In short: for a second drive in a home PC, 1-2 TB is usually enough; for a game library and media, 2-4 TB; for an 8-camera recorder, about 4 TB; for NAS, 4 TB per drive and up. If you just need to revive an old PC or laptop, 500 GB is the cheapest workable option. Leave headroom: a drive more than 85-90 % full runs slower, and data has a habit of piling up.
Why a new drive shows less than the label
Manufacturers count capacity in decimal units (1 TB = 1,000,000,000,000 bytes), while Windows shows it in binary units (1 "GB" in Windows = 1,073,741,824 bytes). That is why a new drive looks smaller in File Explorer:
| On the box | In Windows |
|---|---|
| 500 GB | ~465 GB |
| 1 TB | ~931 GB |
| 2 TB | ~1.81 TB |
| 4 TB | ~3.63 TB |
| 8 TB | ~7.27 TB |
It is not a defect or a trick, just a different way of counting. No space has gone missing.
How much space different tasks need
Old PC or laptop: 500 GB - 1 TB
Office apps, a browser, documents, a few years of phone photos. If the drive replaces an old one that has started failing, 500 GB is the best value. A laptop needs a 2.5" drive.
Second drive in a PC: 1-2 TB
The classic setup: system on an SSD, everything else on an HDD. 1 TB covers documents, photos and a few games, while 2 TB gives room for several years.
Game library and media: 2-4 TB
Modern games take tens of gigabytes each, and big titles over a hundred. High-quality films also eat space quickly. A sensible minimum is 2 TB; 4 TB is comfortable.
CCTV: 1-8 TB
Depends on the number of cameras, bitrate and how far back you keep footage. Eight 2 MP H.265 cameras recording 24/7 need about 1.2 TB per week. A detailed table is in How to choose an HDD for CCTV.
NAS and home archive: 4 TB per drive and up
In a RAID array part of the space goes to data protection, so usable capacity is less than the sum of the drives. How to calculate it is explained in How to choose an HDD for NAS. Larger options are in the 6 TB and 8 TB and larger filters.
Backups: twice your data
A backup drive should hold all your important data plus a few previous versions. If you have 1 TB of valuable files, get a 2 TB external drive.
What is the maximum hard drive capacity
The largest production HDDs for data centers already exceed 20 TB in a single 3.5" drive, and the limit moves up every year. Home users rarely need that much, but 8-16 TB drives are common for NAS and archives. Before buying a very large drive, check that your motherboard, NAS or recorder supports it.
One big drive or several smaller ones
- One big drive - fewer ports and less space in the case, lower price per terabyte in the mid-range.
- Several smaller drives - a single failure affects only part of your data, but you need more SATA ports and power connectors.
For most home PCs, the best setup is one 2-4 TB drive plus a separate external drive for backups.
