Data Transfer Calculator

Runs in browser New Internet & Network

Give a size (4.7 GB, 700 MiB, 2 TB) and a link speed (100 Mbps, 12.5 MB/s) to get the transfer time at that speed, with an optional estimate of the protocol overhead, the same transfer at eight common speeds from 10 Mbps to 10 Gbps, and the size in both SI and IEC units. The second calculation turns a speed and the hours it runs each day into the data moved per hour, day, week and period, for data caps and transfer bills. The formulas are shown with your numbers; it is arithmetic in your browser, not a measurement.

Privacy: This tool runs entirely in your browser. Your input never leaves your device.

How do you work out bandwidth and transfer time correctly?

Line speeds are quoted in bits per second with SI prefixes (1 Mbps = 10^6 bit/s), file sizes in bytes, often with binary prefixes (1 MiB = 1,048,576 bytes), so the first step is always ×8 or ÷8 in the right units: a 100 Mbps line moves at most 12.5 MB/s. Transfer time is size × 8 ÷ rate, and the rate a transfer needs is size × 8 ÷ time, plus the header overhead on the wire (about 5–8% for TCP over Ethernet). Real transfers are slower than this arithmetic because of latency, congestion and TCP's window: one connection carries at most its window divided by the round-trip time (RFC 6349, RFC 7323).

How to use Data Transfer Calculator

  1. Leave the calculation on transfer time, type the data size (for example 4.7 GB) and the link speed (for example 100 Mbps).
  2. Choose a protocol-overhead estimate if you want the header bytes counted; leave it on none for the pure payload time.
  3. Read the transfer time and the table of common speeds to see what a faster or slower line would change.
  4. For a monthly volume, switch to data moved at a steady rate and fill in the hours a day and the number of days.

Data Transfer Calculator features

  • Transfer time for any size (B to PB, SI or IEC, or in bits) at any link speed written in bits or bytes per second
  • An optional protocol-overhead estimate that adds the header bytes of TCP/IPv4 or TCP/IPv6 over Ethernet, with the bytes that actually cross the wire and the effective data rate
  • The same transfer at eight common link speeds from 10 Mbps to 10 Gbps, with your own speed slotted into the list
  • The size in SI and IEC units side by side (4.7 GB = 4.38 GiB), which explains why a download program shows smaller numbers
  • Data volume at a steady rate: per hour, per day, per week and over a period of your choice, in SI and IEC units, for data caps and transfer bills
  • The formula and the same formula with your numbers in it, for every result

Inputs and outputs

Inputs

  • Calculation: Time to transfer a file or backup, or Data moved at a steady rate (daily and monthly volume).
  • Data size (transfer time), Link speed (both), Protocol overhead estimate (transfer time), Hours a day at that rate and Days (volume).

Outputs

  • At a glance: transfer time, size in SI and IEC, effective data rate and bytes on the wire; or the volume over the period and per day.
  • Formula and figures: the formula, your numbers in it, and the size and speed in bytes and bits.
  • The same size at common link speeds (transfer time and data rate in bytes), or the volume by period, both downloadable as CSV.

Data Transfer Calculator example

A 4.7 GB disc image over a 100 Mbps line, with the TCP/IPv4 overhead estimate

Input:

Calculation: Time to transfer a file or backup · Data size: 4.7 GB · Link speed: 100 Mbps · Protocol overhead estimate: TCP/IPv4 over Ethernet (5.3%)

Output:

Transfer time: 6 min 36 s (includes the overhead estimate: 5.3%) · Data size: 4.7 GB (4.38 GiB) · Effective data rate: 95.0 Mbps · Bytes on the wire: 4.95 GB
Formula: t = S × 8 × (1 + o ÷ 100) ÷ R · Substituted: 4,700,000,000 × 8 × 1.053 ÷ 100,000,000 = 395.928 s
At common speeds: 10 Mbps 1 h 05 min · 50 Mbps 13 min 12 s · 1 Gbps 39.6 s · 10 Gbps 4 s

How to read the Data Transfer Calculator results

The transfer time is the best case for that speed: the line is fully used by this one transfer from start to finish. Treat it as a lower bound and add a margin for real conditions.

The effective data rate is what remains for your file after the header bytes; it is the figure to compare with the speed a download program reports, once both are in the same unit.

If a download program shows MiB or MiB/s, compare with the IEC figures (the hints under the SI ones): 11.9 MiB/s is the full speed of a 100 Mbps line.

In the volume table, the whole-period figure is what a data cap or a transfer bill counts if the link really runs at that rate for those hours.

