Bulk density of wood pellets

What bulk density is

Bulk density (The mass of fuel loosely poured into a container of known volume, divided by that volume — including the voids between the pellets.)The mass of fuel loosely poured into a container of known volume, divided by that volume — including the voids between the pellets. is the mass of pellets that fits into a unit of volume when the pellets are loosely poured. It covers both the wood itself and the air in the spaces between the pellets. It is expressed in kilograms per cubic metre and reported as received^[1].

Bulk density depends on several characteristics at once: how densely the pellets themselves are compressed, their length and diameter (short and long pellets pack differently), the amount of dust filling the voids and the moisture content.

Requirements and test method

ENplus requires the bulk density of all classes to fall within 600–750 kg/m³, determined according to ISO 17828^[1]. The current edition of that standard dates from 2025^[20]. The Polish regulation Dz.U. 2025 poz. 618 (Polish Journal of Laws 2025, item 618) adopts the same range for A1 and A2 pellets^[36].

The upper limit is relatively new in the standard: a maximum bulk density was added to the class table in the 2021 edition of ISO 17225-2^[11]. The draft of this standard gives the same range of 600–750 kg/m³^[12]Data require verification: The range was read from the FDIS draft; it needs to be confirmed in the published ISO 17225-2:2021.. The upper limit makes practical sense: boilers and feed systems are designed for fuel with defined properties, and fuel that is markedly heavier than usual would upset dosing just as fuel that is too light would.

The principle of the test: a container of standardised volume is filled with pellets in a specified way, the surface is levelled and the container is weighed. The details — container size, filling and compaction method — are laid down in the standard.

How much space a tonne of pellets takes up

Once you know the bulk density, calculating the storage volume is easy. A tonne of pellets with a density of 600 kg/m³ takes up about 1.67 m³, and at 750 kg/m³ about 1.33 m³. At the lower end of the range, the same mass of fuel needs 25% more space than at the upper end.

Density and volumetric dosing

Most pellet boilers dose fuel by volume: the auger moves a similar volume of pellets with each revolution, and the controller regulates how long the feed runs. The boiler does not weigh the fuel — it “assumes” that a given volume contains a certain mass, and therefore a certain amount of energy.

The energy in a cubic metre of fuel can be calculated by multiplying the bulk density by the net calorific value. At the minimum calorific value of 16.5 MJ/kg, a cubic metre of pellets with a density of 600 kg/m³ contains 9,900 MJ (2,750 kWh), and one of 750 kg/m³ contains 12,375 MJ (about 3,440 kWh). That is a difference of around 25% for the same volume (editorial calculations).

In practice this means that changing supplier may require adjusting the boiler settings, even if both products meet the requirements. Lighter fuel gives less heat per dose — the boiler will run longer and, in a hard frost, may not reach full output. Heavier fuel, on the other hand, can overload the burner, which encourages incomplete combustion. Dosing is also affected by fines, which fill the voids between the pellets.

Bulk density versus particle density

Alongside bulk density there is particle density (The mass of a single pellet divided by its volume — without the voids between pellets, in g/cm³.)The mass of a single pellet divided by its volume — without the voids between pellets, in g/cm³. (the density of individual pellets), which is determined according to ISO 18847 and expressed in g/cm³^[27]. It describes how densely a single pellet is compressed. In ENplus it is an informative parameter: there is no limit, but the value must be stated^[1].

The two parameters are related but not the same. High particle density usually goes with high bulk density and good durability, but bulk density also depends on the shape and length of the pellets. The Polish regulation uses particle density (≥ 0.90 g/cm³) as a requirement for briquettes, and bulk density for pellets^[36].

A rough measurement at home

You can check bulk density approximately yourself. All you need is a bucket of known capacity (e.g. 10 l), bathroom or kitchen scales and a little patience:

1. Weigh the empty bucket.
2. Fill it with pellets, pouring them freely from a low height — without tamping or shaking.
3. Level the surface with the rim by scraping off the excess with a straight edge.
4. Weigh the full bucket and subtract the mass of the empty one.

If the bucket holds 10 l, multiply the mass of the pellets in kilograms by 100 to get the result in kg/m³. Example: 6.8 kg of pellets in a 10-litre bucket is about 680 kg/m³. It is worth repeating the measurement three times and averaging the result.