Computers & Laptops

Backup Storage Calculator

How much storage a backup plan needs, allowing for copies, retained versions and growth.

7 inputs Free, no sign-up

Your figures

How many places each backup goes - an external drive and a cloud account is two.

How many previous backups each destination holds.

Optional. Compounded over the period below.

Optional. Anything the backup software adds on top - indexes, catalogues, 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

The trap in backup sizing is counting the same thing twice. "Three copies" and "four weeks of history" are two multipliers only if they really are independent — three destinations each holding four snapshots genuinely is twelve, but three copies of a four-week history kept on one drive is four.

This page asks for them separately, multiplies them, and says which reading it took so you can correct it if you meant the other.

How to use this calculator

  1. Enter how much data you are backing up.
  2. Enter how many separate destinations each backup goes to.
  3. Enter how many versions each destination keeps.
  4. Add growth and a period if the data is increasing, and overhead if your software needs it.

The formula

Grown source = Source x (1 + growth / 100) ^ years
Full copies  = Destinations x Versions each
Storage      = Grown source x Full copies x (1 + overhead / 100)

Growth compounds rather than adding, because data does not grow linearly and treating it as though it does understates a three-year plan noticeably.

SymbolMeaningUnit
Source Data being backed up GB
Destinations Separate places
Versions History kept on each
Growth Your own estimate % a year

Worked example

500 GB, two destinations, four versions each, 20% growth a year over three years, 10% overhead:

Grown  = 500 x 1.20^3 = 864 GB
Copies = 2 x 4         = 8
Stored = 864 x 8       = 6,912 GB
Total  = 6,912 x 1.10  = 7,603 GB = 7.6 TB

Notes and limits

  • Every copy is counted as a full copy. Incremental and deduplicating backups store far less after the first run — often a small fraction — so this is the safe end of the estimate rather than the likely one.
  • If your versions are shared across destinations rather than held on each, set destinations to 1 and count the versions only.
  • A backup on the same drive as the original is not a backup. The copies figure only means something if the destinations are genuinely separate, with at least one off-site.
  • Growth is your own estimate. Nothing on this page predicts it.

What this calculation assumes

  • Every copy is counted as a full copy; incremental and deduplicated backups use much less.
  • Growth compounds over the period and is your own estimate.
  • Destinations and versions are multiplied together, and the result says so.

Frequently asked questions

How much storage do I need for backups?

Multiply your data by the number of full copies you keep, then allow for growth. 500 GB kept as eight full copies is about 4 TB before growth - though incremental backups will use considerably less in practice.

Should I multiply copies by retention?

Only if they are independent. Three destinations each keeping four versions is twelve full copies. Four versions kept in one place, mirrored to three destinations, is still four versions per destination - so set destinations to 3 and versions to 4 for the first, and think again if your setup is the second.

Do incremental backups need this much?

No, usually far less. After the first full copy, an incremental backup stores only what changed, and a deduplicating one stores each unique block once. This figure is the full-copy worst case.

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