Download Time Calculator

What this calculator does

The gap between the speed on your internet package and the speed a download appears to run at confuses almost everyone, and the explanation is simple. Internet packages are sold in megabits per second. Download managers report megabytes per second. There are eight bits in a byte, so a 12 Mbps connection tops out at about 1.5 MB per second even when everything is working perfectly.

This calculator converts between them and gives you a realistic estimate. Enter the file size and your connection speed in whichever unit you have, adjust the efficiency assumption if you want, and it returns the time at full advertised speed and the time you should actually expect.

It also shows how the same file behaves across a range of connection speeds, which is useful when you are deciding whether to wait for a better connection or whether a package upgrade would make a practical difference.

How the calculation works

First everything is converted to common units. File sizes become megabytes, and connection speeds become megabits per second. Dividing the speed in megabits by eight gives megabytes per second, which is the rate at which the file actually arrives.

Dividing the file size in megabytes by that rate gives the time in seconds. That is the theoretical best case.

Real transfers never reach it. Protocol overhead consumes part of the bandwidth, the server at the other end imposes its own limits, other devices on your network compete for capacity, and mobile networks vary with signal and congestion. The efficiency percentage accounts for all of that in one adjustment, and seventy to eighty per cent is a realistic assumption for most connections. On a congested mobile network it can be considerably lower.

The formula

Megabytes per second = Megabits per second ÷ 8

Effective speed = Advertised speed × Efficiency percentage

Download time in seconds = File size in MB ÷ Effective MB per second

Unit conversions: 1 GB = 1024 MB · 1 MB = 1024 KB · 1 byte = 8 bits

A worked example

A 1.4 GB file on a 12 Mbps connection. The file is 1,433 MB. Twelve megabits per second is 1.5 megabytes per second at full speed, so the theoretical time is 955 seconds, or about 16 minutes.

At 75 per cent efficiency the effective rate falls to 1.125 MB per second and the realistic time rises to about 21 minutes. That is the figure worth planning around.

Now the same file on a 50 Mbps connection. Full speed is 6.25 MB per second, effective speed 4.7, and the download completes in about five minutes. Moving from 12 to 50 Mbps cuts a twenty one minute wait to five, which is the kind of comparison that makes an upgrade decision concrete rather than abstract.

Things worth knowing

Bits and bytes differ by eight

A 100 Mbps package delivers at most 12.5 MB per second. If your download manager shows a number roughly one eighth of your package speed, nothing is wrong. That is what the package actually means.

Test your real speed

Advertised speed is a maximum under ideal conditions. Run a speed test at the time of day you normally download, on the device you normally use, and use that figure here instead of the number on your package.

The server matters too

A download can only arrive as fast as the slowest point in the chain. A busy or distant server will limit you well below your connection speed, which is why the same file downloads at very different rates from different sources.

Watch the data cost

On a metered bundle, the file size is deducted from your allowance regardless of how fast it arrives. Before starting a large download, check what it will cost using the Data Usage Calculator.

Common mistakes to avoid

Treating megabits and megabytes as the same unit, which produces an answer eight times out.
Using the advertised package speed rather than a measured real speed.
Forgetting that other devices on the same connection are sharing the bandwidth.
Assuming the server at the other end can deliver at your full speed.
Overlooking the data allowance cost of a large download on a metered plan.

Frequently asked questions

Why is my download slower than my internet speed?

Usually because the package speed is in megabits and the download shows megabytes, a factor of eight. Beyond that, protocol overhead, server limits, network congestion and other devices sharing the connection all reduce the real rate.

How many MB is 1 GB?

1,024 MB using the binary convention that operating systems apply. Some storage manufacturers use 1,000, which is why a drive sold as 1 TB shows slightly less capacity once formatted.

What speed do I need for video calls?

Roughly 1 to 2 Mbps for standard definition and 3 to 4 Mbps for high definition, in each direction. Stability matters more than raw speed for calls, since a consistent 3 Mbps performs better than an erratic 20.

Does a faster package always mean faster downloads?

Only up to the limits imposed elsewhere. If the server, the website or your device is the bottleneck, upgrading your connection changes nothing. Run a speed test first to see whether you are actually reaching your current package limit.

Why do downloads slow down partway through?

Network congestion, server throttling after an initial burst, thermal limits on the device, or another application starting a transfer. On mobile networks, some plans reduce speed after a threshold of usage within a session.