# Bandwidth and download time calculator

> Download or upload time for a file at a given speed, the data a link moves in a set time, and a website's monthly bandwidth from page views.

Interactive version: https://www.calcopenly.com/programming/bandwidth-data-transfer-calculator
Subject: Programming and tech calculators

Transfer time is the size in bits divided by the speed in bits per second: t = 8 × bytes ÷ (rate × (1 − overhead)). The factor of 8 is there because internet plans quote megabits per second while files are measured in bytes. The other two modes run the same equation the other way, for the data a link moves in a set time, or multiply page views by page weight for a website's monthly traffic.

The default, a 50 GB download on a 100 Mbit/s plan with 5% protocol overhead, takes 1 h 10 min 10 s; with no overhead it would take 1 h 6 min 40 s. A site with 100,000 monthly views of a 2 MB page serves 200 GB a month.

Real downloads can be slower than the result when the server, Wi-Fi or a congested route caps the speed below the plan's rate. Sizes follow IEC 80000-13: 1 GB is 10^9 bytes and 1 GiB is 2^30 bytes.

## Inputs

- **I want to find** (options: Transfer time, Data moved in a time, Website monthly bandwidth)
- **Data size**
- **Connection speed**: Internet plans quote megabits per second (Mbit/s); download dialogs often show megabytes per second (MB/s), 8 times smaller.
- **Time spent transferring**
- **Protocol overhead**: Share of the line rate lost to packet headers. TCP/IPv4 over Ethernet carries 1460 payload bytes per 1538 bytes on the wire, about 5%.
- **Page views per month**
- **Average page weight**: Everything one view downloads (HTML, scripts, images). Your browser's network panel shows the total.
- **Days in the month**

## Results

- Transfer time — main result
- Transfer time in seconds (s)
- Data
- Data
- Payload speed after overhead (Mbit/s)
- Data per day (GB)
- Average speed needed (Mbit/s)

## Formula

$$
\begin{aligned} t &= \frac{8 \times \text{bytes}}{\text{rate} \times (1 - \text{overhead})} \\[4pt] \text{monthly} &= \text{views} \times \text{page weight}\end{aligned}
$$

## Worked examples

### 50 GB at 100 Mbit/s with no overhead

- I want to find: Transfer time
- Data size: 50 GB
- Connection speed: 100 Mbit/s
- Protocol overhead: 0%
- **Transfer time in seconds: 4,000 s**
- **Transfer time: 1 h 6 min 40 s**
- Checked against: Python 3.8: 50e9 × 8 / 100e6 = 4000 s

### Same transfer with 5% overhead

- I want to find: Transfer time
- Data size: 50 GB
- Connection speed: 100 Mbit/s
- Protocol overhead: 5%
- **Transfer time in seconds: 4,210.53 s**
- **Payload speed after overhead: 95 Mbit/s**
- Checked against: Python 3.8 decimal: 4000 / 0.95 = 4210.526315…

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

- I want to find: Transfer time
- Data size: 1 GiB
- Connection speed: 1 Gbit/s
- Protocol overhead: 0%
- **Transfer time in seconds: 8.59 s**
- Checked against: Python 3.8: 2**30 × 8 / 1e9 = 8.589934592 s

### One hour at 100 Mbit/s

- I want to find: Data moved in a time
- Connection speed: 100 Mbit/s
- Time spent transferring: 1 h
- Protocol overhead: 0%
- **Data: 45 GB**
- **Data: 41.91 GiB**
- Checked against: Python 3.8: 100e6 × 3600 / 8 = 45e9 B; 45e9 / 2**30 = 41.90951 GiB

### 100,000 views of a 2 MB page

- I want to find: Website monthly bandwidth
- Page views per month: 100,000
- Average page weight: 2 MB
- Days in the month: 30
- **Data: 200 GB per month**
- **Data per day: 6.667 GB**
- **Average speed needed: 0.617 Mbit/s**
- Checked against: Python 3.8 decimal: 1e5 × 2e6 = 2e11 B; 2e11 × 8 / (30 × 86400) = 617 283.95 bit/s

### Speed given in megabytes per second

- I want to find: Transfer time
- Data size: 700 MB
- Connection speed: 10 MB/s
- Protocol overhead: 0%
- **Transfer time in seconds: 70 s**
- **Payload speed after overhead: 80 Mbit/s**
- Checked against: Python 3.8: 700e6 / 10e6 = 70 s; 10 MB/s × 8 = 80 Mbit/s

## Questions

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

About 80 seconds at the full line rate: 1 GB is 8,000 megabits, and 8,000 ÷ 100 = 80 s. With 5% protocol overhead it takes about 84 s. At 10 Mbit/s the same file takes 13 min 20 s, and at 1 Gbit/s about 8 s, as long as the server and Wi-Fi keep up.

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

Mbps (megabits per second) measures network speed, while MB/s (megabytes per second) is how browsers and download managers show progress. One byte is 8 bits, so divide Mbps by 8: a 100 Mbit/s plan tops out at 12.5 MB/s, and 1 Gbit/s at 125 MB/s. Protocol overhead lowers the figure you see by a further few percent.

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

Part of the gap is protocol overhead: over Ethernet, TCP/IPv4 carries 1,460 bytes of data in each full 1,538-byte frame on the wire, so about 5% of the line rate goes to headers, preamble and inter-frame gap. The rest comes from the server's own limit, Wi-Fi signal, congestion on the route, or other devices sharing the connection.

### How much bandwidth does a website need per month?

Multiply monthly page views by the average page weight, which your browser's network panel shows. 100,000 views of a 2 MB page is 200 GB a month, about 6.7 GB a day or an average of 0.62 Mbit/s. Visits bunch up in busy hours, so size hosting and caching for several times that average.

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

Under the SI and IEC 80000-13 prefixes, 1 GB is 1,000 MB, or 10^9 bytes, while 1 GiB (gibibyte) is 1,024 MiB, or 2^30 = 1,073,741,824 bytes. Network speeds and drive labels use decimal units. Windows counts in binary units but labels them GB, so a 50 GB file shows there as 46.57.

### How accurate is the bandwidth and download time calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 6 worked examples whose answers come from independent sources; for example, “50 GB at 100 Mbit/s with no overhead” is checked against Python 3.8: 50e9 × 8 / 100e6 = 4000 s.

### Where does the method come from?

IEC 80000-13:2008 — decimal (kB, MB, GB) and binary (KiB, MiB, GiB) prefixes for bytes; IEEE 802.3 Ethernet: 8-byte preamble, 14-byte header, 1500-byte payload, 4-byte FCS and 12-byte inter-frame gap (1538 bytes per full frame); NIST — Prefixes for binary multiples.

## Sources

- IEC 80000-13:2008 — decimal (kB, MB, GB) and binary (KiB, MiB, GiB) prefixes for bytes
- IEEE 802.3 Ethernet: 8-byte preamble, 14-byte header, 1500-byte payload, 4-byte FCS and 12-byte inter-frame gap (1538 bytes per full frame)
- [NIST — Prefixes for binary multiples](https://physics.nist.gov/cuu/Units/binary.html)