What Is Inside a Hard Capsule? 5 Common Capsule Contents and How They Are Filled

When you open a hard capsule, you may expect to see powder. Powder is common, but it is far from the only material that can be placed inside a capsule. Depending on the formulation and production method, capsule contents can include powders, granules, pellets, mini-tablets, and suitable liquids or semi-solids.

These fill forms are not simply different in appearance. A fine powder behaves differently from free-flowing pellets, while mini-tablets need to be handled as individual units. Liquid-filled hard capsules require another dosing method and usually additional sealing considerations.

This means the material inside a capsule directly affects how the product is manufactured and what type of capsule filling configuration is required. A capsule filling machine designed for powder should not automatically be assumed to handle pellets, mini-tablets, or liquids with the same dosing mechanism.

The following sections explain what each fill type is, why manufacturers use it, and how the material changes the capsule filling process.

1. What Can Be Put Inside a Hard Capsule?

A hard capsule has two main parts: a body, which receives the fill material, and a cap, which closes over the body after filling. The shell and the material inside it are separate parts of the finished capsule.

The capsule shell can be made from materials such as gelatin or HPMC. The capsule contents, on the other hand, are the formulation placed inside that shell. This distinction matters because discussions about what capsules are “made of” often mix the shell material with the fill material.

Hard capsules can contain powders, granules, pellets, and mini-tablets. Suitable non-aqueous liquids and semi-solid formulations can also be filled into hard capsules when the shell, formulation, capsule filling process, and sealing method are compatible. FDA dosage-form terminology also distinguishes powder-filled, pellet-filled, and liquid-filled capsules.

A capsule can also contain more than one type of material. Some formulations use combination fills, such as pellets with powder or mini-tablets with pellets. This can help keep components physically separate when they have different formulation or release requirements.

The choice of capsule contents therefore starts with the formulation rather than with the capsule filling machine. Manufacturers first determine what goes inside the capsule, how much must be filled, and how the material behaves during production. The appropriate dosing and capsule filling machine configuration can then be selected.

2. 5 Common Types of Capsule Contents

Powder

Powder is one of the most common materials found inside hard capsules. In many products, the capsule powder is a blend of the main ingredient and excipients selected to support processing, stability, or product performance.

Powders do not all behave the same way. Particle size, particle-size distribution, bulk density, moisture condition, cohesiveness, and flowability can affect how powder moves through the capsule filling machine and how consistently it is dosed.

On an automatic capsule filling machine using a dosing-disc system, powder enters the dosing holes and is compacted by filling rods. The dosing-disc thickness remains fixed during operation, while filling-rod depth is one of the adjustments used to control the amount being dosed.

Machine settings are only part of the process. Powder properties, material supply, dosing conditions, mechanical repeatability, and production checks also contribute to consistent filling.

Granules

Granules are larger agglomerated particles formed from smaller powder particles. They are usually more irregular in shape than pellets and can vary in size depending on how they are produced.

Manufacturers may use granulation to improve handling and flow in suitable formulations or reduce segregation between ingredients. Inside a hard capsule, granules often look like coarse grains rather than fine powder.

Although granules and powders are both solid capsule fill materials, they do not feed in the same way. Granule size, bulk density, friability, and flow behavior need to match the feeding and dosing configuration of the capsule filling machine.

This is why a capsule filling machine supplier needs to know that the planned capsule contents are granules rather than only being given the capsule size and target output.

Pellets

Capsule pellets are small particulate units, often relatively uniform in size and more regular in shape than conventional granules. A pellet-filled capsule may contain many individual pellets.

Pellets are useful when manufacturers want to divide a formulation into multiple small units. Different pellet populations can also be manufactured or coated differently and then combined inside the same capsule. This makes pellets useful for multiparticulate formulations and products where components need to remain physically separated.

Pellets also require a different filling approach. Instead of compacting a powder plug, the dosing system meters free-moving particulate units while limiting damage and maintaining a repeatable quantity.

A standard powder configuration on a capsule filling machine should therefore not automatically be treated as a pellet-filling configuration. Dedicated or adapted dosing components are selected according to the product and capsule filling machine design.

Powder, granules, and pellets have different particle forms

Mini-Tablets

Mini-tablets are small compressed tablets designed to fit inside a capsule. They show why the answer to “what is inside a capsule?” is not always powder or loose particles.

A hard capsule can contain one mini-tablet, several mini-tablets, or mini-tablets combined with other capsule contents such as pellets. Separate compressed units can help keep different components physically separated inside one capsule.

From the capsule filling machine perspective, mini-tablets are very different from powder. They are discrete pieces, so the capsule filling machine needs a suitable mechanical metering system that can deliver the required quantity into each capsule body.

The number and dimensions of the mini-tablets therefore matter when specifying a capsule filling machine project.

Liquids and Semi-Solids

Hard capsules can also contain suitable liquids and semi-solid formulations. Although liquid-filled products are often associated with softgels, liquid-filled hard capsules are also an established dosage-form option.

The formulation must be compatible with the capsule shell. Viscosity, composition, filling temperature, interaction with the shell, and leakage risk can all affect production.

A liquid capsule filling machine typically uses a controlled pump dosing system to deliver the required volume into the capsule body. After filling, the cap and body are closed, and liquid-filled hard capsules commonly require an appropriate sealing or banding process to reduce leakage risk.

3. Why Are Different Capsule Contents Used?

There is no single best capsule fill form for every product. Manufacturers select powder, granules, pellets, mini-tablets, or liquids according to the formulation and production process.

Powder can be suitable when ingredients can be blended and dosed consistently as a dry mixture. Granules can provide useful handling characteristics. Pellets and mini-tablets create separate units inside the capsule, while suitable liquid or semi-solid systems provide another option for formulations better handled in that physical form.

