Why use this calculator
Almost everyone who has ever complained that their internet is slower than advertised has run into the same arithmetic rather than a fault. Connection speeds are sold in megabits per second and file sizes are measured in megabytes, and there are eight bits in a byte, so a 100 megabit per second connection downloads at about 12.5 megabytes per second at absolute best.
The download manager showing 11 MB/s on a 100 Mbps plan is performing exactly as promised. This calculator does the conversion properly, applies a realistic efficiency factor for protocol overhead and congestion, and shows what happens when several transfers share the line, so you can tell the difference between a slow connection and a misunderstanding.
How to use it
- Enter the file size exactly as your operating system reports it and choose the matching unit, since storage is always quoted in bytes.
- Then enter your connection speed from your plan or from a speed test, and choose its unit carefully, watching the capitalisation: Mbps means megabits per second and is what plans are sold in, while MB/s means megabytes per second and is eight times larger.
- Set a realistic efficiency, where eighty-five per cent is a fair default for a wired connection and sixty to seventy-five is more honest over Wi-Fi or at peak evening hours.
- Finally, say how many transfers are running at once, since they share the same pipe.
What each result means
- Transfer time
- A realistic estimate including overhead and sharing. Expect variation, because real throughput fluctuates with congestion and server capacity.
- At the full advertised speed
- The theoretical floor with perfect conditions and no overhead. No real transfer achieves this, so treat it as a benchmark rather than a target.
- Time lost to overhead
- The difference between theory and reality. Protocol headers, retransmissions and congestion all consume capacity that never carries your file.
- Effective throughput
- What your browser or download manager will display, in megabytes per second. Compare this against what you actually see to spot a genuine problem.
- File size in bits
- The file multiplied by eight. This single conversion explains almost every complaint that a connection is slower than advertised.
- File size in bytes
- The size as your operating system reports it. Storage has always been measured in bytes, unlike bandwidth.
- Connection speed in bytes
- Your plan speed divided by eight, which is the only form directly comparable to a file size. A 100 Mbps plan is 12.5 MB/s at best.
- Data transferred per hour
- Useful for checking a transfer against a monthly data cap or for planning an overnight backup window.
The formula
File size in bits = size multiplied by its unit factor in bytes, multiplied by 8
Connection speed in bits per second comes from the unit chosen, where Mbps multiplies by one million, Gbps by one billion, and MB/s by one million then by 8.
Effective speed = advertised bits per second multiplied by the efficiency percentage, then divided by the number of simultaneous transfers
Transfer time in seconds = file size in bits divided by that effective per-stream speed
Effective throughput in megabytes per second = effective bits per second divided by 8 million
How this number is used in practice
The bits and bytes distinction is not a marketing trick so much as an accident of history: telecommunications has measured capacity in bits per second since the telegraph, while computing has measured storage in bytes since the byte was standardised at eight bits. Both conventions are older than home broadband and neither is going to change.
The practical rule is simply to divide any advertised speed by eight to get the megabytes per second you will actually see, and anyone who remembers that rule stops being surprised by download managers permanently.
Real throughput rarely reaches the advertised figure and the reasons stack up. TCP and IP headers consume a few per cent before anything else happens. Wi-Fi typically delivers between fifty and eighty per cent of a wired connection depending on distance, interference and standard. Evening congestion in the local network reduces it further, which is why regulators in several countries now require providers to publish typical busy-period speeds rather than theoretical maximums.
Beyond about a hundred megabits the bottleneck also moves: the server you are downloading from, or the disk you are writing to, frequently becomes the limit rather than your connection.
Upload speed is the number that catches people out, because most residential connections are asymmetric by design. A plan advertised as a hundred megabits down may offer only twenty up, which means sending a large video file takes five times longer than receiving one of the same size.
Anyone working with video, running backups to cloud storage or hosting video calls should check the upstream figure specifically, since it is the one that determines whether those tasks are comfortable or painful.
