# 温度換算

> °F = °C × 9/5 + 32 で摂氏と華氏を換算し、両方からケルビンへ変換します。ランキンとレオミュールにも対応し、絶対零度未満は受け付けません。

操作できるページ：https://www.calcopenly.com/ja/conversion/temperature-converter
分野：単位換算ツール

Temperature scales differ in the size of a degree and in where zero sits, so a conversion adds an offset as well as multiplying: °F = °C × 9/5 + 32, and K = °C + 273.15. The converter works through kelvin, whose zero is absolute zero, and rejects any value below it (−273.15 °C or −459.67 °F).

The default converts normal body temperature: 37 × 9/5 + 32 = 98.6 °F. Weather forecasts, oven settings and fever readings are the everyday uses; the 38 °C fever threshold is 100.4 °F.

A temperature difference converts without the offset: a rise of 10 °C is a rise of 18 °F. Rankine is the absolute scale with Fahrenheit-sized degrees, and Réaumur is a historical scale with 80 degrees between the freezing and boiling points of water.

## 入力

- **値**: 変換元の単位で数値を入力するか、単位付きの数値を入力します
- **変換元** (選択肢：kelvin (K), degree Celsius (°C), degree Fahrenheit (°F), degree Rankine (°R), degree Réaumur (°Ré))
- **変換先** (選択肢：kelvin (K), degree Celsius (°C), degree Fahrenheit (°F), degree Rankine (°R), degree Réaumur (°Ré))

## 結果

- 変換後の値 — 主な結果
- 単位換算
- 換算係数
- 正確さ

## 計算式

$$
\begin{gathered} t_{\text{°F}} = t_{\text{°C}} \times \tfrac{9}{5} + 32 \\ T_{\text{K}} = t_{\text{°C}} + 273.15 \\ T_{\text{°R}} = T_{\text{K}} \times \tfrac{9}{5} \\ t_{\text{°Ré}} = t_{\text{°C}} \times \tfrac{4}{5} \end{gathered}
$$

## 計算例

### Body temperature, 37 °C in °F

- 値: 37 °C
- 変換元: degree Celsius (°C)
- 変換先: degree Fahrenheit (°F)
- **変換後の値: 98.6 °F**
- **単位換算: 37 °C = 98.6 °F**
- **正確さ: 正確**
- 照合元：°F = °C × 9/5 + 32 (NIST SP 811 §4.2.1.1): 37 × 1.8 + 32 = 98.6

### −40 is the same in both scales

- 値: -40 °C
- 変換元: degree Celsius (°C)
- 変換先: degree Fahrenheit (°F)
- **変換後の値: -40 °F**
- 照合元：−40 × 9/5 + 32 = −40

### Absolute zero in °F

- 値: 0 K
- 変換元: kelvin (K)
- 変換先: degree Fahrenheit (°F)
- **変換後の値: -459.67 °F**
- **正確さ: 正確**
- 照合元：0 K = −273.15 °C; −273.15 × 9/5 + 32 = −459.67 °F (NIST SP 811 App. B.9)

### 100 °F in °C

- 値: 100 °F
- 変換元: degree Fahrenheit (°F)
- 変換先: degree Celsius (°C)
- **変換後の値: 37.7777777778 °C**
- **正確さ: 定義は正確、結果は循環小数**
- 照合元：(100 − 32) × 5/9 = 340/9 (Python 3.8 fractions from the legal definitions)

### 300 K in °R

- 値: 300 K
- 変換元: kelvin (K)
- 変換先: degree Rankine (°R)
- **変換後の値: 540 °R**
- **換算係数: 1.8**
- 照合元：°R = K × 9/5 (NIST SP 811 App. B.9)

## よくある質問

### How do I convert Celsius to Fahrenheit?

Multiply by 9/5 (1.8) and add 32. For 37 °C, 37 × 1.8 = 66.6, and adding 32 gives 98.6 °F. To go back, subtract 32 and multiply by 5/9: (100 °F − 32) × 5/9 = 37.78 °C. NIST SP 811 states the same relation as t/°F = 1.8 t/°C + 32.

### Is 98.6 °F normal body temperature?

98.6 °F is the exact Fahrenheit value of 37 °C, the average Carl Wunderlich published in 1868; individual readings vary with the person, the time of day and where the thermometer goes. For illness the threshold matters more: the NHS treats 38 °C or above as a high temperature, and the US CDC defines fever as 100.4 °F, the same value.

### At what temperature are Celsius and Fahrenheit equal?

At −40. Setting the two numbers equal in °F = °C × 9/5 + 32 gives x = 1.8x + 32, so x = −40, and −40 °C is exactly −40 °F. It is the only crossing point because the degrees differ in size; above −40 the Fahrenheit number is always the larger one.

### How do I convert Celsius to kelvin?

Add 273.15: 25 °C is 298.15 K, and absolute zero, 0 K, is −273.15 °C. A kelvin is the same size as a Celsius degree and, since 2019, is defined by fixing the Boltzmann constant at 1.380649 × 10⁻²³ J/K (BIPM SI Brochure). Kelvin is written without a degree sign.

### Why is a 10 °C rise an 18 °F rise and not 50 °F?

Because a difference has no zero point to shift. One Celsius degree is 1.8 Fahrenheit degrees, so a change of 10 °C is a change of 18 °F; the +32 applies only to a reading on the scale. The converter's steps show the offset and the per-degree factor separately for this reason.

### 「温度換算」の精度はどのくらいですか？

精度は入力値と計算方法の前提に依存します。十進演算には有効数字50桁を使いますが、推定、数値計算手法、元データの精度はそれより低い場合があります。表示の丸め処理でこれらの制約がなくなるわけではありません。 独立した出典の解答と照合した計算例：5。 例えば、「Body temperature, 37 °C in °F」は°F = °C × 9/5 + 32 (NIST SP 811 §4.2.1.1): 37 × 1.8 + 32 = 98.6と照合しています。

### この計算方法の出典は何ですか？

NIST SP 811 (2008), Guide for the Use of the SI, Appendix B — conversion factors; NIST Handbook 44, Appendix C — General tables of units of measurement; BIPM SI Brochure, 9th edition (2019), §2.3.1 — the kelvin and degree Celsius.

## 出典

- [NIST SP 811 (2008), Guide for the Use of the SI, Appendix B — conversion factors](https://www.nist.gov/pml/special-publication-811)
- [NIST Handbook 44, Appendix C — General tables of units of measurement](https://www.nist.gov/pml/owm/nist-handbook-44-current-edition)
- [BIPM SI Brochure, 9th edition (2019), §2.3.1 — the kelvin and degree Celsius](https://www.bipm.org/en/publications/si-brochure)