If you have ever received a wood chip delivery specification or reviewed a biomass fuel tender, you have likely encountered terms like G50, M35, or G-Class and M-Class. These are not marketing labels — they are standardized quality classifications that define the physical characteristics of wood chips used in energy production, landscaping, and industrial applications.
Understanding the difference between G-Class and M-Class is essential for anyone involved in biomass procurement, boiler operation, or timber supply chains. This article explains both classifications in plain language, compares them side by side, and helps you determine which specification is right for your application.
What Is Wood Chip Classification?
Wood chip classification is a standardized system for describing and measuring the physical properties of chipped wood material. Rather than relying on subjective descriptions like 'fine' or 'coarse,' classification systems assign measurable parameters to each batch of wood chips, enabling consistent procurement, combustion performance, and equipment compatibility.
Why Wood Chip Size Classification Matters in Industry
In industrial and energy settings, the quality of wood chips directly affects equipment performance. Chips that are too large can jam auger feeding systems or cause incomplete combustion in boilers. Chips that are too wet reduce the net calorific value of the fuel and create storage problems including mold growth and self-heating.
For landscaping and mulching applications, chip size determines how quickly the material breaks down and how it performs as a ground cover. In paper and pulp manufacturing, fiber length and chip geometry determine yield and paper quality. Classification makes it possible to specify, procure, and verify the right material for each use case.
Overview of the EN ISO 17225-4 Grading Standard
The primary international standard for wood chip quality classification is EN ISO 17225-4: Solid Biofuels — Fuel Specifications and Classes — Graded Wood Chips. Published by the International Organization for Standardization (ISO) and adopted across European Union member states, this standard defines quality classes for wood chips based on multiple parameters including particle size (G-Class), moisture content (M-Class), ash content, and bulk density.
While other regional standards exist — particularly in North America and Asia — EN ISO 17225-4 is the benchmark most commonly referenced in European biomass contracts, tender documents, and equipment manufacturer specifications.
What Is G-Class Wood Chip?
G-Class refers to the geometric or particle size classification of wood chips. The 'G' stands for the graded size range of the chips, determined by measuring the proportion of material that falls within defined upper and lower size limits. This classification governs the physical dimensions of the chips — their length, width, and thickness.
G-Class Wood Chip Size Classification Breakdown (G30, G50, G100)
Under EN ISO 17225-4, G-Class grades are defined by the nominal upper size limit of the main chip fraction. The most commonly used grades are:
• G30 — Fine chips with a nominal upper size of 30 mm. The majority of the material (typically 80% or more) must pass through a 30 mm screen. Suitable for small automated boilers and fine-feeding systems.
• G50 — Medium chips with a nominal upper size of 50 mm. A widely used general-purpose grade for medium to large biomass boilers.
• G100 — Coarse chips with a nominal upper size of 100 mm. Typically used in large industrial combustion systems with robust feeding mechanisms.
• G200 and beyond — Very coarse material used in specialized industrial processes or as a pre-chipped feedstock for further processing.
Each grade also specifies allowable percentages of oversize and fine material (dust/sawdust), ensuring the chip population meets a defined particle size distribution rather than just a maximum size limit.
How G-Class Affects Feeding Systems and Combustion
The G-Class of wood chips has a direct impact on how chips are handled, fed into a combustion chamber, and burned. Smaller G-Class chips (G30) flow more consistently through screw conveyors and auger systems, making them ideal for automated residential and small commercial boilers. Larger chips (G50, G100) require more robust feeding infrastructure but are compatible with stoker burners and moving grate systems used in industrial plants.
From a combustion perspective, smaller chips burn more completely and quickly, while larger chips may require longer residence times in the furnace. Using chips outside the rated G-Class for a boiler can result in bridging (blockages in the feed system), incomplete combustion, and increased particulate emissions.
G-Class Wood vs Other Chip Grades — What Sets It Apart
G-Class is a size-only classification. It does not tell you anything about moisture content, ash content, or chemical contamination. This is a critical point: a batch of chips can be perfectly graded at G50 while being too wet for efficient combustion (high M-Class value). G-Class certification simply confirms that the particle size distribution meets the specified standard — all other quality parameters must be specified and verified separately.
