# Joules to Watt-hours (J to Wh)

> Convert joules to watt-hours: 1 J = 0.000277778 Wh. Exact conversion factor, formula, 22-row table and worked examples, with JSON API and MCP access.

- Page id: `joules-to-wh` · Quantity: Energy (`energy`, base unit J) · Tool: `convert_units`
- Canonical page: https://tttkmbb.com/conversion/joules-to-wh · This document: https://tttkmbb.com/conversion/joules-to-wh.md · JSON: https://tttkmbb.com/conversion/joules-to-wh.json (add `?value=` for a result)

## Formula

- `1 J = 0.000277778 Wh`
- `watt-hours = joules × 0.0002777777778`
- Reverse: `joules = watt-hours × 3600`

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 J = 0.00138889 Wh
- 12.5 J = 0.00347222 Wh
- 100 J = 0.0277778 Wh

## Conversion table

| Joules (J) | Watt-hours (Wh) |
| --- | --- |
| 1 | 0.000277778 |
| 2 | 0.000555556 |
| 3 | 0.000833333 |
| 4 | 0.00111111 |
| 5 | 0.00138889 |
| 6 | 0.00166667 |
| 7 | 0.00194444 |
| 8 | 0.00222222 |
| 9 | 0.0025 |
| 10 | 0.00277778 |
| 15 | 0.00416667 |
| 20 | 0.00555556 |
| 25 | 0.00694444 |
| 30 | 0.00833333 |
| 40 | 0.0111111 |
| 50 | 0.0138889 |
| 75 | 0.0208333 |
| 100 | 0.0277778 |
| 200 | 0.0555556 |
| 250 | 0.0694444 |
| 500 | 0.138889 |
| 1000 | 0.277778 |

## API

- `GET https://tttkmbb.com/api/v1/convert?value=5&from_unit=J&to_unit=Wh` → 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":"J","to_unit":"Wh"}`
- All energy units: https://tttkmbb.com/api/v1/units?quantity=energy · full converter: [Energy Converter](https://tttkmbb.com/conversion/energy.md)
- Authentication: none. Freshness: `static` (fixed conversion factors).

## FAQ

**How many watt-hours are in one joule?**

1 J = 0.000277778 Wh. 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 joules to watt-hours?**

Multiply the value in joules by 0.0002777777778: watt-hours = joules × 0.0002777777778. For example, 5 J = 0.00138889 Wh.

**How do I convert watt-hours back to joules?**

Multiply by the inverse factor 3600 (joules = watt-hours × 3600), or use the watt-hours-to-joules page.

## Related conversions

- [Watt-hours to joules](https://tttkmbb.com/conversion/wh-to-joules.md) (reverse)
- [Kilocalories (food calories) to Kilojoules](https://tttkmbb.com/conversion/kcal-to-kj.md)
- [Kilojoules to Kilocalories (food calories)](https://tttkmbb.com/conversion/kj-to-kcal.md)
- [Joules to Calories (small, thermochemical)](https://tttkmbb.com/conversion/joules-to-calories.md)
- [Calories (small, thermochemical) to Joules](https://tttkmbb.com/conversion/calories-to-joules.md)
- [Kilowatt-hours to Megajoules](https://tttkmbb.com/conversion/kwh-to-mj.md)
- [Megajoules to Kilowatt-hours](https://tttkmbb.com/conversion/mj-to-kwh.md)
- [Kilowatt-hours to BTU](https://tttkmbb.com/conversion/kwh-to-btu.md)
- [BTU to Kilowatt-hours](https://tttkmbb.com/conversion/btu-to-kwh.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)