MB to GB Converter

Article — MB to GB Converter

MB to GB: decimal versus binary

There are two valid answers to "how many MB in a GB." In the decimal (SI) standard used by drive manufacturers and internet providers, 1 GB = 1,000 MB. In the binary (IEC 80000-13) standard used by RAM makers and the Windows file-size display, 1 GiB = 1,024 MiB. Both are correct. The two definitions differ by 7.4% at the GB level, which is why a 1 TB drive shows up as 931 GB in Windows Explorer.

The converter above offers both standards with a single toggle. The article below covers why two standards exist, which industries use which, and how to read a Windows drive size that disagrees with the label.

How many MB in a GB

1,000 in the decimal standard. 1,024 in the binary standard. The relationship is fixed in each system but the systems themselves are different. The IEC 80000-13 standard formally defines:

• 1 MB (megabyte, decimal SI) = 10⁶ = 1,000,000 bytes
• 1 GB (gigabyte, decimal SI) = 10⁹ = 1,000,000,000 bytes
• 1 MiB (mebibyte, binary IEC) = 2²⁰ = 1,048,576 bytes
• 1 GiB (gibibyte, binary IEC) = 2³⁰ = 1,073,741,824 bytes
• 1 TB = 10¹² bytes; 1 TiB = 2⁴⁰ bytes

The decimal-to-binary gap is 4.86% at the MB level (1 MiB = 1.0486 MB) and 7.4% at the GB level (1 GiB = 1.0737 GB). It grows wider at each step because the ratio compounds.

The MB-to-GB formulas

Both formulas are simple. The trick is knowing which one applies:

Drive labels and ISP plans use decimal. RAM modules and Windows file sizes use binary. macOS since version 10.6 uses decimal across the operating system. Linux defaults to binary in df -h but offers decimal in df -H. iOS shows decimal storage; Android is inconsistent across versions.

Why two MB-to-GB standards exist

The decimal MB-to-GB convention is the natural one for SI units. The "mega" prefix has meant 10⁶ since the 1870s, when SI was first formalised; "giga" has meant 10⁹. Drive manufacturers, ISPs and any industry doing engineering in SI-compatible units treats megabyte and gigabyte the same way.

The binary convention came from early computing. Random-access memory is built from chip die areas that are powers of 2, so 1,024 bytes is the natural unit. Programmers in the 1960s and 70s started using "K" to mean 1,024 instead of 1,000. The convention spread to "M" = 1,048,576 and "G" = 1,073,741,824. By the 1990s the prefixes meant different things in different industries.

MB to GB for hard drives and SSDs

Every hard drive and SSD on the market is labelled in decimal. A "1 TB" drive contains exactly 10¹² = 1,000,000,000,000 bytes. Windows, however, calculates capacity in binary, dividing by 2³⁰ instead of 10⁹. The same drive then shows up as 1,000,000,000,000 / 1,073,741,824 = 931.32 GiB. Most users see "931 GB" because Windows mislabels the binary value as GB instead of GiB.

This is the source of the long-running "missing space" complaint. The bytes are not missing; the manufacturer used the decimal convention and Windows used the binary one. The class-action lawsuits in the mid-2000s against Western Digital, Seagate and Apple all settled on the same outcome: manufacturers must explain the difference in documentation, but the labelling stays.

Binary MB to GB for RAM and OS

RAM uses binary. A "16 GB" stick of memory really holds 16 GiB = 17.18 × 10⁹ bytes — almost 7% more than a "16 GB" hard drive would. The discrepancy never causes complaints because RAM is sold by its binary value across the entire industry and Windows reports the same number. The labelling lie cancels out.

Operating systems are split. Windows reports binary file sizes labelled as MB and GB. macOS switched to decimal in version 10.6 (2009), so an iCloud storage plan of "200 GB" really is 200 × 10⁹ bytes. Linux gives both via df -h (binary) and df -H (decimal). iOS uses decimal throughout.

MB to GB and internet bandwidth

Internet bandwidth is reported in bits per second (Mbps, Gbps), and the prefixes are decimal. A "100 Mbps" plan means 100 × 10⁶ bits per second = 12.5 megabytes per second. Mobile data plans likewise use decimal: a "1 GB" plan provides 10⁹ bytes of transfer.

The bits-versus-bytes confusion is separate from MB-to-GB. 8 bits = 1 byte. A 100 Mbps line transfers 100/8 = 12.5 MB/s decimal. The full-throughput download of a 1 GB file on a 100 Mbps connection takes 1,000 / 12.5 = 80 seconds, assuming no overhead.

KiB, MiB, GiB: the IEC prefixes

The IEC binary prefixes solve the ambiguity by adding a vowel: kibi for "kilo binary," mebi for "mega binary," gibi for "giga binary." The symbols add an "i": KiB, MiB, GiB, TiB. The values: 2¹⁰, 2²⁰, 2³⁰, 2⁴⁰.

Adoption is partial. Most Linux distributions display KiB and MiB by default. Windows continues to display GB for binary values. Browsers, smartphones, console games and most consumer-facing apps stick with the GB label regardless of which underlying definition they are using. For serious engineering work, the IEC prefixes are the unambiguous choice.

MB-to-GB mistakes to avoid

Assuming 1 GB = 1,024 MB everywhere. The 1,024 ratio is binary only. Drive specs and bandwidth are decimal — 1,000 MB per GB.

Confusing the missing-space symptom with a defect. A "1 TB" drive labelled as 931 GB in Windows has no defect. The bytes are all there; Windows just reports them in binary.

Mixing bits and bytes. 100 Mbps is 12.5 MB/s, not 100 MB/s. The capital B means bytes; lowercase b means bits.

Treating MB and MiB as interchangeable in a technical specification. They differ by 4.86% at the MB level and 7.4% at the GB level. For storage capacity planning, the difference is real money.