What Is M-Class Wood Chip?
M-Class refers to the moisture content classification of wood chips. The 'M' stands for moisture, and the class is defined by the maximum allowable moisture content of the chips, expressed as a percentage of the total mass of the sample as received (also called wet basis or ar basis).
M-Class Moisture Content Grades Explained (M10, M20, M35, M55)
EN ISO 17225-4 defines the following M-Class grades:
• M10 — Very dry chips with moisture content of 10% or less. Typically produced from kiln-dried or artificially dried wood. Highest energy density per tonne.
• M20 — Dry chips with moisture content of 20% or less. Common in chips made from air-dried roundwood or well-seasoned timber.
• M35 — Semi-dry chips with moisture content of 35% or less. A widely specified class for chips from freshly felled but partially dried wood.
• M55 — Wet or green chips with moisture content of 55% or less. Typical of freshly chipped green wood or forest residues with little or no drying.
Some specifications also reference M65, covering very wet forest chips used in large industrial boilers with pre-drying systems.
How Moisture Content Impacts Energy Value and Storage
Moisture content has a profound effect on the net calorific value (NCV) of wood chips. Water in the chip mass must be vaporized during combustion, consuming energy that would otherwise contribute to heat output. As a rough guide, the NCV of wood chips drops by approximately 1 MJ/kg for every 5 percentage points increase in moisture content above 0%.
Beyond combustion efficiency, high moisture content creates significant storage challenges. Wet chips are susceptible to biological degradation, producing heat, carbon dioxide, and methane during storage — a process known as self-heating. In large stockpiles, this can lead to spontaneous combustion. Mold growth in wet chip stores also poses health risks for workers.
M-Class vs G-Class — Are They Competing or Complementary Standards?
M-Class and G-Class are entirely independent quality parameters. They measure different physical properties — size and moisture — and neither one implies anything about the other. A chip can be classified as G50 (correct size) and M55 (very wet), or as G30 (fine) and M10 (very dry). In real-world procurement, both parameters are always specified together, and a complete wood chip specification will include both a G-Class and an M-Class designation.
Grand Design M-Class vs G-Class — A Full Comparison
When comparing M-Class and G-Class as quality frameworks, the key insight is that they address completely different dimensions of wood chip quality. Understanding how they differ in measurement approach, operational impact, and specification context is essential for anyone writing or interpreting a wood chip supply contract.
Key Differences in Measurement, Application, and Performance
G-Class is determined through physical screening — passing a sample of chips through screens of defined aperture sizes and measuring the mass proportions in each fraction. M-Class is determined by weighing a sample, drying it in an oven at 105°C until constant mass, and calculating the mass loss as a percentage of the original wet weight.
Operationally, G-Class governs the mechanical compatibility of chips with handling and combustion equipment, while M-Class governs the thermochemical performance and storage characteristics. Both are equally important in biomass energy applications.
Comparison Table: G-Class vs M-Class at a Glance
| Feature | G-Class | M-Class |
| What it measures | Particle size (chip dimensions) | Moisture content (water in chips) |
| Unit of measurement | Millimeters (mm) | Percentage (%) of wet mass |
| Common grades | G30, G50, G100, G200 | M10, M20, M35, M55 |
| Determined by | Screen/sieve analysis | Oven-drying gravimetric test |
| Primary impact | Feeding system compatibility, combustion rate | Energy value, storage stability |
| Secondary impact | Dust and fines generation | Drying cost, transport weight |
| Governing standard | EN ISO 17225-4 | EN ISO 17225-4 |
| Independent of each other? | Yes | Yes |
M-Class vs GL-Class — Clearing Up the Confusion
Some suppliers and older documentation refer to GL-Class (sometimes written as G-L class), which can cause confusion. GL-Class is not a separate standard — it appears in some national or proprietary grading systems as a combined or legacy designation. In the context of EN ISO 17225-4, the authoritative classifications are G-Class (size) and M-Class (moisture), and these should be used in all formal specifications. If you encounter GL-Class in a supplier document, seek clarification on whether it refers to G-Class, M-Class, or a non-standard regional designation.
