. OIML classes are the dominant standard outside the United States and are required for legal metrology purposes in most countries.

The ASTM system (ASTM International, E617 standard) is primarily used in the United States and classifies weights as Class 000, 00, 0, 1, 2, 3, 4, 5, 6, and 7 — where lower numbers indicate higher precision. ASTM Classes 000 through 4 are designed for laboratory and high-precision applications; Classes 5 through 7 cover industrial and commercial use. Most laboratory applications in the US require ASTM Class 4 or better.

The two systems are not identical, but they are broadly comparable at corresponding accuracy levels. Our full range of laboratory calibration weights is manufactured to both OIML R111-2004 and ASTM E617 standards, covering classes E1, E2, F1, F2, and M1 in OIML, and Class 0 through Class 7 in ASTM, with weights ranging from 1 mg to 2,000 kg.

Scale-to-Weight Class Matching Guide

The table below consolidates the one-third rule into practical selection guidance for the most common laboratory and industrial scale types. It is intended as a starting reference; always verify against the scale manufacturer's calibration specification and applicable regulatory requirements for your industry.

For engineering-specific applications — including calibration of force measurement devices, load cells, and testing machines used in structural and materials testing — our engineering series test weights are produced to the tolerance and dimensional specifications required by engineering metrology standards, available from 5 kg through 2,000 kg.

Calibration Weight Materials: Stainless Steel vs Cast Iron

The material from which a calibration weight is manufactured affects its long-term dimensional stability, surface condition, magnetic properties, and resistance to environmental factors — all of which directly influence accuracy over the weight's service life.

Stainless steel is the standard material for all laboratory-grade calibration weights from Class E1 through Class M1. Its key advantages are low magnetic permeability (reducing susceptibility to electromagnetic interference near electronic balances), high corrosion resistance (essential in laboratory environments where chemical vapors are present), and a smooth, dense surface finish that resists contamination and is easy to clean without dimensional change. Precision stainless steel weights are manufactured with a sealed adjusting cavity — typically in the base or under a threaded plug — filled with fine wire or metal granules to bring the weight's actual mass within MPE during manufacture. For analytical and semi-micro balance applications, weights must be handled exclusively with tweezers or clean-room gloves; direct hand contact deposits skin oils and moisture that can shift a 1 g weight by several micrograms.

Cast iron is standard for industrial-grade weights in Class M1 through M3 (ASTM Class 5–7) and for large-denomination weights used on platform and floor scales. Cast iron weights are substantially less expensive than stainless steel equivalents at the same nominal mass, offer adequate accuracy for their intended applications, and are robust enough to withstand the handling conditions of warehouses, logistics operations, and field calibration. Their surface is typically painted or lacquered to prevent rust. Because their MPE is comparatively large, minor surface corrosion does not compromise their accuracy class in most cases. They are not suitable for laboratory balance calibration, however, as their surface condition is difficult to maintain to the standard required for Class F2 and above.

Brass appears in some legacy weight sets and in specific low-denomination weights where its density properties are advantageous. Modern manufacturing has largely replaced brass with stainless steel for new precision weights, but brass weights remain in service in many calibration laboratories and are still produced to customer specification.

Handling, Storage, and Traceability Requirements

A calibration weight is only as reliable as the conditions under which it is maintained. Several handling and storage requirements apply universally across all accuracy classes, with the stringency increasing as the class number decreases (i.e., as accuracy increases).

Handling: E1 and E2 class weights must never be touched by bare hands. Dedicated tweezers, weight forks, or clean-room gloves rated for the application are required. F1 and F2 weights should also be handled with gloves in routine practice. M-class weights for industrial use are less sensitive but benefit from clean handling to prevent surface contamination that accelerates corrosion.

Storage: All precision weights should be stored in their original cases — typically lined with foam or velvet to prevent surface contact and mechanical shock. Storage environments should be stable in temperature (ideally 18–25°C), free from excessive humidity (below 60% RH), and away from chemical fumes, electromagnetic fields, and vibration sources. Never store calibration weights in open air or in contact with reactive materials.

Traceability: In regulated industries — pharmaceutical, food safety, legal metrology, and ISO 9001-compliant quality systems — calibration weights must be accompanied by a calibration certificate documenting the weight's actual measured mass, uncertainty, the date of calibration, and a traceable chain back to a national or international measurement standard. Certificates issued under ISO/IEC 17025 accreditation provide the highest level of documented confidence and are required for many audited quality systems. A weight without valid traceability documentation is not a calibration standard — it is simply a mass of known shape.

Recalibration intervals vary by use frequency, environmental conditions, and accuracy class. As a general guideline, E1 and E2 reference weights used in national and accredited laboratories are recalibrated annually. F1 and F2 weights used in routine laboratory work are typically recalibrated every one to two years. M-class industrial weights may be recalibrated every two to three years in stable conditions, or more frequently in harsh or high-throughput environments.

Selecting the right scale type for your application — and pairing it with the correct class of certified calibration weight — is not a one-time decision. It is a continuous quality management commitment. Our full range of scale calibration system and equipment is available to support every stage of that commitment, from initial instrument verification through ongoing routine testing and scheduled recalibration programs.