Capsule Size Selection in OEM: Matching Shell Size to Bulk Density
When clients approach capsule contract manufacturing, they usually bring a specification reading 300 milligrams of a given extract per capsule and ask which shell size will work. The question looks like a lookup exercise, yet the conflict often only surfaces at sampling: the powder fills half the shell, or leaves an obvious void at the declared capacity. The root cause is that shell capacity is a volume, while the specification is a mass, and the bridge between them is bulk density. This article walks through that conversion chain and explains which end to adjust when a formula will not fit.
1. The basic parameters of shell sizes
Hard capsule shells run from 000 down through 00, 0, 1, 2, 3 and 4. Both gelatin and HPMC shells follow the same numbering, although wall thickness and capacity differ slightly, with HPMC shells typically holding a little less at the same number. Taking size 0 and size 1 as examples, nominal capacity for size 0 is roughly 0.68 millilitres and for size 1 about 0.50 millilitres, but the usable fill volume is smaller because the body and cap overlap when closed. Moulds also differ between suppliers, so at the selection stage it is far better to use the usable fill volume measured on the actual filling line than a generic capacity table; otherwise sampling produces the familiar outcome of a shell that calculates as sufficient but physically will not accept the dose.
2. Bulk density: converting mass into volume
Bulk density is the mass per unit volume of a powder in its natural piled state, normally expressed in grams per millilitre. It is not a single fixed number: the same powder measured loose and after tapping can differ by more than thirty percent, which is why the industry separates tapped and loose bulk density. The ratio of the two, the Hausner ratio, is a widely used indicator of flow and compressibility. Measurement is simple enough. Fill a graduated cylinder or use a dedicated bulk density tester, weigh to obtain loose density, then tap a defined number of times and weigh again for tapped density. For capsule filling it is tapped density that governs, because powder in the hopper and dosing chamber sits in a tapped state. Skipping this measurement reduces the subsequent fill estimate to guesswork.
3. The correct order for estimating fill weight
First, obtain the usable fill volume. Second, measure the tapped density of the blend. Third, multiply volume by density to reach a theoretical maximum fill weight, then apply a safety factor, because no filling process packs every corner. That factor usually sits between 0.80 and 0.90. An example makes it concrete. With a usable fill volume of 0.68 millilitres and a tapped density of 0.60 grams per millilitre, the theoretical fill is about 0.41 grams, and after a 0.85 factor the dependable routine fill is around 0.35 grams. If the formula calls for 0.30 grams per capsule, the size is appropriate; if it calls for 0.45 grams, the shell must move up to 00 or the formula must change. Many projects rework because this estimate was skipped and a nominal capacity was ordered straight from a table, only for the filling line to fail to run it.
4. Which end to change when it will not fit
This is where formulation design meets commercial negotiation, and there are usually four routes. Move up a shell size, which is the most direct answer but changes swallowability and appearance; larger capsules are less friendly to older consumers and paediatric markets, and blister and bottle formats must be resized to match. Increase powder bulk density through granulation, particle size adjustment or a denser raw material form, which is often the best route because dose and appearance stay unchanged, though disintegration and release must be re-verified afterwards. Reduce fill weight and increase capsules per daily dose, which alters usage habits and packaging cost and normally needs the client to reconfirm the consumption scenario. Or fill part of the dose as a liquid or semi-solid, which suits oils and extracts but demands more from equipment and process control.
5. Knock-on effects of bulk density
Bulk density influences the whole line, not just the fill weight. Powders with poor flow produce fill weight variation that must be countered with glidants and moisture control, since a small rise in moisture degrades flow sharply, particularly in humid regions. Powders with poor compressibility behave erratically on dosing disc or dosator machines, where granulated powder or a machine with agitation will hold dose weight more steadily. Light, dusty powders lose material during transfer and add load to workshop dust extraction. For hygroscopic extracts, density itself changes with time, so a measurement taken immediately after milling can differ from one taken two hours later; standardising conditions such as resting time and humidity is therefore essential.
6. A checklist for the sampling stage
Five items are worth locking down during sampling. The target fill weight and its tolerance range. The measured usable fill volume of the selected shell size. Loose and tapped density of the blend under standardised conditions. The measured dose stability of the chosen filling method, normally twenty consecutive capsules weighed with the relative standard deviation calculated. And confirmation that disintegration and release remain acceptable under accelerated conditions. Written into the sampling approval sheet, these five prevent months of re-adjustment later. Shell size selection is not a one-off calculation but a balance between formula, equipment and user experience, and the earlier the data is on the table, the lower the communication cost.
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About Guangzhou Mesen Health Biotechnology Co., Ltd
Guangzhou Mesen Health Biotechnology Co., Ltd is located in the Huashan Science and Technology Park, Huadu District, Guangzhou. The company has focused on OEM and ODM services for the big health industry for nearly 20 years, operating dedicated research, production, sales and management centres, and maintaining long-term strategic partnerships with well-known Chinese medicine colleges and research institutions. Capabilities span pressed candy, gummies, capsules, solid beverages, herbal teas, plant beverages, protein powders, dietary supplements and enzyme jelly - from formulation development through contract manufacturing. For details on minimum order quantities, sampling lead times and quotations, see our manufacturing advantages and product range, or contact us for a tailored proposal.
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