Storage

Backup Storage Calculator

Plan repository capacity for retained full backups and incremental changes. State the source size, how many copies you keep, and an assumed compression reduction. The result separates full and incremental storage before adding headroom.

Your estimate

Full backup storage: 700 GB
Incremental storage: 245 GB
Estimated total backup storage: 945 GB
Recommended capacity: 1,134 GB
Recommended capacity in TB: 1.134 TB

Estimate only. Deduplication, compression, retention dependencies, metadata and backup software affect real storage. Compression is space reduction, not a compression ratio; margin is applied after compression.

How backup storage is estimated

This model treats each retained full backup as a copy of the same source data and each retained incremental as a fixed percentage of that source. It is useful for an initial capacity budget. It does not emulate the chain management, block tracking or retention engine of a particular backup product.

Full and incremental storage formulas

Convert source size to decimal GB: 1 TB = 1,000 GB. Stored fraction = 1 − compression percentage ÷ 100. Full backup GB = source GB × full copies × stored fraction. Incremental GB = source GB × daily change percentage ÷ 100 × retained incrementals × stored fraction.

Total GB = full GB + incremental GB. Recommended GB = total GB × (1 + safety margin percentage ÷ 100). Divide by 1,000 for recommended TB. The recommended value is a capacity target, not a disk model or rounded purchase recommendation.

Compression is a reduction, not a ratio

A 30% reduction means storing 70% of the uncompressed data. Already compressed or encrypted files may yield little additional saving. Zero compression is a useful conservative starting point. A 100% reduction is accepted as a mathematical boundary but is not a realistic way to preserve a backup.

Retention, chains and changed data

Enter the total number of incremental backups retained across your full backup chains. Do not multiply that count by full copies again unless it represents additional retained increments. Each incremental in this model uses the same source size and change percentage; it does not accumulate all earlier changes like a differential backup.

Keep the full backups required by your restore chains. A retention setting expressed in days may not equal a simple count of independent files. Source growth, synthetic fulls, immutability windows, replicas and verification workspaces can require extra capacity.

Deduplication and software limitations

Deduplication can share repeated data across backups, so actual occupied space can be lower than this model. Metadata and operational reserves can make it higher. The calculation does not assume any deduplication savings. Validate estimates against a representative backup cycle and your software's reported repository usage.

Examples

500 GB workstation

Input: 500 GB; 2 fulls; 5% daily change; 14 incrementals; 30% compression reduction; 20% margin.
Result: 700 GB full + 245 GB incremental = 945 GB total. Recommended: 1,134 GB (1.134 TB).

2 TB server

Input: 2 TB; 4 fulls; 3% change; 28 incrementals; 20% compression reduction; 20% margin.
Result: 6,400 GB full + 1,344 GB incremental = 7,744 GB total. Recommended: 9,292.8 GB (9.2928 TB).

10 TB NAS

Input: 10 TB; 2 fulls; 2% change; 30 incrementals; no compression; 25% margin.
Result: 20,000 GB full + 6,000 GB incremental = 26,000 GB total. Recommended: 32,500 GB (32.5 TB).

Larger business backup

Input: 50 TB; 4 fulls; 1% change; 60 incrementals; 40% compression reduction; 20% margin.
Result: 120,000 GB full + 18,000 GB incremental = 138,000 GB total. Recommended: 165,600 GB (165.6 TB).

Frequently asked questions

Does this include deduplication?

No. Repeated blocks may occupy less physical space in a deduplicating repository, but the saving is workload- and software-dependent. Do not assume a fixed deduplication ratio without evidence.

Is the daily change percentage cumulative?

Each retained incremental is modeled independently as the entered percentage of the original source size. This is not a differential backup model and does not model source growth.

Can I estimate full-only backups?

Yes. Set incremental count to zero. Full copies and compression are still calculated, and the safety margin applies to their total.

What does compression percentage mean?

It means space saved. Entering 20% leaves 80% of the modeled data to store. It is not the ratio sometimes reported as 2:1 or 3:1.

Is RAID usable capacity enough for backups?

RAID usable capacity is one part of repository sizing. Allow for formatting, operational reserves, growth and backup retention. RAID itself does not replace an independent backup.

Last reviewed: September 23, 2026