Start from the fill weight.
Every capsule size has a volume, and every powder has a bulk density. Multiply the two and you have the fill weight that size can carry. A size 0 capsule holds about 0.68 ml; a dense mineral powder at 0.9 g/ml fills it with around 600 mg, while a light botanical extract at 0.4 g/ml fills it with less than 300 mg.
That is why the question cannot be answered from the dose alone. A 500 mg dose of a dense material fits comfortably in a size 0. The same dose of a fluffy, low density powder needs a size 00, or a granulation step to make it denser, or a second capsule.
Measure the tapped density of the actual blend, not of the actives on their own. Excipients change it, and a blend that flows well on the filling machine is usually denser than its ingredients suggest.
The sizes people actually swallow.
Size 00 is the largest most adults are comfortable with, and it is already large. Size 000 exists and is used, but it is a real barrier for a daily product and for anyone who struggles with tablets. Sizes 0 and 1 are the comfortable middle, and sizes 2 to 4 suit children, pets and low dose actives.
Serving count is the other half of the decision. One large capsule a day and two small ones are the same dose, and many buyers prefer the two: they are easier to take and the pack looks fuller. Two capsules also open the door to a smaller, cheaper shell.
Market matters too. Some regions have a strong preference for smaller capsules, and a product built for one shelf does not always travel to another without a size change.
Check it on the line before you print.
Order sample shells in the two sizes you are deciding between and run the real blend through them, on the filling machine that will make the product. Fill weight, weight variation and how the powder flows into the body of the capsule are all easier to see than to calculate.
Keep the shell size off the label until that run is done. A pack designed around a size 0 and a product that turns out to need a size 00 means a new bottle, a new label and a new set of documents.
If the fill weight lands between two sizes, ask about the blend before you ask about a bigger capsule. A flow agent, a granulation or a different grade of one excipient can bring a formula down a size, and that is often the cheaper change.
