What this calculator works out
This calculator estimates the capacity a backup repository needs from the protected data, the number of full backups retained, the daily change rate and the incremental retention period, then applies a compression ratio.
Retention is what drives the total. Backing up 1 TB does not need 1 TB of storage — it needs enough for every version you intend to keep.
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Before you size a repository
- Daily change rate is the proportion of the protected data that changes each day. 2 to 5% is typical for general file data; databases and virtual machines can be far higher.
- Compression ratios vary enormously by data type. Documents and databases compress well; photographs, video and already-compressed archives barely at all.
- Deduplication, where available, often saves more than compression and is not modelled here.
- Add headroom. A repository at 95% full cannot complete the run that would free space.
Keeping thirty days of increments means you can recover a file as it was thirty days ago. Ransomware and slow-acting corruption are the reasons that matters — damage is frequently discovered weeks after it started, and a backup chain shorter than the discovery window contains only damaged copies.
How this calculator works
Fulls plus increments, then compression:
Raw = protected data × full backups + protected data × daily change × incremental days
Storage needed = raw ÷ compression ratioThe full backups term dominates for short retention; the incremental term dominates as retention lengthens. Where storage is tight, reducing the number of full copies usually saves more than shortening the incremental chain — and costs more in recovery time.
Worked example: 1 TB protected
Using the default figures — 1,000 GB protected, 2 full backups, 3% daily change over 30 days, 1.5:1 compression:
- Before compression: 2,900 GB
- Storage required: 1,933 GB
Nearly twice the protected data, for a modest retention policy. Extend to 90 days of increments and the raw figure becomes 4,700 GB; add a third full copy and it is 5,700. The instinct to buy a backup drive the same size as the data being protected is the most common sizing error, and it is out by a factor of two before retention is even considered.
Common mistakes
- Buying a backup drive the same size as the source. Retention needs multiples.
- Assuming compression on media files. Photographs and video are already compressed.
- Filling the repository. A full repository cannot complete the job that would clear it.
- Retaining less than the discovery window. Damage found late needs a copy from before it started.
- Counting a sync as a backup. Deletions and corruption propagate.
Frequently asked questions
What is the difference between sync and backup?
A sync makes two locations match, so a deletion or corruption on one propagates to the other. A backup keeps historical versions, so an earlier state can be recovered. Cloud sync services frequently offer limited version history, which helps, but a service designed for sync is not a substitute for one designed for backup.
How long should I keep backups?
Long enough to cover the time it might take to notice a problem. Thirty days is a common minimum for general data; some organisations keep monthly copies for a year or longer. Legal and regulatory retention requirements apply to some categories of data and set a floor rather than a ceiling.
What is an immutable backup?
A copy that cannot be altered or deleted for a defined period, even by an administrator. It exists because ransomware increasingly targets backups first — an attacker with credentials can otherwise delete the recovery path before encrypting anything. The NCSC recommends offline or immutable copies for exactly this reason.
What is the 3-2-1 rule?
Three copies of the data, on at least two types of media, with one kept off-site. Each part defeats a different failure: drive failure, accidental deletion, and fire, theft or ransomware.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
Related tools
References
- National Cyber Security Centre — backup strategy, offline copies and ransomware mitigation
- Information Commissioner's Office — data retention, minimisation and security obligations
Sources are checked at publication and can change — how I choose and check references.