Step one: cost per serving

Price ÷ servings per container = cost per serving.

Servings per container is on the Supplement Facts panel, directly under serving size. One regulatory quirk: a manufacturer may leave "Servings Per Container" off the panel when that information is already stated in the net quantity of contents declaration. If you cannot find it on the panel, check the front of the bottle where it says "60 Capsules," and divide by the serving size yourself.

That is the whole calculation, and doing it converts an incomparable pair of numbers into a comparable one. Do it on your phone in the aisle. It takes ten seconds and it regularly reverses the answer.

Step two: check the serving size, because it is where the difference hides

A serving is not a capsule.

The regulation requires serving size to be expressed in a term appropriate to the form: "tablets," "capsules," "packets," "teaspoonfuls." It does not require it to be one of anything. A serving of two capsules is extremely common. Servings of three, four and six exist.

This matters twice over:

It changes the cost. A 120-capsule bottle with a four-capsule serving is a 30-day supply, not a 120-day supply.

It changes what you are comparing. When two products both say "500 mg," but one serving is one capsule and the other is two, the per-capsule amounts are 500 mg and 250 mg. The panel is not misleading you (the amount shown is per serving, as required), but a fast comparison of the big numbers on the front will get this wrong.

Always locate the serving size before comparing anything to anything.

Step three: the elemental weight trap

This one is non-obvious, and it is where the most money gets wasted.

The regulation is explicit: quantitative amounts on a Supplement Facts panel "shall represent the weight of the dietary ingredient rather than the weight of the source of the dietary ingredient." The example the regulation itself uses is "the weight of calcium rather than that of calcium carbonate."

That is exactly right and exactly helpful when it is on the panel. The confusion comes from the front of the bottle and from the ingredient line, where compound weights often appear.

A mineral is almost always delivered as a compound (a salt, an oxide, a chelate), and the compound weighs considerably more than the mineral inside it. So "1,000 mg magnesium citrate" and "1,000 mg magnesium" are very different statements, and only one of them tells you how much magnesium you are getting.

As a rule of thumb, compare the number on the Supplement Facts panel, not the number on the front of the bottle. The panel gives you the element. The front of the bottle may be giving you the compound.

If a panel shows "Magnesium (as magnesium glycinate) 200 mg," the 200 mg is magnesium. That is the number to divide into your cost.

A related version of the same trap appears with anything measured as a percentage or a ratio. A product delivering "500 mg of a 10:1 extract" and a product delivering "500 mg of whole powdered herb" are not the same product, and neither is "500 mg of extract standardized to 5% marker compound."

Step four: extracts, whole herb, and what the numbers mean

For botanicals, cost per serving is only meaningful once you know what a serving contains.

Whole powdered herb is the dried plant, ground. A 500 mg capsule contains 500 mg of plant.

An extract is the plant processed with a solvent to concentrate certain constituents. The regulation handles these specifically: for an extract where the solvent has been removed, the declared weight is the weight of the dried extract; for a liquid extract where the solvent has not been removed, the declared quantity is the volume or weight of the total extract. Those are different measurements, and a large number on a liquid extract is not comparable to a smaller number on a dried one.

An extract ratio like 10:1 describes how much starting material went into the finished extract, by weight. It is a processing description, not a potency measurement.

Standardization describes the amount of a chosen marker compound. The NIH Office of Dietary Supplements is blunt about the limits of this: there is no legal or regulatory definition of standardization in the United States, US law does not require it, and for most botanicals the constituents responsible for any effect are not known, so the marker being measured is not necessarily the thing that matters.

Standardization does deliver batch-to-batch consistency, which is a real benefit and a good reason to prefer it. It is not a potency guarantee, though, and it cuts against over-precision in a cost comparison.

For your arithmetic, this means you can compare cost per milligram within a category: extract against comparable extract, whole herb against whole herb, standardized preparation against the same standardization. Comparing across those categories on milligrams alone produces a number that looks precise and means very little.

Step five: cost per day, not cost per serving

The number you live with is cost per day.

Cost per serving × servings per day = cost per day.

If a label says to take a serving twice daily, a 60-serving bottle is a 30-day supply. Multiply through and compare monthly costs. This is the figure to weigh against everything else you spend money on.

A worked comparison

Two magnesium products.

