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Vitamin IU Converter

Converts international units to mass and back for vitamin D (1 µg = 40 IU), vitamin A as retinol (1 IU = 0.3 µg) and vitamin E as natural d-alpha-tocopherol (1 IU = 0.67 mg) or synthetic dl-alpha-tocopherol (1 IU = 0.9 mg by mass, 0.45 mg alpha-tocopherol equivalents for RDA purposes).

When to use

A supplement label, prescription or dietary reference gives a vitamin in IU and you need µg or mg, or vice versa.

Do not use when: The vitamin is not D, A or E (IU is not defined for others in current use), or the vitamin A source is beta-carotene, which has different retinol-activity factors.

Formula

mass = IU × factor, with factor 0.025 µg/IU (vitamin D), 0.3 µg/IU (retinol), 0.67 mg/IU (d-alpha-tocopherol), 0.9 mg/IU (dl-alpha-tocopherol, USP mass) or 0.45 mg/IU (dl-alpha, IOM bioactivity); IU = mass / factor

Factors follow the NIH Office of Dietary Supplements fact sheets; for vitamin E the 2000 IOM report counts only the 2R-stereoisomers of the synthetic form, which is why its RDA equivalence (0.45 mg/IU) is half the USP mass definition. Results are informational only and not a substitute for clinical judgement or medical advice.

Inputs

Parameter	Type	Required	Description
`vitamin`	enum: vitamin_d \| vitamin_a_retinol \| vitamin_e_natural \| vitamin_e_synthetic \| vitamin_e_synthetic_iom	yes	Vitamin and form; the conversion factor differs by form.
`direction`	enum: iu_to_mass \| mass_to_iu	default iu_to_mass	Whether the value is in IU or in mass units.
`value`	number	yes	Amount in IU (iu_to_mass) or in mass_unit (mass_to_iu). Range: > 0, ≤ 1000000000
`mass_unit`	enum: ug \| mg	default ug	Unit of the value when direction is mass_to_iu; ignored otherwise.

Outputs

Output	Type	Unit	Description
`iu`	number	IU	Amount in IU.
`micrograms`	number	µg	Amount in micrograms.
`milligrams`	number	mg	Amount in milligrams.
`factor_note`	string		The published conversion applied.

Example

Vitamin D 1000 IU: {"vitamin":"vitamin_d","direction":"iu_to_mass","value":1000} → {"micrograms":25,"milligrams":0.025,"iu":1000}

Vitamin D 20 µg: {"vitamin":"vitamin_d","direction":"mass_to_iu","value":20,"mass_unit":"ug"} → {"iu":800,"micrograms":20}

Vitamin E natural 400 IU: {"vitamin":"vitamin_e_natural","value":400} → {"milligrams":268,"micrograms":268000}

GET https://tttkmbb.com/api/v1/calculate/vitamin-iu-conversion?vitamin=vitamin_d&direction=iu_to_mass&value=1000

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/vitamin-iu-conversion (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/vitamin-iu-conversion · Markdown: https://tttkmbb.com/medical/vitamin-iu-conversion.md · JSON definition: https://tttkmbb.com/medical/vitamin-iu-conversion.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="vitamin-iu-conversion"
OpenAPI operationId: convert_vitamin_iu
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

FAQ

Why do labels show both mcg and IU for vitamin D?

US supplement labels switched to µg in 2020–2021 (with IU optional); 400 IU = 10 µg, 800 IU = 20 µg, 2000 IU = 50 µg, 4000 IU = 100 µg.

Which vitamin E factor should I use?

Use the natural (0.67 mg) or synthetic USP (0.9 mg) factor to know the mass in a product; use the IOM 0.45 mg factor for synthetic vitamin E when comparing with the 15 mg alpha-tocopherol RDA.

Related calculators

Mass / Weight Converter — General mass unit conversion.
Dose by Weight Calculator — Weight-based dosing once the mass is known.