What capsule sizes are standard and how is size chosen?

Beyond Biopharma · Pharmaceutical Gelatin FAQ

Hard gelatin capsules come in a family of standard sizes labelled 000, 00, 0, 1, 2, 3, 4 and 5, where 000 is the largest and 5 the smallest. With a typical tamped powder fill, the larger sizes hold on the order of 0.5-1.0 grams (size 00) down to a few tens of milligrams (size 5). The right size is chosen from the target dosage of the active ingredient, the bulk (tapped) density of the powder or the weight of pellets the fill needs to nest, and a suitable size that stays convenient to swallow, together with the machine capacity for capsules and any regulatory target for the dosage form.

The standard size numbering system

Capsule “sizes” are an older, conventional numbering scheme, not a metric measure. Crucially, lower numbers mean bigger capsules: size 000 is the largest and size 5 is the smallest. Empty hard gelatin capsules are sold in these fixed sizes, and each size has an official length and diameter (for example, size 00 is roughly 23 mm long and 8 mm in diameter, with the exact dimensions defined in each manufacturer’s specification and the pharmacopoeias). Because overall machine tooling, filling pins and sealing rings are made to these sizes, the numbering system is universal across gelatin and HPMC/vegetarian capsules.

Realistic fill capacity by size

What each shell can actually hold depends on the fill — its density and how firmly it is tamped. The table below gives typical fill-weight ranges for a moderately dense, tamped powder, but these are industry benchmarks, not guarantees:

Two fills of the same size never weigh the same if the powder densities differ, which is why fillers pick a size for their actual formulation rather than copying a neighbour’s choice.

What determines the choice of size

1. Target dose of active ingredient — the capsule must physically fit the whole drug load, and the empty volume must be enough for the active plus the excipients.
2. Bulk / tapped density of the fill — a dense powder fits a lot into a small shell; a light, fluffy powder that does not pack down needs a bigger shell for the same weight.
3. Fill weight precision — you fill by volume, so the machine’s tamping and dosing pins must sit comfortably in the capsule for good weight uniformity.
4. Swallowability and patient compliance — a smaller, easier-to-swallow shell is preferred where the dose allows, though patient groups and preferences differ.
5. Dosage-form intent — pellets, mini-tablets, powders or liquids affect how much volume is realistically usable inside the shell.

Why “volume” beats “capacity in mg”

Published capacities in milligrams are only meaningful with a stated fill density. A better mental model is that each shell offers a fixed internal volume (usable filled volume is a little less than the shell’s interior because the two halves lock together). If a powder has a tapped density of 1.0 g/mL, a shell with 0.7 mL usable volume will hold about 0.7 g of that powder; halve the density and the same shell holds about half as much. Formulators therefore calculate needed volume from the dose, density and weight, then pick the smallest standard size whose real fill capacity covers it.

Practical guidance

• Confirm the exact internal volume and fillability of each size with your capsule supplier, since real values depend on the specific shell geometry.
• Always match the shell size to your fill — run small trials and weigh real fills rather than trusting generic tables.
• Choose the smallest shell that comfortably holds the target fill and still runs reliably on your capsule-filling machine.
• Remember size choice interacts with shell wall thickness, which affects both strength and cost; your supplier’s specification is the authoritative reference.