Worked examples
A 25 GB game download on a 100 Mbps connection at 85 per cent efficiency
About 39 minutes, against 33 min 20 s at the full advertised rate
A 4 GB video uploaded on a 20 Mbps upstream link over Wi-Fi at 70 per cent
Roughly 38 minutes, since upload speeds are typically a fraction of download
A 2 TB backup over a gigabit link with four transfers running at once
20 hr 54 min for this file, because four transfers each get a quarter of the link
When people use it
- Estimating how long a large game, film or dataset will take before starting it
- Checking whether the speed your download manager reports matches what your plan should deliver
- Planning an overnight backup or upload window for large media files
- Deciding whether a faster internet plan is worth the extra cost for your actual file sizes
- Working out whether a transfer will fit inside a monthly data allowance
Tips
- Divide any advertised speed by eight to get megabytes per second. A 100 Mbps plan tops out near 12.5 MB/s.
- Watch the capitalisation. Mbps is megabits and MB/s is megabytes, and the difference is a factor of eight.
- Check your upload speed separately. Most residential plans are asymmetric and upstream is often a fifth of downstream.
- Use a wired connection for very large transfers. Wi-Fi typically delivers 50 to 80 per cent of what the cable would.
- Past about 100 Mbps the server or your disk is often the bottleneck, so a faster plan may change nothing.
- Run a speed test at the time of day you actually transfer. Evening congestion can halve what a midday test reports.
What it assumes, and where it stops being accurate
- Uses decimal units, so 1 GB is one billion bytes. Operating systems that report in binary gibibytes will show a slightly smaller figure for the same file.
- Assumes the connection is the bottleneck. Above roughly 100 Mbps the source server or local disk is frequently the real limit.
- The efficiency percentage is a single blunt factor covering protocol overhead, Wi-Fi losses and congestion, all of which vary continuously.
- Assumes a steady rate for the whole transfer. Real transfers ramp up, fluctuate and often slow markedly during peak evening hours.
- Simultaneous transfers are assumed to share bandwidth evenly, which is approximately but not exactly how congestion control behaves.
- Ignores latency, which does not affect large file transfer times materially but dominates the responsiveness of interactive use.
Terms used on this page
- Mbps
- Megabits per second, the unit internet plans are advertised in.
- MB/s
- Megabytes per second, the unit file transfers display; eight times larger than Mbps.
- Throughput
- The data rate actually achieved, as opposed to the advertised link speed.
- Asymmetric connection
- A link with different download and upload speeds, typical of residential broadband.
- Protocol overhead
- Capacity consumed by packet headers and control traffic rather than your file.
Questions people ask
- Why is my download slower than my plan speed?
- Usually because plans are sold in megabits and downloads are shown in megabytes, and there are eight bits in a byte. A 100 Mbps plan gives about 12.5 MB/s at best. Protocol overhead, Wi-Fi losses and congestion reduce it further.
- What is the difference between Mbps and MB/s?
- Mbps is megabits per second, the unit internet plans are sold in. MB/s is megabytes per second, the unit file transfers are displayed in. One megabyte is eight megabits, so MB/s is always eight times smaller than the Mbps figure.
- What efficiency should I use?
- Around 85 per cent for a wired connection under normal conditions, 60 to 75 per cent over Wi-Fi or during peak evening hours, and higher than 90 only on a well-provisioned wired link to a fast server. Run a speed test to calibrate your own figure.
- Why is uploading so much slower?
- Most residential connections are asymmetric, allocating far more capacity to download than upload because that matches typical use. A plan advertised at 100 megabits down may provide only 20 up, so sending a file takes roughly five times longer than receiving it.
- Will a faster plan make my downloads faster?
- Only while your connection is the bottleneck. Above roughly 100 Mbps the limit is often the server you are downloading from or the disk you are writing to, so doubling your plan speed can produce no measurable improvement on everyday transfers.
- Does this work for uploads too?
- Yes. Enter your upload speed instead of your download speed and the calculation is identical. Just check your plan's upstream figure specifically, because it is usually much lower than the headline download number that gets advertised.
Use this with
Download Time Calculator answers one part of the question. These pick up where it stops: Data Storage Converter, Cloud Cost Comparison Calculator, IPv4 Subnet Calculator and IT Downtime Cost Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- Australian Competition and Consumer Commission: broadband performance. Reporting on typical busy-period speeds against advertised plans
- US Federal Communications Commission: Measuring Broadband America. Measured broadband performance against advertised rates
- Ofcom: broadband speeds research. UK measurement of actual against advertised broadband speeds
Download Time Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.