In this article
You buy a 1 TB external hard drive, plug it in, and Windows says 931 GB. Your new 128 GB phone shows even less once it’s set up. Is 69 GB missing? Did you get cheated? No — it’s a units problem that confuses almost everyone.
Two ways to count a gigabyte
Computers work in binary (powers of 2), but the metric system works in powers of 10. So “kilo”, “mega” and “giga” ended up with two meanings:
| Unit | Decimal (SI) | Binary |
|---|---|---|
| Kilo | 1,000 bytes (KB) | 1,024 bytes (KiB) |
| Mega | 1,000,000 bytes (MB) | 1,048,576 bytes (MiB) |
| Giga | 1,000,000,000 bytes (GB) | 1,073,741,824 bytes (GiB) |
| Tera | 1,000,000,000,000 bytes (TB) | 1,099,511,627,776 bytes (TiB) |
- Drive and phone makers use the decimal definition — the official SI meaning.
- Windows (and some other software) calculates in binary but still labels the result “GB”.
The maths
A 1 TB drive contains 1,000,000,000,000 bytes. Divide by 1,073,741,824 (bytes per binary gigabyte):
1,000,000,000,000 ÷ 1,073,741,824 ≈ 931.3
So Windows shows 931 GB — which is really 931 GiB. Every byte you paid for is there.
The gap grows with size: a binary kilobyte is 2.4% bigger than a decimal one, a binary gigabyte about 7.4% bigger and a binary terabyte almost 10% bigger, because each step multiplies the difference between 1,000 and 1,024.
What your devices will show
| Advertised | Shown as (binary) |
|---|---|
| 128 GB | ≈ 119.2 GB |
| 256 GB | ≈ 238.4 GB |
| 512 GB | ≈ 476.8 GB |
| 1 TB | ≈ 931.3 GB |
| 2 TB | ≈ 1.82 TB (1,862.6 GB) |
| 4 TB | ≈ 3.64 TB (3,725.3 GB) |
Why phones show even less
On top of the units difference, phones and computers reserve space for:
- The operating system (Android or iOS) and preinstalled apps
- System files, recovery partitions and caches
That’s why a “128 GB” phone might show roughly 100–110 GB available to you out of the box.
Internet speeds use different units again
Internet speeds are measured in bits, not bytes. 8 bits = 1 byte, so a 100 Mbps connection downloads at most about 12.5 MB per second. That’s why a 1 GB file doesn’t download in 10 seconds on a “100 Mbps” plan.
Quick tips
- When buying storage, expect about 7% less shown in Windows for GB-sized drives and 9% less for TB-sized drives.
- Compare products using the advertised (decimal) sizes — they’re all measured the same way.
- Check free space, not just total capacity, when planning big downloads or backups.
Convert any data unit
Our Unit Converter includes both decimal units (KB, MB, GB, TB) and binary units (KiB, MiB, GiB, TiB), plus bits — so you can see exactly what any storage size or download will look like.
Free toolTry the Unit ConverterConvert length, weight, temperature, area, volume, speed, time and data.Frequently asked questions
Is the manufacturer cheating?
No. They use the official decimal (SI) definition of a gigabyte. The difference comes from the operating system reporting in binary units while still labelling them GB.
Why does macOS show the full amount?
Since 2009, macOS reports storage using decimal units, so a 1 TB drive shows about 1 TB. Windows still uses binary units with GB labels.
Do RAM sizes work the same way?
RAM is built in binary sizes, so 8 GB of RAM really is 8 GiB (8 × 1,073,741,824 bytes).
Written by the Toolsopedia team. Spotted an error? Tell us and we’ll fix it.