# 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).

- Calculator id: `vitamin-iu-conversion` · Category: Medical & Clinical (`medical`) · Tool name: `convert_vitamin_iu`
- Canonical page: https://tttkmbb.com/medical/vitamin-iu-conversion · This document: https://tttkmbb.com/medical/vitamin-iu-conversion.md · JSON definition: https://tttkmbb.com/medical/vitamin-iu-conversion.json

## Purpose

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).

**Use when:** 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.

## Input

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

## Output

| Field | 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. |

## 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.

## Data Sources

- NIH Office of Dietary Supplements – Vitamin D fact sheet for health professionals — https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/ (government, retrieved 2026-09-24)
- NIH Office of Dietary Supplements – Vitamin E fact sheet for health professionals — https://ods.od.nih.gov/factsheets/VitaminE-HealthProfessional/ (government, retrieved 2026-09-24)
- NIH Office of Dietary Supplements – Vitamin A and carotenoids fact sheet for health professionals — https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/ (government, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/vitamin-iu-conversion?vitamin=…&value=…`
- `POST https://tttkmbb.com/api/v1/calculate/vitamin-iu-conversion` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/vitamin-iu-conversion · OpenAPI operationId `convert_vitamin_iu` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "vitamin-iu-conversion", "inputs": {…}}`

## Example

- Vitamin D 1000 IU: inputs `{"vitamin":"vitamin_d","direction":"iu_to_mass","value":1000}` → `{"micrograms":25,"milligrams":0.025,"iu":1000}`
- Vitamin D 20 µg: inputs `{"vitamin":"vitamin_d","direction":"mass_to_iu","value":20,"mass_unit":"ug"}` → `{"iu":800,"micrograms":20}`
- Vitamin E natural 400 IU: inputs `{"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
```

## Limitations

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. 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. All values are computed from the formula above; no measurement or live data is involved.

## 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

- [Mass / Weight Converter](https://tttkmbb.com/conversion/mass.md) — General mass unit conversion.
- [Dose by Weight Calculator](https://tttkmbb.com/medical/dose-by-weight.md) — Weight-based dosing once the mass is known.