How G-Class and M-Class Work Together in Real Specifications
In practice, wood chip specifications always combine G-Class and M-Class into a single quality declaration. Understanding how to read and write these combined specifications is a core competency for procurement managers, boiler operators, and energy contractors.
Reading a Combined Spec: What "G50 M35" Really Means
A specification of G50 M35 means that the wood chips must meet two simultaneous requirements: the particle size distribution must comply with the G50 grade (the majority of chips must pass through a 50 mm screen, with defined oversize and fines limits), and the moisture content must be 35% or less on a wet mass basis.
This combined specification gives a buyer and seller a precise, testable, and legally defensible quality standard. If a delivery fails the G50 screen test, it is out of specification for size regardless of its moisture content. If the moisture is 40% on a batch labeled M35, the delivery fails the M-Class requirement regardless of its size. Both parameters must be met simultaneously.
Wood 5 vs Wood 3 — Understanding Chip Grade Levels in Practice
In some European national standards and procurement frameworks — particularly in Germany (DIN), Austria, and Switzerland — wood chips are also referenced by quality levels such as Wood 3 or Wood 5. These designations often correspond to a bundled quality tier that encompasses G-Class, M-Class, ash content, and sometimes chemical contamination limits in a single grade label.
Wood 3 typically corresponds to higher-quality chips from clean, untreated forestry or sawmill sources, while Wood 5 may allow for a broader range of source materials including recycled or mixed-origin wood. These national tier systems are broadly compatible with but not identical to EN ISO 17225-4 classes. When procuring across borders, always confirm whether the specification references EN ISO 17225-4 directly or a national equivalent.
Other Quality Parameters Beyond G and M Class (Ash, Bulk Density, Contamination)
A complete wood chip quality specification typically includes parameters beyond G-Class and M-Class:
• Ash content — The proportion of non-combustible mineral residue remaining after complete combustion, expressed as a percentage of dry mass. Low ash content (A0.5 to A2.0) is preferred for boilers with automated ash removal.
• Bulk density — The mass of chips per unit volume, typically expressed in kg/m3. Important for logistics, storage silo design, and energy content calculations per delivery volume.
• Chemical contamination — Relevant for chips from recycled or treated wood. EN ISO 17225-4 defines acceptable contamination limits for heavy metals and halogens.
• Nitrogen, sulfur, and chlorine content — Important for emissions compliance, particularly in regions with strict air quality regulations.
These parameters work alongside G-Class and M-Class to form a comprehensive fuel quality profile.
Choosing the Right Wood Chip Class for Your Application
The right G-Class and M-Class combination depends on your specific application, equipment specifications, and budget. Below is a practical guide to the most common use cases.
Small Boilers and Residential Heating — Which Class to Specify
Residential and small commercial biomass boilers (typically under 100 kW) are designed for fine, dry chips. The recommended specification for these systems is G30 combined with M20 or M25. Finer chips feed reliably through small-diameter auger systems, and lower moisture content ensures complete combustion in compact combustion chambers with limited air supply.
Using G50 or higher chips in a boiler rated for G30 risks bridging and jamming in the feed system. Using M35 or higher chips in a high-efficiency condensing biomass boiler reduces efficiency significantly and may cause excessive condensate in the flue system.
Industrial Biomass Plants — Recommended G-Class and M-Class Combinations
Large industrial biomass plants (1 MW and above) are typically designed for G50 or G100 chips, often accepting moisture up to M35 or M45 due to the presence of pre-drying systems or large grate combustion chambers with long fuel residence times. Some plants specifically procure wet chips (M55) when they have on-site chip dryers that use waste heat from the combustion process.
For district heating plants and combined heat and power (CHP) facilities, the specification G50 M35 is a common procurement standard that balances fuel availability, logistics cost, and combustion performance.
Mulching, Landscaping, and Non-Energy Uses of Graded Wood Chips
For mulching and landscaping applications, the G-Class specification remains relevant, but M-Class is less critical. Arboricultural chips used as path cover or tree mulch are typically in the G50 to G100 range. Moisture content affects the initial appearance and weight of the chips but has little effect on their long-term performance as a mulch material.
