Bits vs Bytes: Why Your Download Estimate Is Off by 8×

"My connection is 100 Mbps but this 1 GB file is taking forever." The confusion almost always comes down to one factor: 8.

Bits vs bytes

• Network speeds are measured in bits per second — Mbps (megabits per second).
• File sizes are measured in bytes — MB (megabytes).
• There are 8 bits in a byte.

So a 100 Mbps connection transfers at most 100 ÷ 8 = 12.5 MB/s. A 1 GB (1000 MB) file therefore takes about 80 seconds at full speed — not 10.

The notation is a clue: lowercase "b" means bits (Mbps, Mb/s), uppercase "B" means bytes (MB/s). ISPs quote the big-sounding bits number; your download manager shows the smaller bytes number.

SI vs binary units

A second, smaller gotcha: does "GB" mean 1,000,000,000 bytes or 1,073,741,824?

• SI: 1 KB = 1,000 bytes, 1 GB = 1,000,000,000 bytes.
• Binary: 1 KiB = 1,024 bytes, 1 GiB = 1,073,741,824 bytes.

Storage marketing uses SI; operating systems often report binary. The difference is about 7% at the GB scale — enough to matter for precise estimates.

Real transfers are slower

Even with the math right, real downloads are slower than the theoretical maximum because of TCP overhead, latency, congestion, and disk write speed. Treat the calculation as a best-case ballpark.

Do the math correctly

The Bandwidth Calculator handles both traps: pick your size unit (SI or binary) and your speed unit (bits or bytes), and it applies the ×8 conversion where needed and shows the estimated transfer time.