You buy a 1 TB hard drive and Windows says it has only 931 GB. Missing space? No — it is two different ways of counting. Here is the simple explanation.
In this guide
- Two systems: decimal and binary
- Why a 1 TB drive shows 931 GB
- Comparison chart
- Convert data sizes
Two systems: decimal and binary
- Decimal (SI): 1 KB = 1,000 bytes, 1 MB = 1,000 KB, 1 GB = 1,000 MB. Used by drive makers, macOS and network speeds.
- Binary (IEC): 1 KiB = 1,024 bytes, 1 MiB = 1,024 KiB, 1 GiB = 1,024 MiB. Windows uses binary maths but labels it “GB”.
Why a 1 TB drive shows 931 GB
The maker’s 1 TB = 1,000,000,000,000 bytes. Windows divides by 1,024³ = 1,073,741,824, giving about 931 “GB” (really 931 GiB). Nothing is missing.
Comparison chart
| Advertised | Bytes | Shown in Windows |
|---|---|---|
| 128 GB | 128,000,000,000 | ≈ 119 GB |
| 256 GB | 256,000,000,000 | ≈ 238 GB |
| 500 GB | 500,000,000,000 | ≈ 465 GB |
| 1 TB | 1,000,000,000,000 | ≈ 931 GB |
| 2 TB | 2,000,000,000,000 | ≈ 1.81 TB |
Convert data sizes
Our converters use decimal units: try GB to MB, TB to GB and MB to KB.
Frequently asked questions
So is 1 GB 1000 or 1024 MB?
Officially 1,000 MB. 1,024 MiB is a gibibyte (GiB), though Windows calls it GB.
Does my phone use 1000 or 1024?
Most phones and macOS use decimal (1000), so the numbers match the box more closely.
Is RAM measured differently?
RAM is traditionally sold in binary sizes, so 16 GB of RAM is really 16 GiB.
