Price per dose: comparing a daily supplement with one taken every 3 days

Sticker price is the least useful number on a supplement box. Two products at the same price can cost very different amounts to actually use, because one serving might be one tablet or three, and the directions might say daily or every few days. This post works through the arithmetic for comparing a product taken daily with one taken every three days, with the numbers taken straight from the labels.

The three numbers you need

Everything comes from the Supplement Facts panel and the directions line, both of which are regulated and printed on every box:

1. Serving size. How many tablets or capsules count as one dose. All amounts on the panel are per serving (21 CFR 101.36(b)(1) and (b)(2)).
2. Servings per container. How many doses are in the package. This is the number to divide the price by, not the tablet count.
3. Directions. How often a serving is taken. This is what turns a price per serving into a price per day.

Worked example: a daily product

Take an invented daily product: $29.99 for a bottle of 60 capsules, serving size two capsules, directions "take one serving daily." The bottle holds 30 servings, so the price per serving is $29.99 ÷ 30, or $1.00. Taken daily, that is also $1.00 per day, and the bottle lasts 30 days. Notice the trap: if you divided by the capsule count instead, you'd get $0.50 and think the bottle lasts twice as long as it does.

Worked example: Rocket Fuel, taken every three days

Rocket Fuel's serving is one tablet, and the directions are one tablet every three days. Packs contain two tablets each, so a 1 Pack covers six days and a 12 Pack covers 72. Here is the math for each pack at the one-time prices on the product page as of this writing.

Per-tablet figures are rounded to the cent. Subscription pricing is different and is shown on the product page; the per-day math works the same way with whichever price applies to you.

Putting the two side by side

Compared per serving, the invented daily product ($1.00) looks cheaper than any Rocket Fuel pack ($4.58 to $7.48 per tablet). Compared per day of use, which is the number that reflects what you'd actually spend over a month, the gap narrows: $1.00 per day against $1.53 to $2.49 per day. Over 30 days that is $30.00 for the daily product and, for Rocket Fuel, $45.81 on 12 Pack pricing or $74.75 on 1 Pack pricing, since 30 days is ten tablets.

Which column "wins" depends on what you're comparing. The daily product costs less per month in this example. Rocket Fuel costs less per dose than its sticker price suggests once the three-day direction is counted. Neither number says anything about what either product does; the table is only an exercise in reading the label correctly.

Where this math breaks

Three label features change the result, and all three are printed on the box. First, a serving size larger than one unit, which is the capsule-count trap above. Second, a directions line that gives a range ("take 1 to 2 daily"), where the honest approach is to calculate both ends. Third, a proprietary blend, which lets you calculate price per serving but not price per milligram of any one ingredient inside it. If a label leaves one of these numbers out, the calculation stops there rather than filling the gap with a guess.

How to check any product in a minute

Read servings per container from the panel. Divide the price by it. Read the directions and divide again by the number of days between servings. Write the per-day figure on the box if it helps. Then do the same for the next box. Two products compared on price per day of use, with the same month in view, is a fair comparison; two products compared on bottle price is not.

Our own label Rocket Fuel From $14.95 See the label