Digital Storage Unit Guide: Accurate GB, MB, TB Conversion for US Devices
Understand GB, MB, and TB conversions for US devices. Learn why 1 GB is 1000 MB (decimal) but 1 GiB is 1024 MiB (binary), how to convert storage correctly, and why your drive shows less space than the label.
Why Digital Storage Units Confuse Everyone
A 1 TB hard drive shows up as roughly 931 GB in Windows. Your phone says 128 GB but the system reports 119.2 GB. This is not a defect — it is the difference between two definitions of the prefixes kilo, mega, giga, and tera: decimal (base 10) and binary (base 2).
Drive manufacturers advertise capacity using decimal units, where 1 GB = 1000 MB. Operating systems and memory report using binary units, where 1 GiB = 1024 MiB. The abbreviation confusion (GB vs GiB) is the root of the perceived "missing" space.
Decimal vs Binary: The Two Systems
Decimal (SI prefixes, IEC 80000-13): 1 kB = 1000 B, 1 MB = 1000 kB, 1 GB = 1000 MB, 1 TB = 1000 GB. This is how storage vendors label drives and how file sizes are usually shown in file managers for smaller files.
Binary (IEC prefixes): 1 KiB = 1024 B, 1 MiB = 1024 KiB, 1 GiB = 1024 MiB, 1 TiB = 1024 GiB. This is how RAM, operating system storage reporting, and many technical tools count.
The practical rule: if you see "GB" on a drive box, it is decimal. If Windows shows "GB" for the same drive, it is really counting GiB. The ratio between them is 1000³ / 1024³ ≈ 0.9313.
Exact Conversion Factors
Within the decimal system: 1 TB = 1000 GB, 1 GB = 1000 MB, 1 MB = 1000 kB. Within the binary system: 1 TiB = 1024 GiB, 1 GiB = 1024 MiB, 1 MiB = 1024 KiB.
To convert decimal to binary for the "why is my drive smaller" question: a 1 TB (decimal) drive contains 1,000,000,000,000 bytes. Divide by 1024³ to get 931.32 GiB — the number Windows displays.
Use our GB to TB converter for decimal conversions and the GB to TB guide for worked examples. For MB/GB, see MB to GB converter.
Quick Reference Table
Common decimal conversions used when buying and managing storage:
Storage Capacity Quick Table
1 GB = 1000 MB (decimal) or 1024 MiB (binary)
1 TB = 1000 GB (decimal) or 1024 GiB (binary)
1 TB drive shows ~931 GiB in Windows
512 GB drive shows ~476 GiB in Windows
256 GB drive shows ~238 GiB in Windows
128 GB phone shows ~119 GiB usable
For phone and laptop memory, binary units are used; see our KB to MB guide for the smallest prefixes.
How to Avoid Storage Confusion
When buying a drive, expect the operating system to report about 7% less than the label (0.9313 × advertised GB). This is normal for every manufacturer and not a sign of hidden files or defects.
When converting for file size limits (email attachments, cloud uploads), use decimal: a 25 MB attachment limit means 25,000,000 bytes. When calculating RAM or comparing with technical documentation, use binary.
Never mix the two systems in one calculation. If a spec sheet says GiB, keep everything in binary; if it says GB and is consumer storage, keep everything in decimal. Our MB to GB guide walks through both with examples.
Frequently Asked Questions
How many MB is 1 GB?
In decimal terms, 1 GB = 1000 MB. In binary terms, 1 GiB = 1024 MiB. Drive labels use decimal; operating systems often report binary.
Why does my 1 TB drive only show 931 GB?
The drive contains 1,000,000,000,000 bytes (decimal TB). Windows reports in GiB: 1,000,000,000,000 ÷ 1024³ = 931.32 GiB. This is normal.
What is the difference between GB and GiB?
GB is decimal (10⁹ bytes); GiB is binary (2³⁰ bytes). 1 GB = 1000³ bytes; 1 GiB = 1024³ bytes. GiB is about 7% larger.
How do I convert TB to GB?
Multiply by 1000 for decimal (1 TB = 1000 GB) or by 1024 for binary (1 TiB = 1024 GiB), depending on the system.
Is 512 GB a lot of storage?
For most users, yes: 512 GB holds roughly 100+ full HD movies or tens of thousands of photos. It shows as about 476 GiB in Windows.
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Quick Summary
Digital storage uses two systems: decimal (1 GB = 1000 MB, used by drive labels) and binary (1 GiB = 1024 MiB, used by operating systems). A 1 TB drive shows about 931 GiB in Windows. Match the system to the source to avoid the "missing space" confusion.
Related Guide
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KB to MB Conversion Guide for US Email, Images, and Small Files
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Disclaimer
Unit conversions on this site use internationally recognized, fixed conversion factors (for example, 1 inch = 2.54 cm, 1 pound = 0.45359237 kg, 1 °F = (°C × 9/5) + 32). They are provided for general, informational, and educational use. For applications where accuracy is critical — such as medical dosing, engineering, scientific research, trade, or legal compliance — always verify results against authoritative sources such as the U.S. National Institute of Standards and Technology (NIST) and the official International System of Units (SI) brochure, and follow any applicable local standards.
Exact Constants & Authoritative Sources
Every conversion on this page uses the fixed, internationally agreed constants below — drawn from NIST and the SI Brochure. No values are estimated or rounded as if exact. See the full list on our Sources & Standards page.
- EXACT1 kibibyte (KiB) = 1024 bytes (exact, binary, IEC)
- EXACT1 kilobyte (kB, decimal) = 1000 bytes (exact, SI)
- EXACT1 mebibyte (MiB) = 1,048,576 bytes; 1 megabyte (MB) = 1,000,000 bytes
- EXACT1 gibibyte (GiB) = 1,073,741,824 bytes; 1 gigabyte (GB) = 1,000,000,000 bytes
Common Pitfalls & How to Avoid Them
GB vs. GiB (the "missing" storage space)
Operating systems label gibibytes as "GB". A "500 GB" drive holds 500 × 10⁹ bytes = 465.66 GiB, which Windows shows as ~465 GB. The space is not "lost" — decimal vs. binary units differ.
kB vs. KB capitalization
Strictly, kB = 1000 bytes (SI) and KB is sometimes used for 1024 (binary). To avoid ambiguity use KiB/MiB/GiB for binary and kB/MB/GB for decimal.
A unit-conversion reference site has no manufacturer-specific or device calibration parameters, so "layered tuning" does not apply here. The only real error-handling is input validation: the converter rejects non-numeric, empty, or out-of-range entries and shows a clear message, then falls back to exact constants above. No estimated or rounded figures are ever presented as exact.
Authoritative sources for this page
- IEC (International Electrotechnical Commission) — IEC 80000-13 — Quantities and units for information science and technology (binary prefixes)
Last Updated 2026 | This article was reviewed for accuracy