Computers & Laptops

PC Backup Storage Calculator

How large a backup drive one computer needs, allowing for versions and for the data growing.

7 inputs Free, no sign-up

Your figures

How many previous backups the drive holds before the oldest is dropped.

One external drive is 1. A second off-site drive makes it 2.

How long you want the drive to last before it is full.

What the backup software adds - catalogues, indexes and spare room.

Try an example

Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

A backup drive the same size as your data is not big enough. Keeping any history at all means keeping several copies, and the data grows while the drive does not.

This works out roughly how large a drive to buy so that it lasts the period you have in mind.

How to use this calculator

  1. Enter how much data is on the machine. Your file manager gives the used space.
  2. Say how many versions the backup keeps, and how many separate drives you use.
  3. Set the growth you expect and how long the drive should last.

The formula

Grown data   = Data x (1 + growth / 100) ^ years
Full copies  = Drives x Versions
Storage      = Grown data x Full copies x (1 + overhead / 100)
SymbolMeaningUnit
Data What is on the machine GB
Versions History kept
Growth Your own estimate % a year

Worked example

400 GB on the machine, three versions on one drive, 20% growth over two years, 10% overhead:

Grown  = 400 x 1.20^2  = 576 GB
Copies = 1 x 3          = 3
Stored = 576 x 3        = 1,728 GB
Total  = 1,728 x 1.10   = 1,901 GB

So a 2 TB drive, which is exactly the size people buy by instinct — but now for a reason.

Notes and limits

  • This is an estimated storage requirement, not a guaranteed capacity. Every version is counted as a full copy.
  • Windows File History, Time Machine and most backup software store increments rather than full copies, so real use is usually well below this. That makes this the safe end.
  • A drive sold as 2 TB gives about 1.86 TiB once your operating system counts it. The usable storage calculator shows where that goes.
  • One drive in the same room as the computer protects against drive failure and nothing else. A second copy elsewhere is what protects against theft, fire and flood.

What this calculation assumes

  • Every version is counted as a full copy; incremental backups use far less.
  • Growth compounds over the period and is your own estimate.
  • The result is an estimated requirement rather than a guaranteed capacity.

Frequently asked questions

How big a backup drive do I need?

Enough for your data multiplied by the versions you keep, plus room to grow. 400 GB with three versions and two years of growth points at about 1.9 TB, so a 2 TB drive.

Is twice my data enough?

Only if you keep a single version and the data never grows. Any real history needs more, which is what this page works out.

Do incremental backups need this much?

No - considerably less, because only changes are stored after the first full copy. This figure is the full-copy worst case, which is a reasonable basis for buying a drive you want to last.

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