Bits vs. Bytes: How to Estimate Download Time

Internet plans are sold in megabits per second, but files are measured in megabytes. Confusing the two is why downloads take about eight times longer than people expect.

Bits and bytes

• 1 byte = 8 bits.
• Connection speeds use a lowercase b for bits: Mbps, Gbps.
• File sizes use an uppercase B for bytes: MB, GB.

So a 100 Mbps connection moves at most 100 ÷ 8 = 12.5 MB per second.

The download time formula

A 4 GB game (4,000 MB) on a 100 Mbps connection: 4,000 × 8 ÷ 100 = 320 seconds, or about 5 minutes 20 seconds at full speed.

Real speeds are lower

Network overhead, Wi-Fi interference, busy servers and other devices on your network all eat into the speed. Plan for some headroom: real downloads usually run below the advertised rate, especially over Wi-Fi.

Decimal vs. binary units

Storage makers use decimal units: 1 GB = 1,000,000,000 bytes. Many operating systems count in binary, where 1 GiB = 1,073,741,824 bytes, but still label it "GB". That's why a "1 TB" drive shows up as about 931 GB on a computer. Nothing is missing; it's a different unit.

• 1 KiB = 1,024 bytes
• 1 MiB = 1,024 KiB
• 1 GiB = 1,024 MiB

Upload speed

Many home connections are asymmetric: upload can be much slower than download. Backing up a large video folder uses the upload speed, so check that figure too.

Big numbers

At data-center scale the numbers grow quickly. Scientific notation keeps them manageable: a terabyte is 10^12 bytes and a petabyte is 10^15 bytes.