No, and it is not close. The largest capsule the FDA thinks people should be asked to swallow is a size 00, which holds about 0.95 mL of powder. A single day's potassium, weighed out as the salt it comes in, works out at roughly nine grams. That is ten capsules for one mineral.
The one-a-day capsule is a packaging decision that the formula then has to survive.
The short version
- FDA guidance says capsules should not exceed a standard size 00.
- A size 00 holds 0.95 mL, roughly 600 to 930 mg of powder.
- A day's potassium as potassium chloride weighs about nine grams.
- Microgram nutrients fit in a capsule. Gram-scale minerals do not.
- Every one-a-day capsule is a partial multivitamin by physics.
What a capsule can actually hold
Capsules are sold by volume, not by dose. The UNC Eshelman School of Pharmacy compounding lab publishes the standard sizes and what they hold:
| Capsule size | Volume | Lactose | Aspirin |
|---|---|---|---|
| 000 | 1.37 mL | 1,340 mg | 1,000 mg |
| 00 | 0.95 mL | 929 mg | 600 mg |
| 0 | 0.68 mL | 665 mg | 500 mg |
| 1 | 0.50 mL | 489 mg | 300 mg |
Two powders, same shell, and the fill weight moves by 50 percent. Density decides. That is why nobody can tell you what a capsule holds without knowing what is going in it.
The ceiling is set higher up. In its guidance on the size and shape of generic tablets and capsules, the FDA writes: "We recommend that the largest dimension of a tablet should not exceed 22 mm and that capsules should not exceed a standard 00 size." That guidance governs generic medicines rather than supplements, so no supplement brand is bound by it. It is still the clearest official read on what a person can reasonably be asked to swallow, and the reasoning behind it is blunt. The same document reports that difficulty swallowing pills may affect as many as 40 percent of Americans.
So the working budget for a one-a-day is somewhere under a gram.
Why the minerals break the maths
Minerals do not exist on their own. Calcium arrives bonded to something, and you have to carry the weight of whatever it is bonded to. The percentage that is actual mineral is set by molecular weight, which is public and not negotiable.
Run the FDA Daily Values through the chemistry:
| Nutrient | Daily Value | Common form | Weight needed | Size 00 capsules |
|---|---|---|---|---|
| Potassium | 4,700 mg | Potassium chloride, 52% potassium | 9,000 mg | About 10 |
| Calcium | 1,300 mg | Calcium carbonate, 40% calcium | 3,250 mg | About 3.5 |
| Magnesium | 420 mg | Magnesium chloride, 25% magnesium | 1,650 mg | About 2 |
Three minerals. Fifteen capsules. And that is before a single vitamin, amino acid or gram of vitamin C goes in.
Nine grams for the potassium alone.
The magnesium trap
Magnesium is where the squeeze shows up most clearly, because there is a form that fits.
Magnesium oxide is 60 percent magnesium by weight, from a molecular weight of 40.3 against magnesium's 24.3. A full 420 mg Daily Value needs 697 mg of the stuff, which slides into a size 00 with room to spare. Problem solved, on paper.
Then you read the absorption data. Firoz and Graber measured four commercial magnesium preparations in normal volunteers by tracking the rise in urinary magnesium, published in Magnesium Research in 2001. Magnesium oxide came in at a fractional absorption of 4 percent. Magnesium chloride, lactate and aspartate all did better, and matched each other.
Four percent.
The forms that did better are heavier per unit of magnesium. Magnesium chloride has a molecular weight of 95.2, so it works out at 25 percent magnesium and a full dose runs to about 1,650 mg. Two capsules for one mineral, at the cheap end of the forms that performed. The authors were careful about the wider lesson, concluding that inorganic magnesium salts can match organic ones depending on the preparation, so the useful split runs by preparation rather than by category. In English: the form that fits a capsule best is the one that came last in the test. If you have ever compared magnesium forms on a label, you already knew which one the capsule brands pick.
What does fit in a capsule
This is where the honest version of the argument lives, because plenty of nutrients fit a capsule perfectly well. Anything dosed in micrograms or single-digit milligrams costs almost nothing in volume. Taking the Daily Values again:
- Vitamin B12 at 2.4 mcg
- Vitamin D at 20 mcg
- Iodine at 150 mcg
- Selenium at 55 mcg
- Folate at 400 mcg
- Zinc at 11 mg
All six together weigh under 12 mg. You could fit a year of vitamin D into a capsule and still have space.
So a one-a-day capsule can be genuinely complete on the small nutrients. On the heavy ones it carries a token amount and lets the percentage column carry the impression. A label reading 4% DV potassium is telling you exactly what happened, in the smallest print on the panel. Most people read the ingredient list and skip the numbers, which is roughly the opposite of how to read a supplement label.
The trade nobody advertises
Every daily formula picks one of three ways out of this. Shrink the doses until they fit one capsule. Split the formula across four or six capsules and hope people take all of them. Or drop the capsule and use a powder, which costs you the convenience and, if you dose it properly, the taste.
Fireblood took the third one. The scoop is 10.12 g. Nearly 7.9 g of that, about 78 percent, is the twelve amino acids plus 500 mg of MSM and 500 mg of vitamin C. Those alone would be eight or nine size 00 capsules. Nobody is swallowing nine capsules before breakfast, so in a capsule format those ingredients get cut to a dusting or cut entirely.
And the same arithmetic still bites us. Fireblood carries 100 mg of potassium and 50 mg of calcium against Daily Values of 4,700 and 1,300. A ten gram scoop cannot hold a day's potassium either. Nothing you swallow once can. That is what food is for.
Ask the bottle a physical question
Next time a one-a-day claims to be complete, weigh the claim instead of reading it. Pick the heaviest nutrient on the panel, find the form it is in, work out what that form weighs at a full dose, and compare it to the roughly 900 mg the capsule can hold.
The answer is on the panel already. It is just written in a column most people scroll past.
Fireblood is a 10.12 g daily powder with 39 ingredients, every dose printed on the label, in the forms we would take ourselves. It fills the gaps a normal week leaves behind. Bad sleep and bad food are still your problem. The full panel is on the product page if you want to check the maths yourself.
