Bandwidth Calculator

This page holds four bandwidth tools: converting between units of data size, working out how long a download or upload takes, estimating the bandwidth a website uses, and converting between a monthly data allowance and the sustained rate it corresponds to.

What bandwidth means

Bandwidth means different things in different contexts. In computing it is the bit-rate of available or consumed information, normally expressed in bits per second along with its metric multiples. Even within computing it splits into network bandwidth, data bandwidth and digital bandwidth.

The most common everyday use relates to the internet: the volume of information per unit of time that a transmission medium can handle. Note that the stated channel capacity is not necessarily the maximum amount of data the channel will actually move. Because of protocols and encryption, above all TCP, a channel stated at X bits per second will not transmit useful data at X.

Bits and bytes

A bit is the smallest unit of information and holds one of two values, 0 or 1. A byte is eight bits, and can represent values from 0 to 255.

The bit is the unit of data transfer, so a connection with a bandwidth of 8 megabits per second moves 1 megabyte per second. The byte is the unit of data storage, so 8 GB of capacity holds 8 gigabytes of information, which is 64 gigabits.

That difference is behind the most common complaint about internet speed. A 100 Mbit/s connection downloads at about 12.5 MB/s, and a download manager reporting 12 MB/s on a 100 Mbit line is performing exactly as advertised. The factor of eight is not a shortfall.

The convention is worth being careful with in writing: a lower-case b is bits and an upper-case B is bytes, so Mb/s and MB/s differ by a factor of eight.

Decimal and binary units, and why your drive looks small

There is a second factor-of-something problem that catches more people than the bit-byte one. Storage and network figures use decimal prefixes, where a megabyte is 1,000,000 bytes and a gigabyte is 1,000,000,000. Operating systems have historically used binary ones, where the same prefixes mean 1,048,576 and 1,073,741,824.

The gap grows with each step: 2.4% at kilo, 4.9% at mega, 7.4% at giga and 10.0% at tera. This is exactly why a drive sold as 500 GB shows up as about 465 GB once formatted, and why a 1 TB drive shows about 931 GB. Nothing is missing; the two figures are counting in different bases.

The unambiguous binary units are the kibibyte, mebibyte, gibibyte and tebibyte, written KiB, MiB, GiB and TiB. The calculators on this page work in decimal units throughout, which is the convention for network bandwidth and for the way storage is sold.

Why a transfer never hits the advertised rate

The download calculator gives a best case, and real transfers come in slower for several reasons that stack.

Protocol overhead. Every packet carries TCP and IP headers, and the receiver sends acknowledgements back. On a typical connection that costs a few percent of the raw rate before anything else happens.

The slowest link. End-to-end throughput is limited by the narrowest point on the path, which is frequently the server or a peering link rather than your own connection.

Latency and window size. On a long path TCP can be limited by how much data it is allowed to have in flight rather than by bandwidth at all. A high-latency satellite link can be slow to download a single file while still having plenty of capacity.

Contention. A shared medium, whether that is a cable segment, a Wi-Fi channel or a mobile cell, divides its capacity among everyone using it at that moment.

As a rule of thumb, take about 90% of the advertised rate for a realistic single-stream transfer, and rather less over Wi-Fi at distance.

Estimating what a website needs

The website calculator multiplies page views by average page size and spreads the result over the period. Take 5,000 views a day at 500 KB a page: that is 2.5 GB a day, which works out at 0.2314814815 Mbit/s sustained, or 76.09 GB a month.

Three things make real usage higher than that arithmetic suggests.

Bots. Search engine crawlers, uptime monitors, scrapers and security scanners all fetch pages, and on a small site they frequently generate more requests than human visitors do. They belong in the page-view figure.

Average page size is usually underestimated. A page view is not one file. It is the HTML plus stylesheets, scripts, fonts and every image, and a modern page commonly runs to two or three megabytes rather than 500 KB. Measure it in a browser's network panel rather than guessing.

Traffic is not flat. Averaging a day's traffic over 86,400 seconds understates the peak considerably. Most sites see a large share of their traffic in a few hours, so the busiest minute can be several times the average. That is what the redundancy factor is for: a factor of 2 doubles both the rate and the monthly figure, giving headroom for peaks and growth.

Caching pulls in the other direction. A CDN or browser caching serves repeat requests without touching your server, so the bandwidth your host actually bills can be well below the calculated total.

Monthly allowances and sustained rates

Hosting is usually sold as a monthly transfer allowance while connections are sold as a rate, and the fourth tool converts between them. A month here is 30.4375 days, which is a year divided by twelve, or 2,629,800 seconds.

On that basis 1,000 GB a month is 3.0420564301468 Mbit/s sustained. Read the other way, a continuously saturated 1 Mbit/s link moves about 329 GB in a month, which is a useful figure to keep in mind when a host advertises unlimited bandwidth on a shared line.

Common internet connection bandwidth

Mobile broadband connection bandwidth

Mobile figures are peak theoretical rates for the technology rather than what a handset sees in practice, which depends on signal strength, cell load and how far away the mast is.

How much bandwidth do you actually need

It depends entirely on what you plan to do. Browsing and email need very little. Streaming and hosting large amounts of video need far more, and the requirement is per simultaneous stream rather than per household.

Streaming video is the usual driver: standard definition needs a few megabits, high definition around 5 to 8, and 4K commonly 15 to 25 depending on the codec. Video calls sit in the low megabits each way. Online gaming needs surprisingly little bandwidth but is sensitive to latency and packet loss, which bandwidth figures say nothing about.

Add up the simultaneous streams a household expects rather than sizing for one, and remember that upload matters as much as download for calls, backups and anything hosted at home.

Common mistakes

The first is confusing bits with bytes. A 100 Mbit/s connection gives about 12.5 MB/s, and the eight-fold difference is the single most frequent source of confusion in this whole subject.

The second is comparing a decimal figure with a binary one. A 500 GB drive showing 465 GB has lost nothing.

The third is sizing a website's bandwidth from the daily average. Peaks are what saturate a connection, which is what the redundancy factor exists to cover.

The fourth is using 500 KB for average page size out of habit. Measure your own; it is very often several times that.