# ダウンロード時間計算

> Calculate how long a file takes to download or upload from its size in MB, GB or TB and your internet speed in Mbps, allowing for protocol overhead.

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

Download time is file size divided by the useful data rate. The calculator turns the size into bits (1 byte = 8 bits; GB means 10⁹ bytes and GiB means 2³⁰ bytes), takes the connection speed in bits per second, and removes the share of each packet taken by protocol headers: time = bits ÷ (speed × (1 − overhead)).

People use it before downloading a game or a cloud backup, and to judge whether a faster plan is worth paying for. The default, a 50 GB file on a 100 Mbit/s line with 5% overhead, takes 1 h 10 min 10 s; at 1,000 Mbit/s the same file takes about 7 minutes.

The result is a best case. The 5% default is the framing cost of TCP/IPv4 over Ethernet; the server's own limit, Wi-Fi signal and other traffic are not modelled. For an upload, enter your upload speed, which on asymmetric lines such as ADSL is lower than the download speed.

## 入力

- **File size**
- **Connection speed**
- **Protocol overhead**: TCP/IPv4 over Ethernet carries 1,460 bytes of data in every 1,538 bytes sent, about 5 % overhead. Use 0 for the raw line rate.

## 結果

- Download time — 主な結果
- Time in seconds (s)
- Effective speed (Mbit/s)
- Effective speed (MB/s)

## 計算式

$$
t = \frac{\text{size (bits)}}{\text{speed (bit/s)} \times (1 - \text{overhead})}
$$

## 計算例

### 50 GB at 100 Mbit/s with 5 % overhead

- File size: 50 GB
- Connection speed: 100 Mbit/s
- Protocol overhead: 5%
- **Time in seconds: 4,210.53 s**
- **Effective speed: 95 Mbit/s**
- 照合元：50 × 8 × 10⁹ bits ÷ (100 × 10⁶ × 0.95) bit/s = 4210.526… s (Python fractions)

### 1 GB at 100 Mbit/s, no overhead

- File size: 1 GB
- Connection speed: 100 Mbit/s
- Protocol overhead: 0%
- **Download time: 1 min 20 s**
- **Time in seconds: 80 s**
- 照合元：8 × 10⁹ / 10⁸ = 80 s

### 1 GiB at 1 Gbit/s (binary size, decimal speed)

- File size: 1 GiB
- Connection speed: 1 Gbit/s
- Protocol overhead: 0%
- **Time in seconds: 8.58993 s**
- 照合元：2³⁰ × 8 bits / 10⁹ bit/s = 8.589934592 s

### 4.7 GB DVD image at 25 Mbit/s with 10 % overhead

- File size: 4.7 GB
- Connection speed: 25 Mbit/s
- Protocol overhead: 10%
- **Time in seconds: 1,671.11 s**
- 照合元：4.7 × 8 × 10⁹ / (25 × 10⁶ × 0.9) = 1671.11 s (Python fractions)

### Empty file

- File size: 0 GB
- Connection speed: 100 Mbit/s
- Protocol overhead: 5%
- **Time in seconds: 0 s**
- 照合元：0 bits take 0 s

## よくある質問

### How long does it take to download 1 GB at 100 Mbps?

About 84 seconds with 5% protocol overhead, or 80 seconds at the raw line rate. 1 GB is 8,000 megabits, and 8,000 ÷ 100 = 80 s. Halving the speed doubles the time: 160 s raw at 50 Mbps. At 1 Gbps the same gigabyte takes 8 s raw, about 8.4 s with overhead.

### What is the difference between Mbps and MB/s?

Mbps (megabits per second) is how connection speeds are sold; MB/s (megabytes per second) is how browsers and download managers show progress. A byte is 8 bits, so divide Mbps by 8: a 100 Mbps line moves at most 12.5 MB/s, and about 11.9 MB/s after 5% overhead. Both use decimal prefixes, so 1 Mbit = 10⁶ bits (NIST).

### Why is my download slower than my internet speed?

Part of every packet is headers, not your file. With a 1,500-byte Ethernet MTU, TCP/IPv4 carries 1,460 bytes of data in every 1,538 bytes on the wire (IEEE 802.3 framing plus 20-byte IP and TCP headers), a 5.1% loss; the 12-byte TCP timestamp option cuts the data to 1,448 bytes, 5.9%. Beyond that, the server's upload limit, Wi-Fi signal and other devices sharing the line slow it further.

### Why does a 50 GB download show as 46.6 GB on my computer?

Because Windows File Explorer counts in binary units but labels them GB: it divides by 2³⁰ (1,073,741,824) bytes, so 50 × 10⁹ bytes appears as 46.6 GB. Stores and ISPs use decimal gigabytes (10⁹ bytes, IEC 80000-13), and macOS has shown decimal sizes since Mac OS X 10.6 in 2009. Choose GiB in the size field for a size copied from Windows.

### 「ダウンロード時間計算」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、「50 GB at 100 Mbit/s with 5 % overhead」は50 × 8 × 10⁹ bits ÷ (100 × 10⁶ × 0.95) bit/s = 4210.526… s (Python fractions)と照合しています。

### この計算方法の出典は何ですか？

NIST — Prefixes for binary multiples (IEC 80000-13): 1 byte = 8 bits, GB vs GiB; IEEE 802.3 Ethernet framing (1,500-byte MTU + 38 bytes of preamble, header, FCS and gap) with RFC 791 / RFC 793 20-byte IPv4 and TCP headers.

## 出典

- [NIST — Prefixes for binary multiples (IEC 80000-13): 1 byte = 8 bits, GB vs GiB](https://physics.nist.gov/cuu/Units/binary.html)
- IEEE 802.3 Ethernet framing (1,500-byte MTU + 38 bytes of preamble, header, FCS and gap) with RFC 791 / RFC 793 20-byte IPv4 and TCP headers