Getting capsule size wrong is expensive. Too small and your formulation will not fit. Too large and you are paying for empty space, and your customers are swallowing a bigger capsule than they need to. The good news is that the calculation is simple once you know the one number most people forget to measure.
It is not about weight. It is about volume.
Capsules are specified by the volume they hold, not the weight. A size 0 capsule holds 0.68 mL of anything. Whether that 0.68 mL weighs 400 mg or 800 mg depends entirely on your powder.
That property is bulk density: how much a given volume of your blend weighs, measured in grams per millilitre. A light, fluffy herbal extract might sit around 0.4 g/mL. A dense mineral salt can exceed 1.2 g/mL. Same capsule, three times the dose.
The formula
Fill weight (mg) = capsule volume (mL) × bulk density (g/mL) × 1000
Capsule size calculator
Enter your dose and bulk density. The chart shows what every size holds with your blend, and which one fits.
Typical blends
Recommended size
0
Indicative only. Actual fill depends on tapped density, flow and your filling equipment. Always confirm with a trial run.
Measure your density, do not guess it
This is the step that gets skipped, and it is the reason most size selections go wrong. Weigh a known volume of your final blend, after all excipients are added, not the active ingredient alone. A 100 mL measuring cylinder and a balance will do it in two minutes.
Measure tapped density rather than loose. Powder settles during filling, so loose density will make you choose a capsule larger than you need. Tap the cylinder on the bench around 100 times, or use a tapped density tester if you have one, then read the volume again.
Three other things that decide the size
Who swallows it
Sizes 00 and 0 are the most widely used because they balance capacity against comfort. For paediatric or veterinary products, size 3 and smaller are usually preferred. A technically correct size 000 that nobody wants to swallow is not a good choice.
Leave headroom
Filling to the absolute limit causes problems on an automatic line. Aim to use around 90 percent of the capsule volume so the blend settles without resistance and the cap seats cleanly.
Two smaller beats one huge
If your dose only fits a size 000, consider splitting it across two size 1 capsules instead. Compliance improves, and you gain flexibility on dosing later.
A worked example
You need 600 mg of a granulated blend with a tapped density of 0.85 g/mL. Your required volume is 600 divided by 850, which is 0.71 mL. A size 0 holds 0.68 mL, so it is marginally too small. Size 00 holds 0.90 mL, leaving comfortable headroom. Size 00 is your capsule.
Now change one thing. If that same 600 mg had a density of 1.10 g/mL, you would need only 0.55 mL, and a size 1 would do. Same dose, two sizes smaller, purely because of density.
Not sure which size fits your blend?
Send us your dose and bulk density and we will recommend a size, then supply samples so you can confirm it on your own filling line.
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