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Why does your 1TB drive only show about 931GB? The answer involves math, marketing and two different ways computers and manufacturers count bytes. Once you know the difference between decimal (1000) and binary (1024) counting, the mystery clears up. This guide explains the KB vs KiB naming, gives quick mental math tricks, shows common file sizes (photos, videos, documents) and touches on how storage changed over time. I’ll also cover cloud storage quirks and why advertised capacity differs from what your OS reports.
1Why a 1TB Drive Often Shows ~931GB
When manufacturers sell a "1TB" drive they usually mean 1,000,000,000,000 bytes (decimal). Most operating systems report sizes using powers of 1024, so they divide bytes by 1024^3 to show gigabytes. That mismatch is the main reason a 1TB (decimal) drive looks like about 931GB in your OS. The real reason this matters is that two different counting systems are in play: one for marketing and one for many OS displays. Knowing which one your device or software uses avoids surprises when you copy large backups or install software.
How the numbers line up
1 decimal TB = 1,000,000,000,000 bytes. If you divide that by 1024^3 (the number OSes often use to show GB) you get about 931.32. Manufacturers count in thousands; many operating systems (and older tools) count in powers of two.
Quick mental trick
To estimate: multiply a decimal TB value by 0.93 to get the approximate GB your OS will show. Example: 1 TB ≈ 0.93 × 1000 GB = 930–931 GB.
2Binary (1024) vs Decimal (1000): The Confusion
Computers natively work with powers of two. Historically, file systems and many tools used 1024 (2^10) as the step between KB → MB → GB. Manufacturers, however, prefer decimal for cleaner numbers and marketability. That creates the 1000 vs 1024 mismatch readers often see.
Where 1024 comes from
1024 = 2^10. For a long time, tech people casually called 1024 bytes a 'kilobyte' because it was close to 1000. That shorthand became widespread, especially in operating systems and file system tools.
Decimal marketing numbers
Saying '1 TB' as 1,000,000,000,000 bytes looks nicer on the box than 931 GB. It’s a simple labeling choice but one that leads to confusion when you plug the drive into a computer.
3KB vs KiB — The Technical Difference
To reduce ambiguity, standards bodies introduced distinct names: KiB (kibibyte), MiB (mebibyte), GiB (gibibyte) use powers of 1024. KB, MB, GB in strict SI usage mean 1000-based units. Many apps still mix the terms, so it's useful to recognize both.
Where the KiB names come from
IEC introduced binary prefixes like kibibyte (KiB) in the late 1990s to make the difference explicit. Official standards and some modern tools use KiB/MiB/GiB when they mean 1024-based units.
What your OS and apps do
Windows historically showed sizes using decimal labels but binary math; macOS switched over time and often shows decimal units. Many Linux tools (ls, du) use binary or provide flags. Check a tool's documentation if exact meaning matters.
4Common File Sizes: Practical Examples
Knowing typical file sizes helps plan backups and choose drives. Below are approximate ranges for common files in 2020s consumer devices. Real sizes vary with resolution, compression and format.
Photos and documents
Smartphone photos: 2–8 MB each (compressed JPEG). Scans and high-quality RAW images: 20–50+ MB. PDFs and Word documents: often under 1–5 MB unless they include many images.
Audio and video
MP3 songs (~3–5 minutes): ~3–10 MB at typical bitrates. Streaming-quality video: mobile SD ~500 MB/hour, 720p ~1–2 GB/hour, 1080p ~3–5 GB/hour, 4K can be 10–20+ GB/hour depending on codec and bitrate.
Large uses: backups and games
Full system backups, game installs and VM images can be tens to hundreds of GB. Modern AAA game installs commonly eat 50–150 GB per title; some reach 200+ GB.
5Storage Capacity Over Time and Moore's Law
Storage density has risen dramatically, much like processing power grew under Moore's observation. While Moore's Law originally described transistor counts, storage improvements have followed their own curve: areal density, new materials and multi-layer flash have increased capacity while lowering price per byte.
A brief history
Hard drives in the 1980s had megabytes; by the 2000s consumer drives reached hundreds of gigabytes. Solid-state drives (SSDs) and advances in flash memory then accelerated adoption of terabyte-class drives for consumers in the 2010s.
What to expect next
Manufacturers push higher capacities and lower costs, but practical limits (error rates, wear, heat) shape product choices. For most users, the change means more affordable terabyte drives and larger cloud plans.
6Cloud Storage, Advertised Capacity and Real-World Use
Cloud providers and device makers use different conventions, and extra space is used for system data or redundancy. Also, file system overhead, reserved sectors and pre-installed software reduce usable capacity from the raw advertised bytes.
Cloud provider quirks
Cloud plans usually advertise decimal sizes (GB/TB). But what you actually get can depend on how the provider measures usage (binary vs decimal), whether they count snapshots, and if redundancy (replication) is considered separate from user-available space.
Other reasons capacity looks smaller
File systems reserve space for metadata and performance; some devices reserve a portion for firmware. Preinstalled recovery partitions, the file system block size, and hidden system files all reduce the space shown as 'available.'
A famous unit-mismatch mistake
Unit mismatches have caused big problems in tech history. The Mars Climate Orbiter was lost in 1999 because two teams used different units. That story is a reminder: consistent units matter, whether for space missions or counting bytes.
Pro Tips
- 1Quick conversion: 1 decimal TB ≈ 0.931 × 1024-based GB (so 1,000,000,000,000 bytes ≈ 931.32 GiB)
- 2Remember the prefixes: KB/MB/GB often mean 1000-based; KiB/MiB/GiB mean 1024-based
- 3Estimate photo storage: plan ~5 MB per smartphone photo (JPEG), ~25–50 MB for RAW
- 4When buying drives, expect usable space to be lower due to file system and reserved areas
- 5For cloud storage, read whether the provider measures usage in decimal or binary and how snapshots are counted
The 931GB mystery is mostly a labeling and counting issue: manufacturers use decimal (1000) for marketing while many tools and OSes report sizes using binary (1024). Once you know which system you’re looking at, storage math becomes straightforward. Try the converters on this site to switch between bytes, KiB, MB and TB depending on whether you want decimal or binary results. When buying or provisioning storage, check how the vendor counts units and factor in system overhead and backups.
