Metric Prefixes and Data Units: Why Your 500 GB Drive Shows 465 GB
October 21, 2026
Metric prefixes are a system for scaling units by powers of ten: kilo means a thousand, mega a million, giga a billion, and so on. They apply to meters, grams, watts, and also to data. In computing there is a wrinkle: memory is naturally organized in powers of two, so the prefix was used loosely for 1,024 instead of 1,000. The result is the confusing situation in which a 500 GB hard drive shows up as about 465.7 GB in some operating systems.
Understanding the two systems and the difference between bits and bytes explains drive capacity mismatches, internet speeds, and download times.
Skip the math and get your answer instantly:
Open the Weight Converter →The SI Prefixes
The International System of Units defines prefixes as exact powers of ten. Using the same pattern across units makes conversions easy: 1 kilometer is 1,000 meters, 1 milligram is 0.001 grams, 1 megawatt is 1,000,000 watts.
| Prefix | Symbol | Factor | Example |
|---|---|---|---|
| tera | T | 10^12 | 1 terabyte = 1,000,000,000,000 bytes |
| giga | G | 10^9 | 1 gigahertz = 1 billion cycles per second |
| mega | M | 10^6 | 1 megawatt = 1,000,000 watts |
| kilo | k | 10^3 | 1 kilogram = 1,000 grams |
| milli | m | 10^-3 | 1 millimeter = 0.001 meters |
| micro | µ | 10^-6 | 1 microsecond = 0.000001 seconds |
| nano | n | 10^-9 | 1 nanometer = 0.000000001 meters |
Decimal vs. Binary Data Units
Storage makers use decimal units: 1 GB = 1,000,000,000 bytes. Operating systems have long displayed binary units: 1 GiB (gibibyte) = 1,024³ = 1,073,741,824 bytes, though some label it GB. A 500 GB drive holds 500,000,000,000 bytes, which is 500,000,000,000 ÷ 1,073,741,824 = 465.66 GiB. A 1 TB drive shows as 931.3 GiB, and a 256 GB phone shows 238.4 GiB before the system files are even considered.
The IEC introduced binary prefixes (kibi, mebi, gibi, tebi) to resolve the ambiguity. Windows still tends to display binary sizes with decimal labels, while macOS and many other systems use decimal units.
Bits and Bytes
A byte is 8 bits. Internet speeds are typically advertised in megabits per second (Mbps), while file sizes are in megabytes (MB). To convert, divide by 8: a 100 Mbps connection can transfer at most 100 ÷ 8 = 12.5 MB per second. A 1 GB file (8,000 megabits) therefore takes about 80 seconds at 100 Mbps, ignoring overhead, or 8 seconds at 1,000 Mbps.
Case matters: lowercase b is bits and uppercase B is bytes. Mb ≠ MB, and Mbps ≠ MB/s.
Download Time Formula
Time (seconds) = file size in megabytes × 8 ÷ speed in Mbps. A 4 GB (4,000 MB) movie at 50 Mbps takes 4,000 × 8 ÷ 50 = 640 seconds, about 10.7 minutes. Real speeds are usually 5% to 20% lower because of protocol overhead and network conditions, so add a margin.
For uploads, many home connections are slower than their download speed, so check the upload rate separately.
Other Places Prefixes Appear
Processor speeds in gigahertz, camera sensors in megapixels, radio frequencies in megahertz, nutrition labels in milligrams and micrograms, and distances in nanometers. A common mistake is confusing milli (10⁻³) and micro (10⁻⁶): 1,000 micrograms make a milligram. In medicine, a unit mix-up between mg and mcg is a serious error, so always read labels carefully.
Frequently Asked Questions
Why does my 1 TB drive show 931 GB?
Manufacturers count 1 TB as 10¹² bytes, while the system displays binary units where 1 GiB is 1,073,741,824 bytes.
How many MB per second is 100 Mbps?
12.5 MB per second, since you divide megabits by 8.
What is a gibibyte?
A binary unit equal to 1,073,741,824 bytes (1,024³), as opposed to a gigabyte of 10⁹ bytes.
What is the difference between milli and micro?
Milli is one thousandth, micro is one millionth.
How long does a 2 GB download take at 200 Mbps?
About 80 seconds at best: 2,000 × 8 ÷ 200.