When you need this conversion
Comparing drives, backup plans and cloud storage tiers. Prices are often per terabyte, while the files you need to store are counted in gigabytes.
Why drives look smaller than advertised
Drive capacity is sold in decimal: 1 TB = 1,000,000,000,000 bytes. Windows divides by 1,024 at each step and shows 931 GB. macOS uses decimal and shows 1 TB. Both describe the same number of bytes. Use GB to GiB to see the Windows figure for any size.
Planning a backup
Add up what you need to store, then leave room to grow:
- 500 GB of photos and documents fits on a 1 TB drive with space to spare.
- A 4 TB drive holds roughly 1,000 hours of 1080p video at 8 Mbps.
- For versioned backups, plan for 1.5–2 times the data you have today.
Common mistakes
Expecting the full capacity to be free. Formatting and recovery partitions use some space.
Mixing TB and TiB in price comparisons. A plan listed in TiB is about 10% larger than one listed in TB.
For smaller units, see GB to MB.