Product A: $14.99, 100 capsules, serving size 2 capsules, panel shows Magnesium 200 mg per serving. That gives 50 servings per container and $0.30 per serving. At 200 mg magnesium per serving, that is $1.50 per gram of magnesium.

Product B: $27.99, 120 capsules, serving size 1 capsule, panel shows Magnesium 150 mg per capsule. That gives 120 servings per container and $0.23 per serving. At 150 mg magnesium per serving, that is $1.55 per gram of magnesium.

The two are nearly identical on cost per unit of magnesium, but Product B is meaningfully cheaper per serving and lasts about two and a half times as long per bottle, while Product A delivers more magnesium in each dose. Which is better depends on how much magnesium you want per day, and now you can answer that question instead of guessing at it.

The shelf price told you that A costs half of B. That was true and useless.

Bigger bottles, subscriptions, and the shelf-life trade-off

Cost per serving almost always falls as bottle size rises, which makes the large size look like the obvious move. Two things complicate that.

Will you finish it before it degrades? A 360-count bottle at a one-per-day dose is a year's supply. For a stable vitamin in a cool dark cupboard, that is fine. For fish oil, for a probiotic, or for anything where oxidation or viability is the issue, a year in an opened bottle is a real quality question and the saving may be false. Buy the large size of the stable things and the small size of the fragile ones.

Will you still be taking it in six months? The most expensive supplement is the one you stop taking after three weeks. If you are trying something new, the small bottle at a worse per-serving price is often the better purchase, and you can size up once you know.

Subscriptions usually improve cost per serving, sometimes substantially. The arithmetic is the same (convert to cost per day and compare), with one addition: check what happens if you pause. A subscription that is hard to stop is not a discount. It is a commitment, and the real price includes the bottles you did not want.

Where per-serving comparison breaks down

Multivitamins cannot be compared on cost per serving in any meaningful way, because you are buying twenty-odd ingredients at once and the products differ on every one of them. One has more vitamin D and no iron; another has a different form of folate; a third includes botanicals the first two do not. There is no single denominator.

The only sensible approach is to decide which two or three ingredients you care about, find those on the panel, and compare on those, treating the rest as a bonus of uncertain value. If you cannot name what you want from a multivitamin, cost per serving will not rescue the decision.

The same problem, in smaller form, affects any multi-ingredient formula. And it is one more practical reason that proprietary blends are expensive in a sense that has nothing to do with price: they make the arithmetic impossible, so the only remaining basis for choosing is how the label makes you feel.

What the arithmetic cannot tell you

This is the part that keeps cost-per-serving honest, because a purely numerical comparison can steer you wrong.

It cannot tell you whether the amount is sensible. The cheapest cost per milligram in the store might be on a product delivering an amount far below anything used in research. Cheap per unit of something you do not need is not value.

It cannot tell you whether the ingredient is what it says. That depends on the manufacturer's identity testing, on third-party verification, and on whether you can get a certificate of analysis. A company that does all of this costs more to run than one that does not, and some of the price difference between products is that work.

It cannot compare forms of an ingredient. Different mineral salts, different botanical preparations and different delivery forms are not interchangeable on weight, and the evidence for absorption differences is uneven: sometimes well established, more often modest and marketed harder than it deserves. Treat strong claims about a particular form's superiority with the same skepticism as any other benefit claim.

It cannot account for proprietary blends. If you cannot see how much of each ingredient is present, you cannot calculate cost per unit of anything. This is a concrete reason to prefer fully disclosed panels: they are the only ones you can do arithmetic on.

It cannot value the things that do not appear on the panel: testing, sourcing, a company that answers the phone. Those are real costs and they buy something real.

So the right use of this arithmetic is not "buy the lowest number." It is to stop paying a premium you did not know you were paying, and to make two products properly comparable before you weigh everything else.

The checklist

Before you buy, on the back of the bottle:

1. Servings per container: from the panel, or divide the count by the serving size
2. Price ÷ servings = cost per serving
3. Serving size: how many capsules, tablets or scoops
4. The amount on the panel, not on the front (for minerals, remember the panel gives you the element)
5. Servings per day → cost per day, then cost per month
6. What form the ingredient is in, so you are comparing like with like
7. Whether you can see the amounts at all: a blend defeats the whole exercise