In paper and pulp manufacturing, chip quality is specified differently — fiber length, species, and density are more important than the G-Class and M-Class system used for biomass energy. However, the underlying measurement methodology for particle size is similar.
Common Mistakes When Specifying Wood Chip Quality
Specifying Only One Class and Ignoring the Other
One of the most common errors in wood chip procurement is specifying only a G-Class or only an M-Class without the other. A contract that specifies G50 but omits M-Class leaves the buyer with no recourse if a delivery arrives at 60% moisture content. Similarly, specifying M20 without a G-Class allows a supplier to deliver improperly sized chips that may jam equipment even though they are dry.
Always include both G-Class and M-Class in any formal wood chip supply specification. If additional quality control is required, include ash content and bulk density requirements as well.
Accepting Self-Reported Quality Without Third-Party Verification
Supplier-issued quality certificates may not reflect the actual quality of each delivery. Moisture content in particular can vary significantly between batches and even within a single delivery, especially for freshly chipped or stored wood chips. For high-volume or high-value contracts, require third-party sampling and testing at the point of delivery, using accredited laboratories following EN ISO 14778 (sampling) and EN ISO 18134 (moisture measurement) protocols.
Overlooking Seasonal Moisture Variation in Fresh Wood Chips
Wood moisture content changes seasonally. Trees felled in late spring and summer typically have higher sap moisture content than those felled in autumn or winter. Chips produced from freshly felled summer timber may arrive at M45 to M55 even if the supplier's standard grade is M35. If your boiler or process is sensitive to moisture variation, consider including moisture spot-testing on arrival or negotiating a seasonal moisture allowance in your supply contract.
Frequently Asked Questions About G-Class vs M-Class Wood Chips
Can Wood Chips Pass G-Class but Fail M-Class?
Yes. G-Class and M-Class are entirely independent parameters. A batch of wood chips can have a perfect particle size distribution meeting G50 requirements while having a moisture content of 60%, which would fail an M35 specification. Both parameters must be tested and reported independently. Passing one class provides no information about compliance with the other.
Is G-Class Wood Always Better Than M-Class Wood?
This question reflects a common misunderstanding. G-Class and M-Class are not competing quality hierarchies — they measure completely different things. A chip cannot be 'better in G-Class than M-Class' any more than a person can be 'better in height than weight.' The appropriate G-Class and M-Class for a given application depends on equipment specifications, energy requirements, and logistics considerations. Neither classification is universally superior.
Where Can I Find the Full EN ISO 17225-4 Standard?
The full text of EN ISO 17225-4 can be purchased through the International Organization for Standardization at iso.org, or through national standards bodies such as BSI (British Standards Institution), DIN (Deutsches Institut für Normung), or AFNOR (Association Française de Normalisation). Some national standards bodies also make preview versions or summaries available free of charge. Equipment manufacturers and major biomass fuel suppliers often include relevant extracts from the standard in their product documentation.
How Often Should Wood Chip Quality Be Tested?
Testing frequency depends on the scale and sensitivity of your application. For small residential boiler users purchasing chips from a single trusted supplier, visual inspection and periodic spot-testing (once or twice per heating season) may be sufficient. For industrial biomass plants or district heating operators purchasing thousands of tonnes per year, testing every delivery — or at minimum sampling every third delivery — using accredited laboratory methods is best practice. Continuous online moisture measurement systems are available for large-scale operations.
Conclusion — Getting Wood Chip Classification Right
Wood chip quality classification is not complicated once you understand that G-Class and M-Class measure entirely different things. G-Class defines the particle size of the chips and determines whether they will work in your handling and combustion equipment. M-Class defines the moisture content and determines how much energy you will get from each tonne and how safely you can store the material.
For any biomass energy application, always specify both G-Class and M-Class in your procurement contracts. Reference EN ISO 17225-4 as your quality standard, require third-party testing for significant volumes, and verify compliance at delivery. Taking these steps protects your equipment, ensures energy efficiency, and gives you legal recourse if a delivery falls short of specification.
Whether you are managing a district heating network, operating a small farm boiler, or procuring chips for an industrial site, understanding the G-Class vs M-Class distinction is the foundation of effective wood chip quality management.