Internet Speed Calculator

Mbps vs MB/s: the 8x that confuses everyone

Internet speeds are sold in megabits per second. File sizes and download windows are shown in megabytes. There are 8 bits in a byte, so a 100 Mbps connection delivers 100 ÷ 8 = 12.5 MB/s, not 100 MB/s.

That single division by 8 is behind almost every "my internet is broken" moment. A 1 Gbps plan is 125 MB/s, not 1,000 MB/s. A 25 Mbps plan is 3.1 MB/s, which is why a 10 GB game takes over an hour on it.

The calculator lets you enter either unit, so you can paste in the number your ISP advertises or the number your download manager reports and get the same answer. Lowercase b means bits, uppercase B means bytes, if you remember nothing else about broadband marketing, remember that.

How download time is calculated

The arithmetic is one line long. Convert the file size to megabits, divide by the connection speed in Mbps, and you have seconds.

Worked example: a 5 GB file on a 100 Mbps line. First convert to megabits: 5 GB = 5,000 MB = 5,000 x 8 = 40,000 megabits. Then divide: 40,000 ÷ 100 = 400 seconds, or 6 minutes 40 seconds. Add the realistic 8% overhead and you get about 7 minutes 15 seconds.

Second example, going the other way: you have a 25 Mbps plan and a 50 GB backup to upload. 50 GB = 400,000 megabits, and 400,000 ÷ 25 = 16,000 seconds = 4 hours 27 minutes. Upload the same backup on a 1 Gbps symmetric fibre line and it takes 6 minutes 40 seconds, because 400,000 ÷ 1,000 = 400 seconds.

Overhead is not a rounding error you can ignore. Protocol headers, TCP acknowledgements, retransmissions, Wi-Fi contention and the server at the other end all take a bite. Real downloads typically achieve 85 to 95% of the theoretical speed, which is why the calculator defaults to an 8% allowance.

Download times at a glance

The table shows how long the same file takes across the speed tiers US households buy. Times assume a full-speed connection with no overhead.

Two things stand out. The jump from 100 Mbps to 1 Gbps cuts a 50 GB download from over an hour to under 7 minutes, a tenfold speed-up for the same file. And below 50 Mbps, file size starts to dictate behaviour: on a 25 Mbps line you plan downloads around them, on a 1 Gbps line you do not think about them at all.

Worked example: a 60 GB game on 200 Mbps

A modern AAA game is around 60 GB and your plan is 200 Mbps. Step by step:

• 200 Mbps ÷ 8 = 25 MB/s of actual throughput
• 60 GB = 60,000 MB
• 60,000 ÷ 25 = 2,400 seconds = 40 minutes
• Allowing 8% overhead: about 44 minutes

Now change one variable. If the console is on Wi-Fi two rooms away and only gets 60% of the wired speed, the effective rate drops to 15 MB/s and the same download takes 67 minutes. If three people in the house are streaming 4K at the same time, each stream eats 15 Mbps, so 45 Mbps of the 200 disappears and the download stretches past an hour.

That is why the number that matters is not the plan speed but the speed reaching the device. A wired connection to the console, or a mesh node in the same room, often does more for download times than paying for the next tier up.

What speed do you need?

Speed requirements are driven by how many things are happening at once, not by a single task. These are the working numbers:

Add the rows together for your household. Two 4K streams, two HD video calls and a game update running in the background is 15 + 15 + 5 + 5 + 3.5 + 3.5 = 47 Mbps of simultaneous demand, which is why a 50 Mbps plan feels tight and a 100 Mbps plan feels fine. The FCC's consumer guide to broadband explains the 100/20 benchmark and the FTC polices ISPs that advertise speeds they cannot deliver.

Why your speed test result is lower than your plan

You pay for 300 Mbps and the test says 240. That is normal, and it is usually one of five things:

• Overhead. Protocol and Wi-Fi overhead takes 5 to 15% off the top before any data reaches you.
• Wi-Fi versus Ethernet. Wi-Fi 5 on a busy 2.4 GHz band can lose half the speed. Plug in a cable and the number usually jumps.
• Distance and walls. Every wall between you and the router costs signal, and 5 GHz falls off faster than 2.4 GHz even though it is faster up close.
• Other devices. Backups, cloud sync, smart TVs and phones all share one pipe. One device running a 4K stream can knock 15 Mbps off everyone else.
• The server at the other end. A speed test to a nearby server measures your line; a download from a busy server measures the server. Try two or three test servers before blaming the ISP.

If the wired number is well below the plan, gather evidence before you call: run three tests at different times of day, note whether the connection is wired or wireless, and compare against the plan's advertised speed. The FCC broadband consumer guide sets out what your provider has to tell you about real-world performance.