Why a 500 GB drive shows 465 GB

Nothing is missing, and nobody cheated you. The drive maker and your operating system count in different units, and both write them as "GB". Once you know which unit each side uses, drive capacities, upload limits, download times and video sizes all start adding up.

Two kinds of "kilo"

• Decimal (SI): 1 KB = 1,000 bytes, 1 MB = 1,000 KB, 1 GB = 1,000 MB, 1 TB = 1,000 GB. Storage is sold in these units, network speeds use them, and "kilo" means a thousand everywhere else in life.
• Binary (IEC): 1 KiB = 1,024 bytes, 1 MiB = 1,024 KiB, 1 GiB = 1,024 MiB, 1 TiB = 1,024 GiB. Computers address memory in powers of two, so much software counts this way, usually while still writing "KB", "MB" and "GB".

The names KiB, MiB and GiB were introduced in 1998 to end the confusion. They are used in some places and ignored in most.

Run the numbers for a 500 GB drive: 500,000,000,000 bytes ÷ 1024³ = 465.66 GiB. The gap grows with each step up, because each step multiplies the difference again:

• kilo: 1 KiB is 2.4% larger than 1 KB
• mega: 4.9%
• giga: 7.4%
• tera: about 10%

So a 1 TB drive shows as about 931 GB in Windows, and a 2 TB drive as about 1.82 TB.

The converter has both families in its Data category. Type the number printed on the box in GB and convert to GiB to see what your computer will report.

Who counts how

• Drive and memory card makers: decimal. A "256 GB" card holds 256 billion bytes.
• Windows: binary, labelled KB, MB, GB. This is where the 465 GB figure comes from.
• macOS and iOS: decimal since 2009. That is why the same file looks slightly larger on a Mac than on a Windows PC: the Mac is dividing by 1,000 instead of 1,024.
• Linux: depends on the program. Desktop file managers often use decimal, many command-line tools default to binary.
• RAM: always binary. 16 GB of memory really is 16 GiB.
• Network speeds: decimal and in bits, not bytes.
• Tooly: file sizes and upload limits on this site are calculated in binary units and labelled MB, the same way Windows shows them. A "50 MB" limit here accepts a file Windows shows as 50 MB.

Formatting and system space

The unit difference explains most of the gap, but not all of it. A formatted drive also reserves space for the file system's own bookkeeping. On a phone or a computer, the operating system and preinstalled apps take a further share before you store anything. A "128 GB" phone that shows only around 100 GB free on day one is the system using its share, plus the decimal-to-binary difference; nothing is wrong with it.

Bits and bytes

A byte is 8 bits. Internet speeds are quoted in megabits per second (Mbit/s or Mbps), file sizes in megabytes (MB). Divide by 8 to compare:

• A 100 Mbit/s connection moves at most 12.5 MB per second.
• A 1 GB download over 50 Mbit/s takes at least 1,000 × 8 ÷ 50 = 160 seconds, a bit under three minutes, before protocol overhead.
• Upload speeds are often much lower than download speeds on home connections. A 20 Mbit/s upload sends a 500 MB video in about 200 seconds at best.

Real transfers are a little slower than these figures because of protocol overhead and network conditions, so add 10 to 20%.

Video sizes follow from bitrate

Video files are easiest to reason about through bitrate, which is also quoted in bits per second:

size in MB = bitrate in Mbit/s × duration in seconds ÷ 8

• A one-minute clip at 16 Mbit/s: 16 × 60 ÷ 8 = 120 MB.
• The same minute at 4 Mbit/s: 30 MB.
• To fit a 3-minute video into 50 MB, the total bitrate must stay under about 50 × 8 ÷ 180 = 2.2 Mbit/s, sound included.

This is why phone videos are so large and why compression works so well: a phone records at a high bitrate to be safe, and a chat window does not need it. Compress video re-encodes with H.264 and shows the size before and after, so you can check the result against your limit.

Where this bites in practice

Upload limits

A form that says "max 25 MB" may count in either unit, and you rarely know which. The difference at that size is about 1.2 MB. A file your Windows PC shows as 24.5 MB is 25.7 million bytes; a site counting in decimal megabytes rejects it, a site counting in binary accepts it.

The practical rule: when a file is within 5% of a limit, treat it as too big. Aim for 90% of the stated maximum and both kinds of counting are satisfied. For photos, Compress JPG or resizing gets you there quickly; for scanned documents, Compress PDF.

Email attachments

Attachments are encoded as text before sending, which makes them about a third larger in transit. A provider with a 25 MB message limit therefore accepts attachments totalling roughly 18 to 19 MB, and less if the message has several files or a long body. If an email bounces as too large even though the files add up to "only" 22 MB, this is why. Send a share link from your cloud storage instead, or compress the files first.

Memory cards and USB sticks

Many cards and sticks come formatted as FAT32, which cannot hold a single file of 4 GiB or more, whatever the free space. A long video or a disk image fails to copy with a "file too large" message while the card shows plenty of room. Reformat the card as exFAT, which almost all current devices read, after copying off anything you need.

Cloud storage and phone plans

Storage plans are sold in decimal gigabytes. Mobile data allowances are usually counted in decimal too, but carriers do not always say. Leave a margin rather than planning to the last megabyte.

Quick conversions

• 1 GB = 0.931 GiB; 1 GiB = 1.074 GB.
• 1 TB = 0.909 TiB, or about 931 GiB.
• 1 MB = 0.954 MiB; 1 MiB = 1.049 MB.
• 100 Mbit/s = 12.5 MB/s; 1 Gbit/s = 125 MB/s.
• 1 minute of video at 8 Mbit/s = 60 MB.

Troubleshooting

• "File too large" although it looks smaller than the limit. The site counts in decimal units, or the limit includes encoding overhead. Get the file to 90% of the limit.
• A new drive is smaller than advertised. Check the size in bytes in the drive's properties and divide by 1,000,000,000. It will match the label.
• The same file has different sizes on two computers. One shows decimal units, the other binary. The byte count, shown in the file's properties, is the same.
• Downloads are slower than the plan's speed. The plan is in megabits; divide by 8 for megabytes per second.

Checklist

1. Look at the exact byte count when a limit is close.
2. Divide network speeds by 8 to get megabytes per second.
3. Calculate video size from bitrate × duration ÷ 8.
4. Keep files at 90% of any stated upload limit.
5. Expect email attachments to grow by a third in transit.
6. Use exFAT for cards and sticks that carry files of 4 GB or more.