When to use Data Transfer Calculator

  • Estimating how long a cloud backup, a virtual machine image or a database dump takes to upload before starting it.
  • Choosing between a faster line and a longer transfer window by comparing the time at common link speeds.
  • Checking whether a camera stream, a sync job or a mirror running all day fits within a monthly data cap.
  • Explaining to users why a "4.7 GB" file shows as 4.38 GB on their computer and why the download took longer than the line speed suggests.

Troubleshooting

  • "Enter a link speed greater than zero": a speed of 0 means the transfer never ends; enter the real rate, such as 20 Mbps.
  • The result looks 8 times off: check bits against bytes; Mbps is megabits, MB/s megabytes.
  • "Unknown size unit": use B, kB, MB, GB, TB, KiB, MiB, GiB, TiB or bit units such as Gbit; a bare number is read as bytes.
  • The hours a day must be more than 0 and at most 24, and the days between 1 and 3660.

Frequently asked questions about Data Transfer Calculator

How long does a 4.7 GB file take to download at 100 Mbps?

4.7 GB is 37.6 gigabits, and 37.6 ÷ 0.1 Gbit/s = 376 seconds, about 6 minutes 16 seconds, if the line runs at its full rate for the whole download. With the 5.3% TCP/IPv4 overhead estimate it becomes about 6 minutes 36 seconds. A real download usually takes longer still, for the reasons in the next answer.

Why does my download take longer than the calculator says?

The calculator assumes the whole line rate goes to your file. In practice the server may send more slowly, TCP takes time to speed up, other devices share the line, Wi-Fi loses capacity with distance, and congestion anywhere on the path slows things down. Measure what your line really delivers with the Internet Speed Test and use that rate instead.

Why does my computer show 4.38 GB for a file sold as 4.7 GB?

They count in different units. 4.7 GB means 4,700,000,000 bytes (SI); many operating systems divide by 1,073,741,824 and show 4.38 "GB", which is really 4.38 GiB. The bytes are the same. The calculator shows both forms so you can match the one your program uses.

How much data does a link move in a month?

Choose "Data moved at a steady rate", enter the speed, the hours a day it runs at that speed and the number of days. A 20 Mbps stream for 8 hours a day over 30 days is 2.16 TB (1.96 TiB). Using your average rate rather than the line speed gives a figure closer to a real bill or data cap.

What does the protocol overhead option change?

It adds the header bytes that travel with your data: about 5.3% for TCP/IPv4 over Ethernet with full-size packets, 6.2% with TCP timestamps, 7.7% for IPv6, or a rough 10% for a VPN or small packets. The transfer time grows by that share and the effective data rate shrinks. It is an estimate of header cost, not a measurement of your network.

Technical notes

Transfer time is t = S × 8 × (1 + o ÷ 100) ÷ R, with S in bytes, R in bit/s and o the overhead as extra bytes on the wire per payload byte; the effective data rate is R × 100 ÷ (100 + o). Sizes and rates are read by @engines/bandwidth with exact prefixes: k, M, G, T, P are powers of 1000, Ki, Mi, Gi, Ti, Pi powers of 1024, b a bit and B a byte.

The overhead presets are the header cost of full-size 1500-byte Ethernet frames: 38 bytes of Ethernet framing (preamble, header, FCS, inter-frame gap) plus 40 bytes of IPv4 and TCP headers per 1460 bytes of data (5.3%), 6.2% with TCP timestamps and 7.7% with IPv6 headers and timestamps. Real transfers also lose time to TCP slow start and to the window limit on long paths (RFC 6349), which no fixed percentage captures.

The volume is V = R ÷ 8 × 3600 × h × d bytes for h hours a day over d days, shown per hour, day and week and for the whole period, in SI and IEC units.

Limitations and methodology

It measures nothing: the speed is the one you type, and the result is arithmetic. Your real speed depends on your line, the server and the path in between; the Internet Speed Test measures your line to the nearest Cloudflare edge.

The overhead estimate covers header bytes only; latency, TCP slow start, retransmissions, server limits and other traffic on the line are not modelled, so real transfers take longer.

The volume calculation assumes a constant rate for the hours given; real usage varies through the day.

Security and privacy

Everything is calculated in your browser; the sizes and speeds you enter are not sent anywhere and nothing is stored.

References

  • Metric (SI) PrefixesNISTThe SI prefixes and their exact powers of ten: kilo is 10^3, mega 10^6, giga 10^9, tera 10^12 — the meaning of k, M, G and T in Mbps, Gbps and GB.
  • units(7) — decimal and binary prefixesLinux man-pages projectLists the decimal prefixes (k = 1000) beside the binary ones (Ki = 1024, Mi = 1,048,576…) and notes in passing the b = bit, B = byte convention.
  • RFC 6349: Framework for TCP Throughput TestingIETF · 2011How to measure TCP throughput on a path: why a single connection is bounded by its window and the round-trip time (the bandwidth-delay product), and which metrics describe a test fairly.