Megabits to Megabytes
Convert megabits per second to megabytes per second and back — the eight-fold difference that explains why a 100 Mbps connection downloads at about 12 MB/s.
Divide by 8: bits measure connection speed, bytes measure file size. A "gigabit" (1,000 Mbps) plan downloads at ~125 MB/s.
| Mbps | MB/s |
|---|---|
| 8 | 1 |
| 16 | 2 |
| 25 | 3.125 |
| 50 | 6.25 |
| 100 | 12.5 |
| 200 | 25 |
| 300 | 37.5 |
| 500 | 62.5 |
| 940 | 117.5 |
| 1000 | 125 |
This is one of the very few converters whose arithmetic is exact
Not accurate — exact, with an error of zero. Most conversion factors cannot be stored exactly, because a computer holds numbers in binary and can only represent fractions whose denominators are powers of two. One tenth repeats forever in binary the way one third does in decimal, so 0.1 is stored slightly off and every calculation using it inherits a tiny error. Eight is a power of two, 0.125 is 0.001 in binary, and multiplying by it only shifts the exponent — the digits themselves do not move.
| Sum | What a computer returns | Exact? |
|---|---|---|
| 0.1 + 0.2 | 0.30000000000000004 | no |
| 0.125 + 0.25 | 0.375 | yes |
Converting to megabytes and back across every whole figure from 1 to 2000 Mbps, and every thousandth from 0.001 to 2, returns the input unchanged in all 4,000 cases. Zero failures either way. A grams-to-ounces converter, doing the same test on the same machine, loses the last digit on 96% of whole-gram inputs.
Where it does break, the boundary is exactly 3 decimal places
The boxes show 6 decimal places, and dividing by eight adds exactly 3 of them to any number: 1 becomes 0.125, 0.7 becomes 0.0875, 0.001 becomes 0.000125. So 6 decimals in means 9 out, and 3 of them are gone. The cut-off is not approximate — it lands precisely between the third and fourth decimal place of input.
| Input (Mbps) | True MB/s | Shown | Typed back |
|---|---|---|---|
| 0.001 | 0.000125 | 0.000125 | 0.001 |
| 0.0001 | 0.0000125 | 0.000013 | 0.000104 |
Nobody has a tenth of a kilobit connection, so this costs nothing in practice. It is a good illustration of where converter error actually lives: the constant is usually fine, and the number of boxes on the screen is what bites.
Every megabytes-per-second figure ends one of only eight ways
A consequence of the arithmetic being exact. Divide any whole number of megabits by 8 and the answer has at most 3 decimal places, and its fractional part can only be one of 8 values: .0, .125, .25, .375, .5, .625, .75, .875. Sweeping every whole figure from 1 to 1,000 Mbps produces exactly those and nothing else, because the remainder on division by 8 has only 8 possible values and each maps to one eighth. The .5 and .25 and .125 you keep seeing on speed-test results are not a rounding convention. They are the only endings there are.
Taking 28 speeds people actually buy, from 10 to 10,000 Mbps, 12 give a whole number of megabytes and 16 give a fraction. The clean ones are exactly the multiples of 8.
| Mbps | MB/s | Whole? |
|---|---|---|
| 10 | 1.25 | no |
| 15 | 1.875 | no |
| 20 | 2.5 | no |
| 25 | 3.125 | no |
| 30 | 3.75 | no |
| 40 | 5 | yes |
| 50 | 6.25 | no |
| 60 | 7.5 | no |
| 75 | 9.375 | no |
| 100 | 12.5 | no |
| 150 | 18.75 | no |
| 200 | 25 | yes |
| 250 | 31.25 | no |
| 300 | 37.5 | no |
| 400 | 50 | yes |
| 500 | 62.5 | no |
| 600 | 75 | yes |
| 750 | 93.75 | no |
| 800 | 100 | yes |
| 940 | 117.5 | no |
| 1,000 | 125 | yes |
| 1,200 | 150 | yes |
| 1,500 | 187.5 | no |
| 2,000 | 250 | yes |
| 3,000 | 375 | yes |
| 5,000 | 625 | yes |
| 8,000 | 1,000 | yes |
| 10,000 | 1,250 | yes |
Every multiple of 1,000 Mbps is automatically clean, because 1,000 is 8 × 125. It is tempting to shorten that to “gigabit and above is clean”, and that is wrong: 1,500 Mbps is well above a gigabit, is not a multiple of 1,000, and lands on 187.5. These 28 tiers between them cover seven of the eight endings. The one nobody sells is .625, which would need a speed five above a multiple of 8 — 125 Mbps, or 205, or 445. Marketing picks round decimal numbers, and round decimal numbers are rarely five past a multiple of eight.
How to use
- Enter a speed in megabits or megabytes per second.
- Read the converted rate.
- Note the case: Mb is megabits, MB is megabytes.
- Expect real transfers to run below the theoretical figure.
Frequently asked questions
Why is my 100 Mbps connection only downloading at 12 MB/s?
Because it is not slow — those are the same speed. A byte is eight bits, so 100 megabits per second is 12.5 megabytes per second. Internet plans are advertised in bits and download managers report bytes, which makes every connection look eight times slower than the number you bought.
What is the difference between Mb and MB?
The capital B means bytes; the lower-case b means bits. One byte is eight bits. The distinction is entirely carried by the case of a single letter, which is why it is misread so often, and why network speeds and file sizes seem never to line up.
Why do ISPs advertise in bits?
Partly convention, since network hardware has always been rated in bits per second, and partly because the number is eight times larger. Both explanations are true, and the industry has shown no enthusiasm for changing.
Will I ever hit the advertised speed?
Rarely, and not because of a conspiracy. Protocol overhead consumes a few per cent, and beyond that the limit is usually your wifi, the far server, or congestion between them. Getting 80 to 90 per cent of the rated speed over a wired connection to a fast server is a good result.
How long will a 5 GB download take?
On a 100 Mbps connection, about seven minutes at the theoretical rate — 5 GB is 40,000 megabits, divided by 100. In practice, allow half again for overhead and the fact that the source is rarely as fast as your connection.
What about gigabit?
A 1 Gbps connection is 125 MB/s in theory. In practice, older devices often cannot write to disk that fast, and a single connection frequently will not saturate it — gigabit shows its value most when several devices are busy at once rather than in any one transfer.
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