MB to GB Converter

MB to GB: the two conversion rules

There are two correct ways to convert megabytes to gigabytes, and the one you pick depends on who is doing the counting:

• Decimal (SI): 1 GB = 1,000 MB. This is the system drive makers, phone makers and the NIST SI standard use, because the metric prefixes (kilo, mega, giga) always mean powers of 1,000.
• Binary (IEC): 1 GiB = 1,024 MiB. This is what operating systems such as Windows report, because memory is addressed in powers of two. The proper unit names here are mebibyte (MiB) and gibibyte (GiB).

Two worked examples, one in each system:

Example 1 (decimal): a photo library holds 87,500 MB. In gigabytes that is 87,500 / 1,000 = 87.5 GB.

Example 2 (binary): a game install reports 87,500 MiB. In gibibytes that is 87,500 / 1,024 = 85.45 GiB.

Both answers are correct. The gap comes from the divisor (1,000 vs 1,024), not from missing data. The calculator above shows both numbers at once so you never have to guess which system a label used.

Quick conversion table

Use this table for fast lookups. The decimal column divides by 1,000; the binary columns divide the same byte total by 1,024 (twice for MiB, three times for GiB):

Reading the table: 1,000 MB is exactly 1 GB in decimal, but the same bytes count as only 0.9313 GiB, about 6.9% less. That percentage is the whole story of the "missing storage" complaint. The binary columns redivide the byte total by 1,024 at every step (bytes to KiB to MiB to GiB), which is why the gap grows as the numbers get bigger.

Why your drive shows less than the label

A 1 TB drive contains 1,000,000,000,000 bytes. Nothing is hidden; that is exactly what you bought. Windows reads that byte total and divides by 1,024 three times because it reports storage in binary units:

1,000,000,000,000 / 1,024 = 976,562,500 KiB, then 976,562,500 / 1,024 = 953,674.32 MiB, then 953,674.32 / 1,024 = 931.32 GiB.

So the "missing" 68.68 GB was never there; the label counted in decimal and the operating system counted in binary. The same math explains a 500 GB drive showing about 465.66 GiB, and a 128 GB phone showing about 119.21 GiB.

Since 1998 the IEC 60027-2 standard has provided the binary prefixes (KiB, MiB, GiB, TiB) precisely to end this confusion. macOS reports file sizes in decimal units since version 10.6, which is why the same drive can look "smaller" when you plug it into a Windows machine than when it was attached to a Mac. Neither number is wrong; they use different rulers.

The standards behind MB and MiB

The megabyte has a documented history of double duty. In the early decades of computing, memory sizes were powers of two, and engineers loosely borrowed the metric prefix mega for 1,024 x 1,024 = 1,048,576 bytes. Meanwhile storage makers and telecommunications engineers kept using mega in its strict SI sense of 1,000,000.

Two standards bodies fixed the naming:

• IEC 60027-2 (1998, amended 2005) introduced the binary prefixes kibi (Ki), mebi (Mi), gibi (Gi) and tebi (Ti). Under this standard, 1 MiB is exactly 1,048,576 bytes and 1 MB stays at 1,000,000 bytes.
• NIST SP 811, the US Guide for the Use of the International System of Units, adopts the same position: SI prefixes mean powers of 1,000, and binary multiples get the IEC prefixes. The companion NIST SP 330 is the US edition of the SI Brochure.

IEEE 1541, the IEEE standard for binary prefixes, says the same thing for computing practice: KiB = 1,024 bytes, MiB = 1,024 KiB, GiB = 1,024 MiB. In day to day use, Windows still prints GB when it means GiB, which keeps the calculator useful: enter a value, read both columns, and compare like with like.

MB, Mb, Mbps: do not mix them up

The single most common unit mistake in tech is reading megabits as megabytes. The capital B matters:

• MB (capital B) is a megabyte, 8 megabits, the unit file sizes use.
• Mb (lowercase b) is a megabit, one eighth of a megabyte.
• Mbps is megabits per second, the unit internet plans are sold in.

A worked example ties them together. You want to download a 4,000 MB file on a 200 Mbps connection:

4,000 MB x 8 = 32,000 megabits. At 200 Mbps the download takes 32,000 / 200 = 160 seconds, or 2 minutes 40 seconds, in the ideal case.

Real downloads run slower than the plan speed because of protocol overhead, Wi-Fi losses and the server's own limits, so treat the ideal time as a floor. If a provider advertises 1,000 Mbps, that is 1,000 / 8 = 125 MB per second at best, which is 125 x 60 = 7,500 MB (7.5 GB) per minute under perfect conditions.

Practical examples: files, photos and streaming

Conversions become useful when you plan storage. Here is a realistic budget for a 128 GB phone (128,000 MB decimal):

The total is 600 + 2,700 + 7,800 + 6,000 + 22,000 = 39,100 MB, leaving 128,000 - 39,100 = 88,900 MB (88.9 GB) free. Streaming consumes data at a different pace: a 4K stream at 45 Mbps uses 45 / 8 = 5.625 MB per second, which is 5.625 x 3,600 = 20,250 MB (about 20.25 GB) per hour.

Decimal vs binary: which should you use

Both systems are correct; each fits a different job:

• Use decimal (1 GB = 1,000 MB) when buying storage, reading spec sheets, checking phone capacity or comparing drive sizes. Manufacturers and the SI standard both use decimal, so comparing products in decimal compares like with like.
• Use binary (1 GiB = 1,024 MiB) when reading what an operating system reports, planning memory upgrades or checking how much space Windows says is free.

The gap between the systems grows with size, because it compounds once per prefix step:

The percentage is constant at about 6.9% once you compare a decimal label against a binary readout two steps apart (GB vs GiB), because (1,000/1,024)^3 = 0.9313. If a seller and an operating system agreed on units, the number would match; the "loss" is pure unit conversion.

Frequently miscalculated conversions

Three conversion mistakes show up again and again. Each one comes from mixing the two systems in the same calculation:

Mistake 1: dividing by 1,024 when the label is decimal. A 256 GB phone does not hold 256,000 / 1,024 = 250 GB of apps in any meaningful sense; the label counts 256,000,000,000 bytes, so the usable space in Windows terms is 256,000,000,000 / 1,024 / 1,024 / 1,024 = 238.42 GiB.

Mistake 2: multiplying megabits by 1,024. Internet speeds are decimal by definition (1 Mbps = 1,000,000 bits per second). A 300 Mbps plan moves 300,000,000 bits per second, which is 37.5 MB per second, not 36.6.

Mistake 3: converting twice. Going from 3,000 MB to GiB and back to MB with the wrong divisor ends in drift: 3,000 / 1,024 = 2.93 GiB, and 2.93 x 1,000 = 2,930 MB, which is not where you started. Convert from bytes (or stick to one system end to end) and the round trip stays exact.

The safe habit: decide which system you are in before you touch a calculator, and carry the same divisor through every step. The tool above labels both columns so mixed-system answers stop being a surprise.