Download Time Calculator
How long a file really takes, with the megabit and megabyte confusion sorted out.
In short
Divide the file size in megabytes by the connection speed in megabytes per second. Connections are advertised in megabits, and there are eight bits in a byte, so a hundred megabit connection delivers about twelve and a half megabytes per second at best.
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Runs entirely in your browser. Nothing you type here is uploaded, stored or logged.
The bits and bytes problem
This single confusion accounts for most of the gap between what people expect and what they get.
Internet connections are sold in megabits per second. Files, download managers and operating systems all work in megabytes. There are eight bits in a byte.
100 Mbps / 8 = 12.5 MB per secondSo a 100 Mbps connection downloads a 1 GB file in about 82 seconds at absolute best, not the 10 seconds the number superficially suggests. Nobody is being deceived exactly, but the choice of unit is not accidental either: megabits produce a larger number for the same speed.
Why even that is optimistic
The theoretical figure assumes you receive your full advertised speed with zero overhead. Several things prevent that:
- Protocol overhead. TCP/IP headers, acknowledgements and retransmissions typically consume 5 to 10%.
- The server. You cannot download faster than the other end will send, and popular files are often rate limited.
- Wifi. Advertised speed is to your router, not across your house. Walls, distance and interference all reduce it, often dramatically.
- Contention. Home connections are shared with your neighbours, and evenings are slower than mornings.
- Everything else on your network. Streaming, backups and updates all take a share.
- Disk speed. Rarely a limit now, but a slow external drive can bottleneck a fast connection.
The realistic figure here applies a discount for these. It is a rule of thumb rather than a measurement, and on a well-configured wired connection you will do better.
Typical file sizes
| What | Roughly |
|---|---|
| Photo from a phone | 3 to 5 MB |
| Song, high quality | 10 MB |
| Album | 100 MB |
| Hour of standard definition video | 700 MB |
| Hour of 1080p video | 2 to 3 GB |
| Hour of 4K video | 7 to 10 GB |
| Large game | 50 to 150 GB |
| Operating system update | 3 to 6 GB |
What speed you actually need
Less than most people are sold. Streaming is the usual driver, and it needs surprisingly little:
- Standard definition video: about 3 Mbps
- 1080p: about 5 to 8 Mbps
- 4K: about 25 Mbps
- Video call: 2 to 4 Mbps
- Online gaming: under 5 Mbps, but latency matters far more than bandwidth
A household streaming three 4K films simultaneously needs about 75 Mbps. Beyond roughly 200 Mbps, most people are paying for a number rather than an experience, and the money is usually better spent on a decent router or a wired connection to wherever it matters.
Latency is not speed
Bandwidth is how much data flows per second. Latency is how long a single packet takes to get there and back.
For downloading, bandwidth dominates. For video calls, gaming and how responsive browsing feels, latency matters far more. A 1 Gbps satellite connection with 600 ms of latency is worse for a call than a 20 Mbps fibre line with 10 ms.
Which is why "upgrade your speed" is often the wrong answer to a connection that feels slow. Test the latency before paying for more bandwidth.
Frequently asked questions
Why is my download slower than my connection speed suggests?
Mostly because connections are sold in megabits and files are measured in megabytes, and there are eight bits in a byte. A 100 Mbps connection delivers about 12.5 MB per second at absolute best. Protocol overhead, the server and your wifi take more.
What is the difference between Mbps and MBps?
The capital B. Mbps is megabits per second, which is how connections are advertised. MBps is megabytes per second, which is what download managers display. Divide megabits by eight to get megabytes.
How realistic is the estimate?
The theoretical figure assumes you get your full advertised speed with no overhead, which never happens. The realistic figure applies a discount for protocol overhead and typical conditions, and is usually much closer to what you will actually see.
Why does wifi make such a difference?
Because advertised speeds are for the connection into your home, not across your house. Wifi is shared, affected by walls and interference, and the speed drops with distance from the router. A gigabit connection over poor wifi can deliver less than a tenth of it.
Does upload speed matter?
For downloading, barely. It matters for video calls, sending large files, cloud backup and online gaming. Many home connections are heavily asymmetric, with upload a fraction of download, which is why sending a video takes far longer than receiving one.