# データ容量換算

> GB から MB、TB から GB、GiB から GB に換算します。十進接頭辞（kB、MB、GB、TB）と二進接頭辞（KiB、MiB、GiB、TiB）を区別し、ビットとバイトにも対応します。

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

Data sizes use two sets of prefixes. The decimal ones follow the SI: 1 kB = 1,000 bytes, 1 MB = 10⁶ and 1 GB = 10⁹ bytes. The binary ones, approved by the IEC in December 1998 and now part of IEC 80000-13, step by 1,024: 1 KiB = 1,024 bytes, 1 MiB = 2²⁰ and 1 GiB = 2³⁰ bytes. A byte is 8 bits, and every conversion between these units is exact.

The default converts 1 GiB to 1,073.741824 MB. The same gap explains why a new drive looks smaller: a 1 TB drive holds 10¹² bytes, which software counting in binary units shows as 931.32 GiB, often labelled “931 GB”.

The gap grows with each prefix: 2.4% at kilo, 7.37% at giga, 9.95% at tera and 20.9% at yotta. Memory chips come in powers of two, so RAM sizes are binary even when written GB, while drive makers and network speeds use decimal prefixes.

## 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：bit, nibble, byte (B), kilobyte (kB), megabyte (MB), gigabyte (GB), terabyte (TB), petabyte (PB), exabyte (EB), zettabyte (ZB), yottabyte (YB), kibibyte (KiB), mebibyte (MiB), gibibyte (GiB), tebibyte (TiB), pebibyte (PiB), exbibyte (EiB), zebibyte (ZiB), yobibyte (YiB), kilobit (kbit), megabit (Mbit), gigabit (Gbit), terabit (Tbit), kibibit (Kibit), mebibit (Mibit))
- **変換先** (選択肢：bit, nibble, byte (B), kilobyte (kB), megabyte (MB), gigabyte (GB), terabyte (TB), petabyte (PB), exabyte (EB), zettabyte (ZB), yottabyte (YB), kibibyte (KiB), mebibyte (MiB), gibibyte (GiB), tebibyte (TiB), pebibyte (PiB), exbibyte (EiB), zebibyte (ZiB), yobibyte (YiB), kilobit (kbit), megabit (Mbit), gigabit (Gbit), terabit (Tbit), kibibit (Kibit), mebibit (Mibit))

## 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

$$
\begin{gathered} x_{\text{to}} = x_{\text{from}} \times \dfrac{f_{\text{from}}}{f_{\text{to}}} \\ f = \text{one unit expressed in } \text{bit} \end{gathered}
$$

## 計算例

### 1 GiB in MB

- 値: 1 GiB
- 変換元: gibibyte (GiB)
- 変換先: megabyte (MB)
- **変換後の値: 1,073.741824 MB**
- **正確さ: 正確**
- 照合元：2³⁰ bytes = 1,073,741,824 bytes (IEC 80000-13); ÷ 10⁶

### A 1 TB drive in GiB

- 値: 1 TB
- 変換元: terabyte (TB)
- 変換先: gibibyte (GiB)
- **変換後の値: 931.322574615 GiB**
- 照合元：10¹² / 2³⁰ = 244140625/262144, a terminating decimal (Python fractions)

### 1 KiB in kB

- 値: 1 KiB
- 変換元: kibibyte (KiB)
- 変換先: kilobyte (kB)
- **変換後の値: 1.024 kB**
- 照合元：1024 bytes / 1000 bytes

### Largest gap: 1 YiB in YB

- 値: 1 YiB
- 変換元: yobibyte (YiB)
- 変換先: yottabyte (YB)
- **変換後の値: 1.20892581961 YB**
- 照合元：2⁸⁰ / 10²⁴ = 1.208925819614629174706176 exactly (Python fractions)

### 1 byte in bits

- 値: 1 B
- 変換元: byte (B)
- 変換先: bit
- **変換後の値: 8 bit**
- 照合元：1 byte = 8 bits (IEC 80000-13)

## よくある質問

### Why does my 1 TB drive show only 931 GB?

Because the maker and the operating system count differently. The drive's 1 TB is 10¹² bytes; Windows divides by 2³⁰ and shows 931.32 while labelling it GB. Nothing is missing, since 10¹² ÷ 1,073,741,824 = 931.32; formatting and recovery partitions take a little more. macOS has counted in decimal since OS X 10.6 and shows the same drive as 1 TB.

### Is 1 GB 1,000 MB or 1,024 MB?

Under the SI and IEC 80000-13, 1 GB is exactly 1,000 MB (10⁹ bytes), and the binary unit is 1 GiB = 1,024 MiB (2³⁰ bytes). Drive makers and macOS use the decimal meaning; RAM and Windows use the binary one while writing GB. The “1.44 MB” floppy disk mixed the two: 1,440 × 1,024 bytes.

### How many bits are in a byte?

Eight. IEC 80000-13 defines the byte as 8 bits, also called an octet. File sizes are counted in bytes (B) and network speeds in bits per second (bit/s), so a 100 Mbit/s connection moves at most 12.5 MB per second; a capital B marks bytes.

### What is a GiB?

A gibibyte, 2³⁰ = 1,073,741,824 bytes. The IEC created the binary prefixes kibi, mebi, gibi and tebi so that kilo, mega and giga could keep their SI meanings of 10³, 10⁶ and 10⁹. Many Linux tools and technical specifications use GiB; Windows shows the same quantity labelled GB.

### How many GB are in 1 TB?

1,000 GB in decimal units, which is how drives are sold. In binary units 1 TiB is 1,024 GiB, and a decimal 1 TB is 931.32 GiB. The converter keeps TB, TiB, GB and GiB separate, so you can see that 1 TiB is 1,099.51 GB, about 10% more than 1 TB.

### 「データ容量換算」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、「1 GiB in MB」は2³⁰ bytes = 1,073,741,824 bytes (IEC 80000-13); ÷ 10⁶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; NIST — Prefixes for binary multiples (IEC 80000-13).

## 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)
- [NIST — Prefixes for binary multiples (IEC 80000-13)](https://physics.nist.gov/cuu/Units/binary.html)