Different fill forms can also support more complex product designs. Separate pellet populations can remain distinct inside the same capsule, while mini-tablets can be combined with pellets or other suitable fills.

These are not fixed one-to-one rules. The final choice depends on formulation development, product performance, manufacturability, capsule compatibility, and the available capsule filling process.

For capsule filling machine buyers, the key point is: capsule contents are decided by the product first, and the capsule filling machine configuration follows from that decision.

4. How Are Different Capsule Contents Filled?

The basic job of a capsule filling machine is to separate the empty capsule body and cap, place the required contents into the body, close the capsule, and discharge the finished product. The dosing method changes with the material.

Powder Filling

Powder can be handled by a dosing-disc system. Powder enters the dosing area, fills the dosing holes, is compacted by filling rods, and is transferred into the capsule bodies.

For dosing-disc capsule filling machines, filling-rod depth is an important adjustment used to control fill amount, while the dosing-disc thickness remains fixed during operation.

Granule Filling

Granules are generally filled using volumetric, non-compression dosing rather than being compressed into a powder plug.

Free-flowing granules can enter a defined dosing chamber or slide-gate metering space mainly under gravity. Once the required volume has been measured, the granules are transferred into the capsule body.

Granule size, size distribution, bulk density, friability, and flow behavior influence how consistently the dosing volume fills. The exact dosing mechanism depends on the capsule filling machine design and the characteristics of the granules.

Pellet Filling

Pellets are also commonly filled using volumetric, non-compression dosing. Depending on the capsule filling machine design, free-flowing pellets and granules can use similar slide-gate, dosing-chamber, or other volumetric metering principles.

The main difference is that pellets, especially coated pellets, often require gentler handling to protect their shape and surface coating. Pellet size, flow behavior, coating characteristics, target quantity, and production speed should therefore be provided when specifying the capsule filling machine.

Mini-Tablet Filling

Mini-tablets are handled as individual solid units. A capsule filling machine can use a slat system or metering-drum system, which performs counting-by-volume. Cavities or metering spaces are designed to receive a defined number of mini-tablets and then transfer them into the capsule bodies.

Liquid Filling

A liquid capsule filling machine uses a pump-based dosing arrangement rather than a powder dosing mechanism. The system meters the formulation into the open capsule body, after which the capsule is closed and, when required, sealed or banded.

Temperature and viscosity can be important for certain semi-solid formulations because the material must remain suitable for pumping and dosing.

Different capsule contents require different dosing methods

5. What Should You Consider When Choosing a Capsule Filling Machine?

If you are purchasing a capsule filling machine, specifying only the capsule size and required output is not enough. The supplier first needs to understand exactly what will be placed inside the capsule.

1. Fill Material

State whether the capsule contains powder, granules, pellets, mini-tablets, liquid, semi-solid material, or a combination.

For powder, bulk density and flow behavior are useful. For pellets and granules, particle size and size distribution become more important. For liquids, viscosity and filling temperature can affect the dosing approach.

2. Capsule Size

Capsule size affects available internal volume and tooling. Common projects use capsule sizes from #000 to #5, but the selected size must also provide enough space for the required capsule contents.

3. Target Fill Amount

For powder, granules, or pellets, the supplier needs the required fill weight or volume. For mini-tablets, the required number of units and corresponding slat or metering-drum configuration need to be defined.

4. Production Capacity

The capsule filling machine must match planned output. Capacity should be considered together with the material and dosing method rather than as an isolated specification.

5. Single or Combination Filling

If the capsule contains more than one component, explain this early. Powder plus pellets, several mini-tablets, or multiple pellet populations can require additional dosing stations or a different capsule filling machine configuration.

6. Liquid Filling and Sealing

For liquid-filled hard capsules, the supplier should know the formulation type, viscosity range, capsule-shell compatibility, operating temperature, dosing requirements, and planned sealing process.

A conventional powder capsule filling machine should not be purchased on the assumption that it can later become a liquid capsule filling machine simply by changing ordinary powder tooling.

7. Product and Size Changeovers

Manufacturers producing several products should consider how often they will change capsule sizes, fill materials, or dosing requirements. Frequent changeovers affect tooling, cleaning, adjustment time, and production scheduling.

As a pharmaceutical and packaging equipment supplier, Rich Packing works with manufacturers to match capsule contents, capsule size, target fill amount, production output, and special dosing requirements with the appropriate capsule filling machine configuration. When needed, the capsule filling process can also be considered together with downstream packaging requirements to build a more practical production setup.

Conclusion

A hard capsule can contain much more than powder. Capsule contents commonly include powders, granules, pellets, mini-tablets, and suitable liquids or semi-solids.

Each fill form has different physical characteristics and manufacturing requirements. Those differences determine how the material must be measured, fed, and transferred into the capsule.

For manufacturers and capsule filling machine buyers, the sequence is:

define the capsule contents → understand the material properties → determine the dosing method → select the appropriate capsule filling machine configuration.

Starting with what goes inside the capsule makes it easier to select a capsule filling machine that matches the actual production process rather than choosing one based only on capsule size or rated speed.

FAQ

References

1. U.S. Food and Drug Administration. Capsule Dosage Form Questions.
2. U.S. Food and Drug Administration. Dosage Forms.
3. U.S. Food and Drug Administration. Dosage Form Drug Manufacturers cGMPs (10/93) - Inspection Guide.
4. Jovičić-Bata J, et al. Liquid- and Semisolid-Filled Hard Gelatin Capsules Containing Alpha-Lipoic Acid. Pharmaceutics, 2024.
5. Cole ET, Cadé D, Benameur H. Challenges and Opportunities in the Encapsulation of Liquid and Semi-Solid Formulations into Capsules for Oral Administration.