# Kilometers to Astronomical units (km to au)

> Convert kilometers to astronomical units: 1 km = 6.68459e-9 au. Exact conversion factor, formula, 22-row table and worked examples, with JSON API and MCP access.

- Page id: `km-to-au` · Quantity: Length (`length`, base unit m) · Tool: `convert_units`
- Canonical page: https://tttkmbb.com/conversion/km-to-au · This document: https://tttkmbb.com/conversion/km-to-au.md · JSON: https://tttkmbb.com/conversion/km-to-au.json (add `?value=` for a result)

## Formula

- `1 km = 6.68459e-9 au`
- `astronomical units = kilometers × 6.684587122e-9`
- Reverse: `kilometers = astronomical units × 149597870.7`

The factor follows from the SI definitions of both units (NIST SP 811); values below are rounded to 6 significant figures, the API returns up to 12.

## Worked examples

- 5 km = 3.34229e-8 au
- 12.5 km = 8.35573e-8 au
- 100 km = 6.68459e-7 au

## Conversion table

| Kilometers (km) | Astronomical units (au) |
| --- | --- |
| 1 | 6.68459e-9 |
| 2 | 1.33692e-8 |
| 3 | 2.00538e-8 |
| 4 | 2.67383e-8 |
| 5 | 3.34229e-8 |
| 6 | 4.01075e-8 |
| 7 | 4.67921e-8 |
| 8 | 5.34767e-8 |
| 9 | 6.01613e-8 |
| 10 | 6.68459e-8 |
| 15 | 1.00269e-7 |
| 20 | 1.33692e-7 |
| 25 | 1.67115e-7 |
| 30 | 2.00538e-7 |
| 40 | 2.67383e-7 |
| 50 | 3.34229e-7 |
| 75 | 5.01344e-7 |
| 100 | 6.68459e-7 |
| 200 | 0.00000133692 |
| 250 | 0.00000167115 |
| 500 | 0.00000334229 |
| 1000 | 0.00000668459 |

## API

- `GET https://tttkmbb.com/api/v1/convert?value=5&from_unit=km&to_unit=au` → unified JSON envelope with `result.values.result`, `formula`, `factor`, `sources`, `freshness`
- MCP: server `https://tttkmbb.com/mcp`, tool `convert_units` with `{"value":5,"from_unit":"km","to_unit":"au"}`
- All length units: https://tttkmbb.com/api/v1/units?quantity=length · full converter: [Length Converter](https://tttkmbb.com/conversion/length.md)
- Authentication: none. Freshness: `static` (fixed conversion factors).

## FAQ

**How many astronomical units are in one kilometer?**

1 km = 6.68459e-9 au. The factor comes from the SI definitions of both units (NIST SP 811 / BIPM), so it is exact or correct to at least 10 significant figures.

**What is the formula to convert kilometers to astronomical units?**

Multiply the value in kilometers by 6.684587122e-9: astronomical units = kilometers × 6.684587122e-9. For example, 5 km = 3.34229e-8 au.

**How do I convert astronomical units back to kilometers?**

Multiply by the inverse factor 149597870.7 (kilometers = astronomical units × 149597870.7), or use the astronomical units-to-kilometers page.

## Related conversions

- [Astronomical units to kilometers](https://tttkmbb.com/conversion/au-to-km.md) (reverse)
- [Kilometers to Miles](https://tttkmbb.com/conversion/km-to-miles.md)
- [Miles to Kilometers](https://tttkmbb.com/conversion/miles-to-km.md)
- [Meters to Feet](https://tttkmbb.com/conversion/meters-to-feet.md)
- [Feet to Meters](https://tttkmbb.com/conversion/feet-to-meters.md)
- [Centimeters to Inches](https://tttkmbb.com/conversion/cm-to-inches.md)
- [Inches to Centimeters](https://tttkmbb.com/conversion/inches-to-cm.md)
- [Millimeters to Inches](https://tttkmbb.com/conversion/mm-to-inches.md)
- [Inches to Millimeters](https://tttkmbb.com/conversion/inches-to-mm.md)

## Data Sources

- NIST Special Publication 811 – Guide for the Use of the International System of Units (SI), Appendix B conversion factors — https://www.nist.gov/pml/special-publication-811 (standard, retrieved 2026-09-24)
- BIPM – The International System of Units (SI Brochure, 9th edition) — https://www.bipm.org/en/publications/si-brochure (standard, retrieved 2026-09-24)
- Wikipedia – Conversion of units — https://en.wikipedia.org/wiki/Conversion_of_units (reference, retrieved 2026-09-24)