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Field guide
Reading a Vitamin D Label: IU, Micrograms and the 600 IU Reference
A vitamin D3 bottle can say "5,000 IU" on the front and "125 mcg" on the Supplement Facts panel, and both describe the same softgel. Labels print vitamin D in international units (IU), micrograms (mcg), or both, and the reference this site audits against is written in IU.
IU and micrograms for vitamin D3
For cholecalciferol (vitamin D3), the IU is defined against a fixed mass: 1 IU = 0.025 mcg. Turned around, 1 mcg = 40 IU, and 1 mg = 40,000 IU. Some common label figures:
- 10 mcg = 400 IU
- 15 mcg = 600 IU
- 25 mcg = 1,000 IU
- 125 mcg = 5,000 IU
- 250 mcg = 10,000 IU
The factor belongs to this one vitamer; it is not a general rule for turning micrograms into IU. Some panels print both figures, such as "125 mcg (5,000 IU)"; others print only micrograms. Check the serving size line too: if it says "2 Softgels", every figure beneath it is for two.
The reference amount: 600 IU a day
| Ingredient | Reference amount | Source |
|---|---|---|
| Vitamin D3 (Cholecalciferol) | 600 IU | Institute of Medicine, Dietary Reference Intakes for Calcium and Vitamin D (2011) |
The Institute of Medicine's 2011 Dietary Reference Intakes for Calcium and Vitamin D gives an RDA of 600 IU/day for ages 1-70 and 800 IU/day for 71 and older. The threshold uses the 600 IU figure, which is 15 mcg. It is a population reference intake, not a personalised one, and it assumes minimal sun exposure. Every threshold the site uses, with its citation, is on how we rate.
Panels that print only micrograms
Readers transcribe what the panel prints and do not convert units. When a vitamin D3 row carries micrograms and no IU, the site converts it with the factor the reference states for cholecalciferol, 40,000 IU per mg, so a "125 mcg" panel is audited as 5,000 IU.
The conversion has a ceiling: 100,000 IU per serving, twice 50,000 IU, the highest vitamin D strength a label states. A result above it is read as a unit mix-up, not a dose, and the row stays undosed. Without that bound, "5,000" typed into the milligram field would be read as 200,000,000 IU and audited as a pass.
One listing, several strengths
One store listing often sells the same softgel at several strengths, each at its own price. The BulkSupplements Vitamin D3 Softgels listing offers 1,000, 2,500, 5,000, 10,000 and 50,000 IU, and the panel we read prints 125 mcg. When variants state different strengths, only a variant whose strength the panel prints may price it: 125 mcg converts to 5,000 IU, so only the 5,000 IU variants count. If none matched, no price would be published, because the listing would not say which product was read.
D3 with K2
Many labels pair vitamin D3 with vitamin K2, usually as menaquinone-7 (MK-7). The reference has no entry for vitamin K2, so its row has no threshold and is not scored, but it still counts as a dosed row the audit could not check.
The panel for BulkSupplements Vitamin D3 and K2 Softgels lists vitamin D3 at 125 mcg (5,000 IU) and vitamin K2 (as MK7) at 100 mcg. One of its two dosed rows can be checked; half is enough, so it is scored on its vitamin D3 alone. A label that adds a dosed vitamin A row has one of three rows checkable, and below half no dose score or cost per dosed serving is published.
From label to cost per dosed serving
Vitamin D3 sits in the health support category, where the value score is built on cost per fully dosed serving: what it costs to reach the reference amount using that product.
- Price per serving: the store price divided by the servings in the package. Where more than one size may price the product, the lowest price per serving is used.
- Dose ratio: the serving's vitamin D3 in IU divided by 600, capped at 1.
- Cost per dosed serving: price per serving divided by the dose ratio.
A worked example
The two bottles below are hypothetical, with round numbers chosen to show the arithmetic. They are not real products.
Bottle A costs $12.00 for 120 servings. Each serving lists 10 mcg (400 IU).
- Price per serving: $12.00 ÷ 120 = $0.10
- Dose ratio: 400 ÷ 600 = 0.667
- Cost per dosed serving: $0.10 ÷ 0.667 = $0.15
Bottle B costs $14.40 for 120 servings. Each serving lists 125 mcg (5,000 IU).
- Price per serving: $14.40 ÷ 120 = $0.12
- Dose ratio: 5,000 ÷ 600 = 8.33, capped at 1
- Cost per dosed serving: $0.12 ÷ 1 = $0.12
Per serving, Bottle A looks cheaper. But a serving of A holds two thirds of the reference amount, so reaching 600 IU costs $0.15 with A and $0.12 with B.
Going past 600 IU earns nothing extra
Bottle B's serving holds 5,000 IU, more than eight times the reference amount, and its ratio is still 1. A 1,000 IU (25 mcg) serving at the same price would get the same cost per dosed serving: the number measures what it costs to reach the reference amount, not the size of the figure on the label. Where a label carries more than one recognised ingredient, such as vitamin D3 in a fish oil softgel, the dose ratio is the average across them, each capped at 1.
Cost per dosed serving is ranked against every other product in the category to give the value score. See the vitamin D3 health support rankings, or the whole shelf in the health support rankings.